United States Office of Science March 2006 

Environmental Protection 

Agency 

and Technology 

 Washington, D.C. 

EPA Office of Water

Aeromonas:

Human Health Criteria Document

Electron Micrograph of Aeromonas, courtesy of Northwest Fisheries Science Center 
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United States Office of Science March 2006 

Environmental Protection 

Agency 

and Technology 

 Washington, D.C. 

EPA Office of Water

Aeromonas:

Human Healh Criteria Document

Prepared for 

Office of Ground Water and Drinking Water 

Office of Water 

Prepared by 

Health and Ecological Criteria Division 

Office of Science and Technology 

Office of Water 

U. S. Environmental Protection Agency 

Washington, DC 20460 

Under contract to 

The Cadmus Group, Inc 

EPA Contract No. 68-C-02-026 
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Aeromonas: Human Health Criteria Document 

DISCLAIMER 

This document has been reviewed in accordance with U.S Environmental Protection Agency 

policy and approved for publication and distribution. The project described here was managed by 

the Office of Science and Technology, Office of Water, US EPA under EPA Contract 68-C-02-

026 to the Cadmus Group, Inc. 

Mention of commercial products, trade names, or services in this document or in the references 

and/or endnotes cited in this document does not convey, and should not be interpreted as 

conveying official EPA approval, endorsement, or recommendation. 

EPA/OW/OST/HECD v 
Aeromonas: Human Health Criteria Document 

FOREWORD 

The mission of the United States Environmental Protection Agency’s (EPA) Office of Water 

(OW) is to protect public health and the environment from adverse effects of pollutants (e.g., 

toxic chemicals and microbial pathogens) in ambient water, drinking water, wastewater, sewage 

sludge and sediments. The Safe Drinking Water Act (SDWA) requires the US EPA to regulate 

disease-causing organisms (pathogens) and toxic chemicals in drinking water. 

SDWA, as amended in 1996, requires the US EPA to publish a list of unregulated 

microbiological and chemical contaminants that are known or anticipated to occur in public 

water systems and that may need regulation with a national primary drinking water regulation 

(NPDWR). SDWA also requires that the Agency make regulatory determinations on at least five 

contaminants on the list every five years. EPA published this list of unregulated contaminants, 

referred to as the Contaminant Candidate List (CCL), in March 1998 (63 FR 10273). SDWA 

requires a new CCL to be published every five years thereafter. The Agency published the 

second CCL (CCL 2) in February 2005 (70 FR 9071). Both 1st and 2nd CCLs (CCL 1 and CCL 2) 

contain Aeromonas hydrophila (A. hydrophila), a member of the genus Aeromonas, as a potential 

waterborne pathogen. For each contaminant on the CCL, the Agency will need to obtain 

sufficient data to conduct analyses on the extent of occurrence and the risk posed to populations 

via drinking water. This information will, ultimately, assist the Agency in determining the 

appropriate course of action (e.g., develop a regulation, develop guidance, or make a decision not 

to regulate the contaminant in drinking water). 

This Aeromonas Human Health Criteria Document constitutes a comprehensive review of 

published literature on the genus Aeromonas providing all relevant information on the 

Aeromonas group of organisms, including but not limited to, the general characteristics of 

Aeromonas, its occurrence in human and animal populations and in water, the health effects 

associated with Aeromonas infection, outbreak data, and an assessment of risk. This document 

also includes information about analytical methods used to detect and quantify Aeromonas in 

water and the effectiveness of removal for various water treatment practices. This document will 

be useful as the Agency evaluates Aeromonas through its CCL and regulatory determinations 

efforts. 

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Aeromonas: Human Health Criteria Document 

ACKNOWLEDGEMENTS 

The development of this criteria document was made possible through an effort led by Dr. Jafrul 

Hasan, EPA Project Manager, U.S. Environmental Protection Ageny, Office of Science and 

Technology, Office of Water. This criteria document was prepared for the U.S. Environmental 

Protection Agnecy, Office of Ground Water and Drinking Water, Office of Water. EPA 

acknowledges the valuable contributions of US EPA Internal Technical Reviewers who reviewed 

this document: Dr. Paul Berger, US EPA Office of Ground Water and Drinking Water, Office of 

Water; Dr. Gerald Stelma, US EPA National Exposure Research Laboratory, Office of Research 

and Development; Dr. James Sinclair, US EPA Office of Ground Water and Drinking Water, 

Office of Water; and Dr. Keya Sen, US EPA Office of Ground Water and Drinking Water, 

Office of Water. 

The project described here was managed by the Office of Science and Technology, Office of 

Water, US EPA under EPA Contract 68-C-02-026 to the Cadmus Group, Inc. EPA also wishes to 

thank Dr. George Hallberg, Cadmus Project Manager, Dr. Nelson Moyer, Lead Technical Expert 

Microbiologist and Contracting Author, and Ms. Susan Bjork, Cadmus Analyst for their 

contributions and invaluable support. 

EPA also thanks the External Peer Reviewers (see External Peer Review Workgroup) for their 

excellent review and valuable comments on the draft document. 

The primary contact regarding questions or comments to this document is: 

Jafrul Hasan, Ph.D. 

Microbiologist 

USEPA Headquarters 

Office of Science and Technology, Office of Water 

1200 Pennsylvania Avenue, N. W 

Mail Code: 4304T 

Washington, DC 20460 

Phone: 202-566-1322 

Email: hasan.jafrul@epa.gov 

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Aeromonas: Human Health Criteria Document 

EXTERNAL PEER REVIEW WORKGROUP 

The External Peer Review was managed by the Office of Science and Technology, Office of 

Water, US EPA under EPA Contract No. 68-C-02-091 to Versar Inc. The following 

professionals were part of the External Peer Review Workgroup that provided excellent technical 

and scientific review on the May 2005 Draft regarding the content and technical approach in 

response to EPA Charge to the Peer Reviewers. 

Dr. Ashok K Chopra, University of Texas Medical Branch, Galveston, TX 

Dr. Amy J Horneman, University of Maryland School of Medicine, Baltimore, MD 

Dr. John M Janda, Microbial Disease Laboratory, Richmond, CA 

Their comments were reviewed and incorporated where appropriate to develop this final human 

health criteria document on Aeromonas. 

Potential areas for conflict of interest were investigated with the Peer Reviewers and review of 

their current affiliations. No conflicts of interest were identified. 

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Contents 

Disclaimer.....................................................................................................................................................................v

Foreword .....................................................................................................................................................................vi

Acknowledgements ................................................................................................................................................... vii

External Peer Review Workgroup ......................................................................................................................... viii

1.0 Executive Summary ......................................................................................................................................1 

1.1 General Information..................................................................................................................................1 

1.2 Occurrence.................................................................................................................................................2 

1.3 Health Effects in Humans.........................................................................................................................2 

1.4 Health Effects in Animals .........................................................................................................................5 

1.5 Risk Assessment .........................................................................................................................................5 

1.6 Methods......................................................................................................................................................6 

1.7 Water Treatment ........................................................................................................................................7 

1.8 Indicators of Occurrence and Treatability................................................................................................7 

2.0 General Information .....................................................................................................................................8 

2.1 History........................................................................................................................................................8 

2.2 Taxonomy ................................................................................................................................................11 

2.3 Description of the Genus .........................................................................................................................15 

2.4 Methods....................................................................................................................................................19 

2.4.1 Isolation and Enumeration from Environmental Samples ...................................................................23

2.4.2 Isolation from Clinical Specimens.......................................................................................................24

2.4.3 Culture Identification Methods ............................................................................................................25

2.4.4 Molecular Detection Methods..............................................................................................................29

2.4.5 Typing Methods...................................................................................................................................32

2.5 Summary ..................................................................................................................................................34

3.0 Occurrence...................................................................................................................................................35

3.1 Worldwide Occurrence ............................................................................................................................36

3.1.1 Occurrence in Human Population ........................................................................................................36

3.1.2 Occurrence in Animal Population........................................................................................................38

3.1.3 Occurrence in Water ............................................................................................................................41

3.1.4 Occurrence in Food..............................................................................................................................46

3.1.5 Occurrence in Other Media..................................................................................................................48

3.2 Environmental Factors Affecting Survival.............................................................................................48

3.2.1 Survival in Water .................................................................................................................................49

3.2.2 Survival in Food ..................................................................................................................................51

3.2.3 Viable but Non-culturable State...........................................................................................................53

3.2.4 Preservation in Stock Cultures.............................................................................................................54

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3.3 Summary ..................................................................................................................................................54

4.0 Health Effect in Humans ............................................................................................................................55

4.1 Clinical Symptoms ...................................................................................................................................56

4.1.1 Mechanism of Virulence......................................................................................................................56

4.1.1.1 Cell-Associated Virulence Factors .............................................................................................59

4.1.1.2 Extracellular Virulence Factors..................................................................................................66

4.1.1.3 Type II Secretion Systems..........................................................................................................76

4.1.1.4 Type III Secretion Systems ........................................................................................................76

4.1.1.5 Siderophores...............................................................................................................................77

4.1.1.6 Quorum Sensing.........................................................................................................................78

4.1.1.7 Opsonins.....................................................................................................................................78

4.1.1.8 Glycosylation .............................................................................................................................78

4.1.1.9 Suicide Phenomenon ..................................................................................................................79

4.1.1.10 Vacuolation ................................................................................................................................79

4.1.1.11 Pathogenicity Islands..................................................................................................................79

4.1.2 Gastrointestinal Infection.....................................................................................................................80

4.1.3 Skin and Soft Tissue Infection.............................................................................................................83

4.1.4 Respiratory Tract Infections ................................................................................................................86

4.1.5 Meningitis ............................................................................................................................................87

4.1.6 Endocarditis .........................................................................................................................................87

4.1.7 Osteomyelitis .......................................................................................................................................87

4.1.8 Hepatobiliary Infections ......................................................................................................................87

4.1.9 Bacteremia/Septicemia ........................................................................................................................88

4.1.10 Peritonitis ........................................................................................................................................88

4.1.11 Hemorrhagic Uremic Syndrome .....................................................................................................89

4.1.12 Ocular Infections.............................................................................................................................89

4.1.13 Septic Arthritis ................................................................................................................................90

4.2 Predisposing Factors ...............................................................................................................................90

4.3 Chronic Conditions/Sequelae..................................................................................................................90

4.4 Dose Response .........................................................................................................................................91

4.5 Immunity/Vaccines ..................................................................................................................................91

4.6 Therapeutic Measures .............................................................................................................................91

4.6.1 Antibiotic Treatment............................................................................................................................92

4.6.2 Other Therapeutic Strategies................................................................................................................95

4.7 Sensitive Sub-populations .......................................................................................................................95

4.7.1 Elderly .................................................................................................................................................95

4.7.2 Children ...............................................................................................................................................96

4.7.3 Immunocompromised ..........................................................................................................................97

4.8 Summary ..................................................................................................................................................99

5.0 Health Effect in Animals...........................................................................................................................100

5.1 Clinical Symptoms .................................................................................................................................101

5.1.1 Mechanism of Action/Disease ...........................................................................................................101

5.1.2 Chronic Conditions/Sequelae.............................................................................................................101

5.2 Dose Response .......................................................................................................................................102

5.3 Immunity/Vaccines ................................................................................................................................102

5.4 Therapeutic Measures ...........................................................................................................................102

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5.4.1 Antibiotic Treatment..........................................................................................................................102

5.4.2 Other Therapeutic Measures ..............................................................................................................102

5.5 Sensitive Subpopulations.......................................................................................................................103

5.6 Summary ................................................................................................................................................103

6.0 Epidemiology and Disease Outbreaks .....................................................................................................103

6.1 Transmission Routes and Mechanisms.................................................................................................103

6.1.1 Foodborne Transmission....................................................................................................................103

6.1.2 Waterborne Transmission ..................................................................................................................104

6.1.3 Person-to-person Transmission..........................................................................................................105

6.1.4 Animal-to-person Transmission.........................................................................................................105

6.1.5 Environmental Transmission .............................................................................................................105

6.2 Disease Outbreaks .................................................................................................................................106

6.2.1 Outbreaks Associated with Drinking Water ......................................................................................106

6.2.2 Outbreaks Associated with Recreational Water.................................................................................107

6.2.3 Outbreaks Associated with Other Water Sources ..............................................................................107

6.2.4 Food ...................................................................................................................................................107

6.2.5 Travelers ............................................................................................................................................109

6.2.6 Other Sources.....................................................................................................................................109

6.3 Summary ................................................................................................................................................109

7.0 Risk Assessment.........................................................................................................................................110

7.1 Risk Assessment Paradigms/Models .....................................................................................................112

7.1.1 Assessment of Virulence Factors as an Indication of Risk ................................................................112

7.1.1.1 Studies Characterizing Virulence Markers in Clinical and Environmental Isolates.................113

7.1.1.2 Studies Characterizing Virulence Markers in Clinical Isolates ................................................115

7.1.1.3 Studies Characterizing Virulence Markers in Environmental Isolates .....................................117

7.1.1.4 Studies Characterizing Virulence Markers in Food Isolates ....................................................119

7.1.1.5 Studies Characterizing Virulence Markers in Animal Isolates.................................................123

7.2 Risk Assessment Case-Control Studies .................................................................................................123

7.2.1 Laboratory Studies.............................................................................................................................123

7.2.2 Studies in Younger Children..............................................................................................................124

7.2.3 Field Studies ......................................................................................................................................125

7.2.4 Animal Studies...................................................................................................................................125

7.2.5 Typing Studies ...................................................................................................................................126

7.3 Risk Factors ...........................................................................................................................................126

7.4 Potential for Human Exposure .............................................................................................................127

7.5 Risk Management ..................................................................................................................................127

7.6 Summary ................................................................................................................................................128

8.0 Water Treatment.......................................................................................................................................129

8.1 Conventional Water and Wastewater Treatment..................................................................................130

8.1.1 Slow Sand Filtration ..........................................................................................................................130

8.1.2 Activated Sludge Plant/Trickling Filter Plant ....................................................................................131

8.1.3 Treatment Plants and Distribution Systems .......................................................................................131

8.1.4 Rapid Gravity Filters .........................................................................................................................132

8.2 Disinfection/Inactivation.......................................................................................................................132

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8.2.1 Disinfection........................................................................................................................................134

8.2.2 Inactivation ........................................................................................................................................135

8.2.3 Distribution Systems..........................................................................................................................135

8.2.4 Survival..............................................................................................................................................136

8.3 Summary ................................................................................................................................................137

9.0 Indicators for Occurrence and Treatability............................................................................................137

9.1 Relationship of Aeromonads to Other Microbial Indicators................................................................138

9.2 Summary ................................................................................................................................................138

10.0 References ..................................................................................................................................................139

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Exhibits 

Exhibit 2.1. Current Genomospecies and Phenospecies of the Genus Aeromonas......................................................14 

Exhibit 2.2. Identification of Genomospecies of the Aeromonas hydrophila Group...................................................18 

Exhibit 2.3. Identification of Genomospecies of the Aeromonas caviae Group..........................................................18 

Exhibit 2.4. Identification of Genomospecies of the Motile Aeromonas.....................................................................19 

Exhibit 2.5. Misidentification of Aeromonads by Commercial Kits ...........................................................................26 

Exhibit 2.6. Recommended Tests for Identification of Aeromonas.............................................................................27 

Exhibit 2.7. Biochemical Tests for Aeromonas spp. Complex Identification (% Positive) .........................................27 

Exhibit 2.8. Biochemical Tests for Separation of A. hydrophila Complex (% Positive) .............................................28 

Exhibit 2.9. Biochemical Tests for Separation of A. caviae Complex (% Positive) ....................................................28 

Exhibit 2.10. Biochemical Tests for Separation of A. sobria Complex (% Positive) ..................................................29 

Exhibit 2.11. PCR Methods for Detection of Aeromonas species ...............................................................................29 

Exhibit 3.1. Aeromonas Population Shift in Activated Sludge....................................................................................45 

Exhibit 3.2. Occurrence of Aeromonas spp. in Foods .................................................................................................47 

Exhibit 4.1. Virulence Factors of Aeromonas species .................................................................................................58 

Exhibit 4.2. Extracellular Enzymes Secreted by Aeromonas spp. ...............................................................................66 

Exhibit 4.3. Distribution of Virulence Genes in Aeromonas spp. Isolated from Patients with Watery Diarrhea ........80 

Exhibit 4.4. Documented Cases of Gastroenteritis Caused by Aeromonas .................................................................82 

Exhibit 4.5. Cases of Gastroenteritis Epidemiologically Linked to Aeromonas..........................................................83 

Exhibit 4.6. Circumstances Promoting Aeromonas Wound Infections........................................................................85 

Exhibit 4.7. Respiratory Tract Infections Caused by Aeromonas................................................................................86 

Exhibit 4.8. Ocular Infections Caused by Aeromonas .................................................................................................89 

Exhibit 6.1. Cases or Outbreaks Caused by Aeromonas spp. ....................................................................................106 

Exhibit 6.2. Foodborne Gastroenteritis Associated with Aeromonas spp..................................................................108 

Exhibit 7.2. Putative Virulence Factors Produced by Aeromonas spp. Isolated from Clinical and Food Sources ....122 

Exhibit 7.2. Distribution of Aeromonas Strains from Various Sources in Bangladesh .............................................124

Exhibit 7.3. Presence of Toxin Genes in Diarrheal Children, Control Children and the Environment .....................124 

Exhibit 8.1. Inactivation of Aeromonas hydrophila by UV Irradiation .....................................................................134 

Exhibit 8.2. Inactivation of Aeromonas hydrophila by Chlorine Dioxide .................................................................134 

Exhibit 8.3. Inactivation of Aeromonas hydrophila by Chlorine...............................................................................135 

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1.0 Executive Summary 

The Safe Drinking Water Act requires the U.S. Environmental Protection Agency (EPA) 

to publish regulations to control disease-causing organisms (pathogens) and hazardous chemicals 

in drinking water. Aeromonas hydrophila (A. hydrophila), a member of the genus Aeromonas, is 

listed on the first and second Contaminant Candidate List (CCL 1 and CCL 2) of potential 

waterborne pathogens (USEPA 1998; 2005). This document constitutes a comprehensive review 

of published literature on the genus Aeromonas, with emphasis on the biology of the organisms, 

their occurrence in the environment, their effects on human health, the methods by which they 

may be detected, and their response to water treatment technologies. 

1.1 General Information 

Members of the genus Aeromonas are facultatively anaerobic, oxidase positive, gramnegative bacteria whose natural habitat is in the aquatic environment. Some species are 

pathogenic for animals and humans (Martin-Carnahan and Joseph 2005). A. hydrophila is listed 

on the CCL, and EPA Method 1605 (USEPA 2001) has been validated for detection and 

enumeration of A. hydrophila in drinking water. 

The taxonomy of the genus Aeromonas has been confusing because of lack of congruity 

between phenotypic and genotypic characteristics of species, and multiple methods are required 

for accurate classification. Currently, 17 DNA-DNA hybridization groups (HG) have been 

described and 14 phenospecies are described in the 2005 edition of Bergey’s Manual (MartinCarnahan and Joseph 2005). Several additional Aeromonas species have been described recently, 

including A. simiae, A. molluscorum, and A. culicicola, though the latter may be a synonym of A. 

veronii. Other species that may be synonyms of recognized species include A. ichthiosmia (A. 

veronii biovar sobria) and A. enteropelogenes (A. trota). Continued use of hybridization groups 

is controversial, as all named phenospecies can be phenotypically identified. The controversies 

involving Aeromonas taxonomy are delineated on the LPSN web page at 

http://www.bacterio.cict.fr/a/aeromonas.html. 

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1.2 Occurrence 

Aeromonas spp. are found worldwide in aquatic environments, including ground water, 

surface waters, estuarine and marine waters, drinking water, and wastewater (Holmes et al., 

1996). 

Aeromonads are found in foods, including fresh grocery produce, seafood, raw meats, 

packaged ready-to-eat meats, cheese, and milk (Palumbo 1996). 

 While Aeromonas spp. are not considered fecal bacteria, they are present in the feces of 

healthy animals and humans, presumably as the result of ingestion of food and water containing 

these organisms (Holmes et al., 1996; Demarta et al., 2000). They are present in high numbers in 

sewage before and after treatment (Monfort and Baleux 1991), thus they have been proposed as 

an indicator of sewage-contaminated surface water. 

Aeromonas spp. may colonize drinking water distribution systems and produce biofilms 

that resist disinfection (Holmes et al., 1996; Bomo et al., 2004). Despite the presence of 

aeromonads in drinking water distribution systems, no point source outbreaks attributable to 

Aeromonas spp. have been reported from ingestion, inhalation, or dermal contact with drinking 

water. 

1.3 Health Effects in Humans 

von Graevenitz and Mensch (1968) reviewed 30 cases of infection and established 

Aeromonas spp. as significant human pathogens causing a variety of extra-intestinal infections in 

various organ systems. Aeromonas infections typically follow trauma in an aquatic environment, 

or dissemination from the gastrointestinal tract as the result of underlying diseases such as 

cancer. Both extra-intestinal and gastrointestinal infections are now known to occur in previously 

healthy hosts as well as immunocompromised or otherwise susceptible populations (Janda and 

Abbott 1996; Joseph 1996; Janda and Abbott 1998). Besides diarrheal illness, Aeromonas spp. 

cause wound infections, septicemia, meningitis, ophthalmitis, endocarditis, aspiration 

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pneumonia, and biliary tract infections. A. hydrophila (HG-1), A. veronii biovar sobria (HG-8), 

A. caviae (HG-4), A. jandaei (HG-9), A. veronii biovar veronii (HG-10), A. schubertii (HG-12), 

and A. trota (HG-14) are isolated from human infections (Janda and Abbottt 1998). Considerable 

strain variation occurs among Aeromonas species, and the proportion of strains within these 

phenospecies that cause human infection is unknown. 

 Virulence of Aeromonas spp. is multifactorial and incompletely understood. Factors 

contributing to virulence include toxins, proteases, hemolysins, lipases, adhesins, agglutinins, 

and various hydrolytic enzymes (Janda and Abbott 1996). Virulence factors are present in two 

forms, cell-associated structures, and extracellular products. Among the cell-associated structures 

are pili, flagella, outer membrane proteins, lipopolysaccharide, and capsules. The major 

extracellular products include cytotoxic, cytolytic, hemolytic, and enterotoxic proteins. 

Aeromonads produce an array of filamentous structures, including short rigid, and long 

wavy pili, and polar and lateral flagella. Short rigid pili are similar to those of E. coli Type I and 

Pap pili (Ho et al., 1992), while long wavy pili belong to a class of Type IV bundle-forming pili 

(Bfp) (Kirov and Sanderson 1996). Removal of pili or neutralization of attachment sites by 

homologous antibody treatment limits or defeats adherence properties in cell culture systems 

(Iwanaga and Hokama 1992; Kirov et al., 1999). The Type IV pili are controlled by the 

tapABCD gene cluster (Pepe et al., 1996). TapD encodes a Type IV peptidase/methyltransferase 

which is responsible for extracellular secretion of aerolysin and other enzymes via a Type II 

secretion system (Strom and Pepe 1999), in addition to its tap pilin-related function. Barnett et 

al. (1997) cloned the tap cluster and demonstrated conclusively that Bfp and Tap pili were 

distinct families. Tap pili have a molecular mass of 17 kilodaltons (kDA), while Bfp pili have a 

molecular mass of 19-23 kDA. Tap pili are thought to facilitate attachment to enterocytes and 

enhance colonization. Bfp pili facilitate adherence to erythrocytes (Kirov et al., 2004). Polar 

flagella and lateral flagella were described by Rabaan et al. (2001) and Kirov et al. (2002). Polar 

flagellins function as adhesions, while lateral flagellins are thought to serve as colonization 

factors (Kirov et al., 2004). 

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Capsule production has been reported for A. salmonicida and A. hydrophila serogroups 

(Martinez et al., 1995), but the function of capsule material is vague. It is presumed to resist 

complement activity and perhaps enhance adherence (Merino et al., 1996). 

S-layers (originally termed A-layer in A. salmonicida) are paracrystalline structures made 

up of identical protein subunits that are translocated across the cell membrane and assembled on 

the cell wall surface via an interaction with O-polysaccharide side chains of lipopolysaccharide. 

Stains producing S-layers are more pathogenic for fish, but the role of S-layer in human infection 

is not clear. Studies suggest that strains containing S-layers autoagglutinate (Kokka et al., 

1991a). 

The hemolysin produced by some Aeromonas species is termed ‘aerolysin’, and it 

posesses both hemolytic and enterotoxic activity. This hemolytic enterotoxin (aerolysin), has 

been shown to share significant homology with the cytotoxic enterotoxin (Act), and two 

cytotonic toxins (Alt and Ast) as reviewed by Xu et al. (1998). Early investigation of Aeromonas 

toxins was responsible for recognition of ‘aerolysin’ in several species of aeromonads; however, 

it appears that ‘aerolysin’ is really a family of toxins whose mechanisms of action are gradually 

being understood. The toxins reported by various investigators have fundamental differences in 

properties, making comparisons difficult (Chakraborty et al., 1987a; Buckley et al., 1981). Act 

increased levels of tumor necrosis factor " (TNF-") and interleukin (IL-1 $) in macrophage cell 

lines and other inflammatory cytokines that result in tissue damage. Prostaglandin activity (PG1 

or PG2) is increased. Aerolysin is released from cells as proaerolysin that is activated by 

proteolytic cleavage of a C-terminal peptide fragment of approximately 40 amino acids. Another 

‘aerolysin’ has been described that is released by a Type II secretion pathway under control of 

exeAB and exeC-N genes. The gene products produce a pore in the outer membrane protein for 

toxin secretion (Howard et al., 1996). This ‘aerolysin’ is a dimer with hemolytic activity for 

mouse erythrocytes. This toxin is involved in channel formation in target cells in a similar 

manner to the way it forms pores in bacterial outer membrane protein, by producing a heptamer 

that inserts into the cell membrane and forms a 1-2 nanometer (nm) channel which results in loss 

of cell permeability, cell leakage, and eventual cell destruction. 

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Aeromonas: Human Health Criteria Document 

Aeromonas spp. produce a lipase, glycerophospholipid cholesterol acyltransferase 

(GCAT), that results in production of cholesteryl esters and phospholipase activity that digests 

plasma membranes of host cells (Buckley 1983). Another class of enzymes, the metallo- and 

serine proteases are involved in toxin activation and have a protective role in inimical 

environments (Rodriguez et al., 1992). Other enzymes that contribute to virulence include 

amylase, chitinase, elastase, lecithinase, and nucleases (Gosling 1996a). 

A. salmonicida strains produce siderophores that facilitate iron acquisition (Chart and 

Trust 1983). Some species of mesophilic aeromonads produce a enterobactin-like siderophore 

under iron-limiting conditions. 

1.4 Health Effects in Animals

 Aeromonads have been reported as pathogens of fish, amphibians, and reptiles (Gosling 

1996b). Aeromonas spp. cause hemorrhagic disease, ulcerative disease, furunculosis, and 

septicemia in fish (Austin and Adams 1996). Aeromonads cause pneumonia, peritonitis, abortion 

and other diseases in birds and domestic animals (Gray 1984; Gosling 1996b). 

1.5 Risk Assessment

 Exposure to Aeromonas spp. through ingestion of food and water is continuous, however 

most environmental strains do not produce gastrointestinal disease in normal humans. No point 

source waterborne disease outbreaks have been attributable to aeromonads through exposure to 

treated drinking water. Case reports suggest that susceptible individuals may acquire 

gastrointestinal illness from chronic exposure to high numbers of aeromonads in untreated water, 

and foodborne disease has been reported. The infectious dose by ingestion is remarkably high, 

based upon bacterial counts in foods implicated in disease. A human feeding study failed to 

induce illness in a significant percentage of volunteers (Morgan et al., 1985), however, this study 

is considered inconclusive since the strain use was selected for the presence of only one 

virulence factor (enterotoxin) and the subjects were healthy adults. In the only risk assessment 

study for exposure via drinking water, Rusin et al. (1997) reported that the oral infectious dose 

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Aeromonas: Human Health Criteria Document 

exceeded 10 log10 colony-forming units per milliliter (CFU/mL) and the risk of infections 

attributable to drinking water was 7.3 per billion exposures. 

The use of water purification devices installed at the tap has become popular, yet devices 

employing activated carbon filtration may actually degrade microbial quality of drinking water. 

Chaidez and Gerba (2004) demonstrated that counts of Pseudomonas and Aeromonas were 

higher in water samples taken after filtration though point-of-use devices. Despite these increases 

in total bacterial load in drinking water, the levels remained substantially below the number 

required to cause gastrointestinal disease. 

Risk assessment studies have demonstrated an increased rate of transmission in daycare 

centers in Ecuador (Sempertegui et al., 1995), nursing homes in the U.S. (Bloom and Bottone 

1990; Sims et al., 1995), and intensive care units in Italy (Torre et al., 1996). Poor personal 

hygiene and reuse of water have been shown to increase the risk of transmission. 

The risk of Aeromonas infections is significant for animals in aquaculture, where 

crowding promotes transmission (Austin and Adams 1996). 

1.6 Methods 

The reconciliation of phenotypic and genotypic methods presented by Martin-Carnahan 

and Joseph (2005) in the Second Edition of Bergey’s Manual of Systematic Bacteriology enables 

identification of most clinical and environmental aeromonads to group and many to species, 

leading Janda and Abbott (1998) to suggest that taxonomic advances make continued use of 

DNA-DNA hybridization groups unnecessary. The myriad of methods for detection, isolation, 

and enumeration of aeromonads from various sources was reviewed by Moyer (1996). Since 

then, the membrane filtration method using Ampicillin Dextrin Agar (ADA) has been published 

in Standard Methods for Examination of Water and Wastewater (APHA 1998), and EPA has 

validated Method 1605: Aeromonas in Finished Water by Membrane Filtration using AmpicillinDextrin Agar with Vancomycin (ADA-V) (USEPA 2001). Ampicillin has been incorporated into 

ADA in various concentrations from 10-30 mg/L by different investigators, but use of ampicillin 

EPA/OW/OST/HECD 6 
Aeromonas: Human Health Criteria Document 

will inhibit the growth ampicillin sensitive aeromonads, including A. trota and some strains of A. 

caviae and A. schubertii. 

Many investigators have published molecular detection methods for one or more species 

of Aeromonas in clinical or environmental samples. Some of these detection systems target genes 

controlling virulence, thereby seeking to differentiate pathogenic from nonpathogenic strains. 

Unless molecular detection methods are preceded by culture enrichment, the detection limit is 

typically greater than 3-4 log10 CFU/mL, rendering these methods too insensitive for direct 

detection of aeromonads in environmental samples. 

1.7 Water Treatment 

Aeromonas spp. are effectively removed by sand filtration and inactivated by 

disinfectants used in water treatment at concentrations commonly employed (Gerba et al., 2003). 

Aeromonads sequestered in biofilms may resist disinfection and persist for long periods of time. 

Use of granulated activated carbon (GAC) in water treatment may provide nutrient sources for 

aeromonads in the form of assimilable organic carbon, contributing to the presence and survival 

of aeromonads in distribution system water. The ability to detect aeromonads in drinking water 

distribution systems is related to temperature, where samples typically test positive only when 

water temperature exceeds 15o C and where chlorine residuals are below 0.2 mg/L. Because grab 

samples may contain cells suspended in the water column, particle-adsorbed cells from filter 

break-though, or microcolonies sloughed from biofilms accumulations, the true number and 

source of aeromonads present in drinking water distribution systems cannot be reliably 

determined. 

1.8 Indicators of Occurrence and Treatability 

Epidemiological evidence does not support the presence of aeromonads in drinking water 

as a threat to public health. A statewide case reporting system in California was discontinued 

after a short time because the number of cases reported was deemed insignificant (King et al., 

1992). 

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Aeromonas: Human Health Criteria Document 

The level of risk aeromonads comprise for immunocompromised hosts from exposure to 

drinking water is unclear, since their primary environmental exposure is from surface water and 

foods (Altwegg and Geiss 1989; Kirov 1993a; Janda and Abbott 1999). Enhanced treatment of 

drinking water using improvements in filtration and disinfection is unlikely to completely 

remove aeromonads from drinking water, since they gain access to water distribution systems 

during construction or maintenance, and maintain populations under favorable conditions of 

temperature and nutrient levels in the absence of disinfection residuals. The number of 

aeromonads typically found in surface source water does not approach an infectious dose for 

humans via ingestion. 

Aeromonas spp. do not correlate with fecal indicators used for monitoring treated 

drinking water. Conventional water treatment processes are effective in removing or inactivating 

aeromonads, however aeromonads may persist in distribution system biofilms when disinfectant 

levels are low (< 0.2 mg/L free chlorine residual). 

2.0 General Information 

 The order Aeromonadales comprises a single family of bacteria, Aeromonadaceae, where 

the genus Aeromonas resides (Martin-Carnahan and Joseph 2005). Aeromonas hydrophila is 

designated as the type species. Aeromonads are naturally occurring inhabitants of aquatic 

environments, namely fresh waters, marine waters, and estuarine waters. Some species of 

aeromonads cause diseases of aquatic animals and opportunistic infections in humans. 

2.1 History

 Bacteria resembling motile Aeromonas species were first isolated from water and 

diseased animals over 100 years ago. The history of their isolation and taxonomy was reviewed 

by Carnahan and Altwegg (1996) and Martin-Carnahan and Joseph (2005). 

Associations of aeromonads with human disease were reported by von Graevenitz and 

Mensch (1968) in a review of 30 cases of Aeromonas infection or colonization, providing 

EPA/OW/OST/HECD 8 
Aeromonas: Human Health Criteria Document 

evidence for their recognition as human pathogens and suggesting that some aeromonads may be 

associated with gastrointestinal disease. Aeromonas spp. are isolated most frequently from fecal 

specimens from children under five years of age, while isolation of aeromonads from other body 

sites typically occurred in adult populations. Aeromonads cause acute diarrheal disease of short 

duration or chronic loose stools in children, the elderly, or the immunocompromised, and they 

have been implicated as a cause of travelers’ diarrhea. Aeromonas spp. cause cellulitis or wound 

infections following traumatic injury in an aqueous environment. They also cause septicemia 

associated with underlying disease such as cirrhosis, leukemia, cancer, and various infections 

associated with hospitalization such as rare urinary tract infections, surgical wound infections, 

meningitis, peritonitis, endocarditis, or other serious infections. Predisposing conditions for 

Aeromonas infection include cirrhosis or other hepatic disease, hematologic malignancies, 

hepatobiliary disease, diabetes, and renal disease. Use of medicinal leaches has resulted in 

infections in patients undergoing reconstructive breast surgery. The species distribution of 

Aeromonas infections as A. hydrophila (48%), with A. sobria, and A. caviae equally distributed 

(25-27%). 

An environmental source of Aeromonas implicated in gastrointestinal infection was first 

proposed by Holmberg et al. (1986), who presented epidemiological evidence to support 

untreated well water as the source of infection in patients with diarrheal disease. A. caviae was 

present in 18 of 34 cases. The role of A. caviae as an agent of gastrointestinal disease remained 

controversial for the next decade until an overwhelming number of case reports overcame the 

last objections. Today, seven species of Aeromonas are recognized to cause a variety of intestinal 

and extra-intestinal infections in humans (Janda and Abbott 1998). Aeromonas spp. have been 

identified as the cause of community acquired infection, nosocomial infection, and travelers’ 

diarrhea (Rautelin et al., 1995). 

The intense interest in Aeromonas spp. in the 1980s lead to the search for improved 

culture methods for their recovery from water, food, clinical specimens, and especially feces, 

since the role of aeromonads as agents of gastrointestinal disease was controversial (Joseph 

1996). Isolation and enumeration methods for all sample types were reviewed by Moyer (1996), 

and phenotypic identification methods were reviewed by Millership (1996). Current 

EPA/OW/OST/HECD 9 
Aeromonas: Human Health Criteria Document 

identification methods for the recognized phenospecies are contained in the Second Edition of 

Bergey’s Manual of Systematic Bacteriology (Martin-Carnahan and Joseph 2005). Methods for 

identification of the newly recognized phenospecies A. hydrophila ssp. dhakensis, A. hydrophila 

ssp. ranae, A. simiae, A. molluscorum, and A. culicicola are not published in this edition of 

Bergey’s Manual. 

An understanding of the mechanisms by which aeromonads cause gastrointestinal disease 

has been frustrated by an inconclusive human feeding study (Morgan et al., 1985) and the lack of 

suitable animal model systems for demonstrating the Koch-Henle postulates (Kelleher and Kirov 

2000). Numerous virulence factors and toxins (Gosling 1996a; Howard et al., 1996; Chopra and 

Houston 1999) have been proposed and examined, but a complete understanding of the multifunctional mechanisms of pathogenesis continues to elude investigators. The first evidence 

supporting a mouse model for Aeromonas virulence studies was published by Sha et al. (2002). 

Other models include the medicinal leech model (Graf 1999) and the blue gourami model (Yu et 

al., 2005). None of these models reproduce gastrointestinal disease in humans. 

Aeromonads are ubiquitous in the aquatic environment (Holmes et al., 1996). Aeromonas 

spp. cause disease in poikilothermic animals, and occasionally in mammals. Diseases caused by 

aeromonads represent a significant source of loss to the aquaculture industry. Aeromonas spp. 

have been found in foods (Palumbo 1996), and they have been shown to cause foodborne 

outbreaks of gastrointestinal disease (Zeng Shan et al., 1988). They have been isolated from 

treated drinking water (LeChavallier et al., 1983; Burke et al., 1984a; van der Kooij 1988; 

Fernandez et al., 2000). Drinking water standards for Aeromonas are in effect in The Netherlands 

(van der Kooij 1988), a bottled water standard is in effect in Canada (Warburton et al., 1998; 

Warburton 2000), and A. hydrophila is listed by EPA on the CCL in the U.S. (USEPA 1998). No 

disease outbreaks associated with treated drinking water have been reported, and the role of 

drinking water as a source of aeromonads causing gastrointestinal disease in not clearly and 

convincingly established. Burke et al. (1984a) demonstrated a seasonal association between the 

presence of aeromonads in a public drinking water supply and the presence of aeromonads in 

human fecal specimens collected from patients with gastroenteritis; however, the causal 

relationship of aeromonads to gastroenteritis was not shown. 

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2.2 Taxonomy 

Until the late 1970s, aeromonads were divided into two groups, based upon physiological 

properties and host range. Motile aeromonads that grew at 35-37o C and were recognized to 

cause human infections were called A. hydrophila. Non-motile aeromonads that grew at 22-28o C 

and infected fish were called A. salmonicida. Besides optimum growth temperature and motility, 

production of indole, and elaboration of a melanin-like pigment on tyrosine agar provided 

reliable phenotypic markers for their differentiation. 

The taxonomy of aeromonads between 1980 and 1990 was complicated by the fact that 

few phenotypic markers were available to reliably differentiate species for the newly recognized 

hybridization groups. Further complication arose from reports of genetically identical strains 

published under different names by different investigators. DNA hybridization groups (HGs) 

were recognized for which there were no reliable phenotypic characteristics, resulting in 

confusion among microbiologists and physicians. Some of the newly recognized aeromonads 

were described from a handful of strains, making it difficult to describe the species 

phenotypically. For example, A. allosacchrophila was described from three isolates (MartinezMurcia et al., 1992), A. encheleia was described from four isolates (Esteve et al., 1995), and A. 

popoffii was described from eight isolates (Huys et al., 1997). 

The current taxonomy of the genus Aeromonas is based upon DNA-DNA hybridization 

and 16S ribosomal DNA relatedness studies. The genera of the family Aeromonadaceae now 

include Aeromonas, Oceanimonas, Oceanisphaera, and Tolumonas (incertae sedis) (MartinCarnahan and Joseph 2005). The current genomospecies and phenospecies within the genus 

Aeromonas are shown in Exhibit 2.1. A. hydrophila ssp. dhakensis (subsp. nov.) (Huys et al., 

2002a), A. hydrophila ssp. ranae (subsp. nov.) (Huys et al., 2003), A. culicicola (sp. nov.) 

(Pidiyar et al., 2002), A. simiae (sp. nov.) (Harf-Monteil et al., 2004), and A. molluscorum (sp. 

nov.) (Minana-Galbis et al 2004a) have been proposed as new species and subspecies of 

Aeromonas, and more will undoubtedly be described. 

EPA/OW/OST/HECD 11 
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The first attempts to identify aeromonads to genotype relied upon differences in 16S 

ribosomal DNA sequences (Martinez-Murcia et al., 1992), and several investigators developed 

probes for detection of various Aeromonas spp. (Demarta et al., 1999; Khan et al., 1999). 

Aeromonas taxonomy based upon 16S ribosomal DNA is complex and investigators have 

examined alternative means to sort out the genotypic maze. Borrell et al. (1997) were able to 

identify ten species using endonucleases AluI and MboI, but needed to add NarI and HaeIII to 

differentiate A. salmonicida from A. encheleia, and their system did not distinguish between A. 

salmonicida and A. bestiarum, nor identify A. popoffii. Figueras et al. (2000) added two 

additional endonucleases NlwNI and PstI to this restriction fragment length polymorphism 

(RFLP) method to differentiate A. salmonicida and A. bestiarum and for recognition of A. 

popoffii. The genetic heterogeneity of aeromonads as the result of cross-over in ribosomal 

sequences makes it unlikely that 16S ribosomal DNA will be a practical means of differentiation 

of species, since additional endonucleases must be added as new species are recognized, 

resulting in an unwieldily and overly complex typing system (Morandi et al., 2005). Genetic 

crossover or recombination may obscure phylogenetic relationships among aeromonads. 

Various other genetic methods are under investigation for species identification. Pidiyar 

et al. (2003a) sequenced the 16S-23S intergenic spacer regions and found that the resulting 

phylogeny did not agree with the results of 16S ribosomal DNA and DNA-DNA hybridization 

studies. The significance of intergenic spacer region analysis is under investigation. The 

sequence of the gyrB gene was used to construct a phylogenetic tree of all 17 hybridization 

groups (Pidiyar et al., 2003b). A. culicicola grouped with A. veronii, based upon gyrB, but it 

grouped with A. jandaei based upon 16S ribosomal gene sequence. From sequence analysis of 

the polymerase chain reaction (PCR) amplicon of gyrB and the gyrB gene, the gyrB gene 

sequence was viewed as a better phylogenetic chronometer than the 16S ribosomal gene. Yanez 

et al. (2003) reported that the gyrB gene agree with the 16S ribosomal data which lead to 

placement of the genus Aeromonas in the family Aeromonadaceae, and gyrB gene sequences 

were useful in resolving discrepancies between 16S ribosomal gene sequences and DNA-DNA 

hybridization results. 

Several other methods have been used to characterize Aeromonas species. Minana-Galbis 

EPA/OW/OST/HECD 12 
Aeromonas: Human Health Criteria Document 

et al. (2004b) examined the genetic diversity between A. hydrophila, A. bestiarum, A. 

salmonicida and A. popoffii by multilocus enzyme electrophoresis (MLEE). By MLEE, A. 

popoffii and A. bestiarum were closely related. MLEE has been used in genomospecies 

determination since 1991 (Altwegg et al., 1991b; Figueras et al., 2000). While some authors are 

enthusiastic about MLEE, few studies have applied the method and none of them have looked at 

the entire population structure of the genus Aeromonas. Multilocus sequence typing (MLST) 

using the four gene loci of 16S rDNA, recA, chiA and gyrB has revealed the taxonomic 

limitations of 16S rDNA alone (Carnahan 2001). 

The current classification of some species is questionable (Martin-Carnahan and Joseph 

2005). A. ichthiosmia is a later synomym of A. veronii, and the taxonomic status of A. 

allosaccharophila is unclear and deserves further investigation, though a later study supports the 

status of A. allosaccharophila as a separate species based upon gyrB gene sequences (Yanez et 

al., 2003). A. allosccharophila and A. ichthiosmia belong to A. veronii complex. The taxonomic 

status of A. enteropelogenes and A. trota is questionable as they share 81-99% relatedness and 

identical APE E20 and API 50CHE biochemical profiles. While A. enteropelogenes has 

taxonomic status, A. trota has been cited more frequently and it should be used preferentially in 

future publication to avoid ambiguity among ampicillin-susceptible aeromonads (Carnahan et al. 

1991a; Huys et al. 2002b). 

A. salmonicida contains four subspecies: salmonicida, achromogenes, masoucida, and 

smithia. However, the lack of congruence between phenotypic and genetic data result in great 

difficulty in accurately assigning isolates to the correct subspecies (Martin-Carnahan and Joseph 

2005). A. salmonicida has not been shown to cause disease in humans, however A. hydrophilalike HG-3 strains that are motile and grow at 37o C have been isolated from clinical specimens 

on rare occassions (Carnahan and Joseph 1993), and its importance relates to animal diseases in 

the marine environment and in aquaculture (Austin and Adams, 1996). 

Other species of Aeromonas have been described, but they have not yet received 

taxonomic recognition. These species include A. arequipensis, A. dechromatica, A. guangheii, 

and A. pastoria (Martin-Carnahan and Joseph 2005). Vibrio proteolytica is improperly published 

EPA/OW/OST/HECD 13 
Aeromonas: Human Health Criteria Document 

in the chemical literature as Aeromonas proteolytica by investigators unfamiliar with the 

taxonomy of the Aeromonadaceae (Holz 2002). 

Exhibit 2.1. Current Genomospecies and Phenospecies of the Genus Aeromonas 

DNA Hybridization Group 

(HG) 

Type Strain Genomospecies Phenospecies Remarks 

1 ATCC 7966 A. hydrophila A. hydrophila Isolated from clinical 

specimens 

1 BCCM/LMG 

19562 

A. hydrophila subsp. 

dhakensis 

A. hydrophila subsp. 

dhakensis 

Isolated from clinical 

specimens 

1 BCCM/LMG 

19707 

A. hydrophila subsp. 

ranae 

A. hydrophila subsp. 

ranae 

Pathogenic for frogs 

2 ATCC 14715 A. bestiarum A. hydrophila-like Isolated from clinical 

specimens 

3 ATCC 33658 A. salmonicida A. salmonicida subsp. 

salmonicida 

Nonmotile fish 

pathogen 

3 ATCC 33659 A. salmonicida A. salmonicida subsp. 

achromogenes 

Nonmotile fish 

pathogen 

3 ATCC 27013 A. salmonicida A. salmonicida subsp. 

masoucida 

Nonmotile fish 

pathogen 

3 ATCC 49393 A. salmonicida A. salmonicida subsp. 

smithia 

Nonmotile fish 

pathogen 

3 CDC 0434-84, 

Popoff C316 

unnamed A. hydrophila-like Isolated from clinical 

specimens 

4 ATCC 15468 A. caviae A. caviae Isolated from clinical 

specimens 

5A CDC 0862-83 A. media A. caviae-like Isolated from clinical 

specimens 

5B CDC 0435-84 A. media A. media 

6 ATCC 23309 A. eucrenophila A. eucrenophila 

7 CIP 7433, NCMB 

12065 

A. sobria A. sobria 

8X CDC 0437-84 A. veronii A. sobria 

8Y ATCC 9071 A. veronii A. veronii biovar sobria Isolated from clinical 

specimens 

DNA Hybridization Group 

(HG) 

Type Strain Genomospecies Phenospecies Remarks 

9 ATCC 49568 A. jandaei A. jandaei Isolated from clinical 

specimens 

10 ATCC 35624 A. veronii biovar veronii A. veronii biovar veronii Isolated from clinical 

specimens, ornatine 

decarboxylase positive 

11 ATCC 35941 unnamed Aeromonas spp. 

(ornithine positive) 

12 ATCC 43700 A. schubertii A. schubertii Isolated from clinical 

specimens 

13 ATCC 43946 Aeromonas Group 501 A. schubertii-like Isolated from clinical 

specimens 

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Aeromonas: Human Health Criteria Document 

Exhibit 2.1. Current Genomospecies and Phenospecies of the Genus Aeromonas, 

continued 

DNA Hybridization Group 

(HG) 

Type Strain Genomospecies Phenospecies Remarks 

14 ATCC 49657 A. trota A. trota Isolated from clinical 

specimens, ampicillin 

susceptible 

15 ATCC 51208, 

CECT 4199 

A. allosaccharophila A. allosaccharophila 

16 ATCC 51020, A. encheleia A. encheleia Pathogenic for eels 

CECT 4342 

17 BCCM/LMG 1754 A. popoffii A. popoffii 

Unassigned MTCC 3249, A. culicicola A. culicicola Isolated from 

NCIM 5147 mosquitoes 

ATCC - American Type Culture Collection, Rockford, MD; BCCM/LGM - Bacteria Collection, Ghent University, Belgium; CDCCenters for Disease Control and Prevention; CIP - Collection de I’Institut Pasteur, Paris, France; NCMB - National Collection of 

Marine Bacteria, Aberdeen, Scotland; CECT - Coleccion Espanola de Cultivos Tipo, University of Valencia, Valencia, Spain; CCUG 

- Culture Collection, University of Gotenborg, Gotenborg, Sweden; MTCC - Microbial Type Culture Collection, Institute of Microbial 

Technology, Chandigarh, India; NCIM - National Chemical Laboratory, Pune, India 

From Martin-Carnahan and Joseph 2005 

2.3 Description of the Genus 

Aeromonas species are autochthonous (natural inhabitants) to the aqueous environment 

where they may be pathogenic for poikilotherms (cold-blooded animals). They are found in soil, 

sediments, and water in freshwater, estuarine, and marine environments. They are present in 

foods, sewage, and biofilms of drinking water distribution systems. Aeromonads are found in the 

intestinal tract of animals and humans, with and without evidence of gastrointestinal disease. 

Some aeromonads are pathogenic for humans, and most human clinical isolates belong to HG-1, 

HG-4, HG-8, HG-9, HG-10, HG-12 or HG-14 (Janda and Abbott 1998). The proportion of 

strains within these hybridization groups that are capable of causing human disease is not known. 

HG-2, HG-3, HG-5, HG-6, HG-7, HG-11, HG-15, HG-16 and HG-17 are isolated from the 

environment or diseased animals, and they are not considered human pathogens. The 

aeromonads causing human diseases are associated with a variety of infections including 

septicemia, wound infections, meningitis, peritonitis, and hepatobilary infections. Some strains 

of Aeromonas produce enterotoxins responsible for causing gastroenteritis in humans; however, 

isolation of aeromonads from feces does not prove pathogenicity, since these bacteria are widely 

distributed throughout the environment in water and foods, especially during summer months 

(Janda and Abbott 1998). 

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 The genus Aeromonas consists of straight, coccobacillary to bacillary gram-negative 

bacteria with rounded ends measuring 0.3-1.0 x 1.0-3.5 Fm (Martin-Carnahan and Joseph 2005). 

They occur singly, in pairs, and rarely as short chains. Motile strains produce a single polar 

flagellum, though peritrichous or lateral flagella may be formed on solid media by some species. 

Aeromonas spp. are facultatively anaerobic, catalase positive, oxidase positive, 

chemoorganotrophic bacteria that exhibit both oxidative and fermentative metabolism on 

carbohydrates. Aeromonas spp. produce a wide variety of extracellular hydrolytic enzymes such 

as arylamidases, amylase, deoxyribonuclease, esterases, peptidases, elastase, chitinase, and lipase 

(Carnahan et al., 1988). Aeromonas spp. grow optimally within a temperature range between 22­

35o C, but growth occurs in a temperature range from 0-45o for some species (Mateos et al., 

1993). Some species, including most nonmotile A. salmonicida strains, do not grow at 35o C 

(Martin-Carnahan and Joseph 2005). They tolerate a pH range from 4.5 to 9.0, but the optimum 

pH range is from 5.5 to 9.0 (Isonhood and Drake 2002), and optimum sodium chloride 

concentration range is from 0 to 4%. 

Serotyping is based upon somatic (O) antigen determinants as described by Sakazaki and 

Shimada (1984). Several typing schema have been proposed (Fricker 1987; Cheasty et al., 1988; 

Thomas et al., 1990), but only one comparison study of two of these schema has been published 

(Shimada and Kosako 1991). The schema of Sakazaki and Shimada recognizes 44 serogroups, 

with an additional 52 provisional serogroups proposed by Albert et al. (1995). Aeromonas spp. 

are found to be serologically heterogeneous, with individual serogroups found in more than one 

species (Janda et al., 1996). Most type and reference strains were not serologically representative 

of a genomospecies. Three serotypes predominate in clinical specimens, O:11 (24%), O:16 

(14%), and O:34 (10%). Korbsrisate et al. (2002) characterized the distribution of A. hydrophila 

serogroups in clinical specimens and developed polyclonal antibodies for rapid identification of 

clinical isolates by direct agglutination. Only 50% of strains fell into the common serogroups 

O:11, O:16, O:18, O:34, or O:83. Rough strains (15.2%) and untypable strains (2.3%) reduced 

the effectiveness of serotyping for identification of clinical strains. A polyvalent antiserum was 

produced that resulted in positive agglutination of 102 or 105 strains, for a calculated sensitivity 

of 97.1% and specificity of 90.7%. This test could be useful in rapid identification of 

aeromonads to genus where they are isolated from samples that may also contain vibrios. 

EPA/OW/OST/HECD 16 
Aeromonas: Human Health Criteria Document 

Motile aeromonads are generally resistant to penicillin, ampicillin, carbenicillin and 

ticarcillin. They are typically susceptible to second and third generation cephalosporins, 

aminoglycosides, carbapenems, chloramphenicol, tetracyclines, trimethoprim-sulfamethoxazole, 

and quinolones (Koehler and Ashdown 1993; Janda and Abbott 1998). Most aeromonads 

produce an inducible chromosomal β-lactamase (Walsh et al., 1997). A. trota and up to 30% of 

A. caviae are susceptible to ampicillin. Antibiotic resistance to streptomycin, chloramphenicol, 

tetracycline, cephalexin, cefoxitin, erythromycin, furazolidone, and sulfathiazole is mediated by 

plasmids (Chaudhury et al., 1996). 

 The genus Aeromonas is differentiated from Plesiomonas and Vibrio by its resistance to 

O/129 (150 Fg) and variable presence of ornithine decarboxylase (Martin-Carnahan and Joseph 

2005). Other key differential characteristics include its inability to grow in the presence of 6.5% 

sodium chloride, gelatin liquefaction, inability to ferment i-inositol, and a negative String Test. 

Additional useful but variable phenotypic characteristics include an inability to grow on 

thiosulfate citrate bile salts sucrose agar (TCBS), and ability of most but not all Aeromonas 

species to ferment D-mannitol, and sucrose. 

Differentiation of species is possible for most species, however the congruity between 

DNA-DNA hybridization group and phenotype is not complete and some strains remain 

unnamed. The biochemical identification of Aeromonas spp. is shown in Exhibits 2.2 to 2.4. 

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Aeromonas: Human Health Criteria Document 

Exhibit 2.2. Identification of Genomospecies of the Aeromonas hydrophila Group 

Motile, mesophilic A. 

A. hydrophila (HG-1) A. bestiarum (HG-2) salmonicida subspecies (HG-3) 

Characteristic 

25o C 35o C 25o C 35o C 25o C 35o C 

D-rhamnose - - + + - -

D-sorbitol - - - - + + 

Lactose - d - D d + 

DL-Lactate d + - - - -

Urocanic acid - - + + + + 

Elastase + d + - + d 

Gluconate nd d - - nd -

oxidation 

Lysine + + + - + d 

decarboxylation 

Maximum 

growth 41o C 38-39o C 38-39o C 

temperature 

HG, hybridization group; +, > 75% of strains positive; d, 26-74% of strains positive; -, < 25% of strains are positive; 

nd, not determined 

From Martin-Carnahan and Joseph 2005 

Exhibit 2.3. Identification of Genomospecies of the Aeromonas caviae Group 

Characteristic A. caviae (HG-4) Unnamed (HG-5A) A. media (HG-5B) 

Lactose d + + 

Cellobiose d + + 

DL-lactate + - + 

Citrate + - -

HG, hybridization group; +, > 75% of strains positive; d, 26-74% of strains positive; -, < 25% of strains are positive 

From Martin-Carnahan and Joseph 2005 

EPA/OW/OST/HECD 18 
Aeromonas: Human Health Criteria Document 

Exhibit 2.4. Identification of Genomospecies of the Motile Aeromonas 

Characteristic 

A. hydrophila

A. bestarium 

Hybridization HG-1 HG-2 HG-4 HG-5 HG-6 HG-7 HG-8 HG-9 HG-10 HG-12 HG-14 HG-15 HG-16 HG-17 

Group 

A. caviae 

Esculin hydrolysis + + + + + - - - + - - d + -

Gas from Glucose 

A. media

+ + - - + + + + + - + + + + 

Voges-Proskauer + + - - - +, + + + d - - - + 

weak 

A. eucrenophila 

Indol + + + d + + + + + - + + + d 

Pyrazinamidase + - + d + nd - - - - - nd nd nd A. sobria

L-arabinose d + + + + - - - - - - d - nd 

D-mannitol + + + + + + + + + - + + + + A. veronii biovar sobria 

sucrose + + + + d + + - + - - + + -

lysine + - - - - +, + + + + + + - - A. jandaei 

decarboxylase weak

ornithine - - - - - - - - + - - d - -

decarboxylase 

A. veroni biovar veroni 

arginine + + + + + - + + - + + d d + 

dihydrolase 

arbutin hydrolysis 

A. schubertii

+ + + + + nd - - + - - nd - nd 

H2S production + + - - + nd + + + - + nd nd nd 

hemolysis + + d - - - + + + + + nd nd - A. trota

ampicillin, R R R S S R R R R R S R R R 

10 :g 

carbenicillin, 

A. allosaccharophila 

R R R nd nd S R R R R S nd R nd 

30 :g 

cephalothin, R R R d S S S S d S R nd nd nd 

30:g 

A. encheleia

colistin, d d S S S nd S R S S S nd nd nd 

4 :g/mL 

HG, hybridization group; +, > 75% of strains positive; d, 26-74% of strains positive; -, < 25% of strains are positive; A. popoffii

nd, not determined; R, resistant; S, sensitive 

From Martin-Carnahan and Joseph 2005 

2.4 Methods 

Isolation and enumeration of Aeromonas spp. from various sources was reviewed by 

Moyer (1996). Clinical specimens are collected and transported according to recommended 

clinical laboratory practice (Altwegg 1996). Environmental samples are collected and handled 

according to the principles outlined in Standard Methods for Examination of Water and 

Wastewater, 20th edition (APHA 1998). Food samples are collected and handled in accordance 

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Aeromonas: Human Health Criteria Document 

with the procedures contained in the FDA Bacteriological Analytical Manual (USFDA 2001). 

Clinical specimens and environmental sample processing may require preparation of serial 

dilutions, and samples may be cultured directly, in dilution, or by membrane filtration, followed 

by incubation of the filter membrane on culture media. 

Broth enrichment methods are frequently used to recover aeromonads from samples 

where they may be present in low numbers together with larger numbers of other bacteria. von 

Graevenitz and Bucher (1983) found that use of APW enrichment increased recovery of 

aeromonads from clinical specimens. APW with or without ampicillin (10 or 30 mg/L) may be 

used for qualitative detection of aeromonads when using the membrane filtration method for 

sample processing (Moyer 1996). Use of ampicillin at the higher concentration may inhibit some 

strains, and A. trota is sensitive to both concentrations of ampicillin. Sachan and Agarwal (2000) 

incorporated cephalothin at 10 mg/L into buffered dextrin broth (BCDB-10) and compared it to 

alkaline peptone water with cephalothin at 10 mg/L (CAPW-10) for selective enrichment of 

aeromonads from chicken samples. BCDB-10 performed slightly better than CAPW-10 for 

naturally infected chickens, while the isolation rate from spiked samples was best for CAPW-10 

followed by BCDB-10, and APW. Optimal recovery may require use of more than one 

enrichment broth, with and without antibiotic to recover strains that may be sensitive to 

antibiotics. 

Aeromonads grow readily on laboratory culture media, and a large number of selective 

and differential culture media have been developed for recovery of Aeromonas spp. from the 

environment, foods, and clinical specimens. Comparative studies suggest that no single medium 

results in optimum recovery of aeromonads, and combinations of media and methods are 

frequently employed by direct plating, membrane filtration, or multiple tube tests for determining 

most probable number. The membrane filtration method, EPA Method 1605, has been validated 

for isolation of Aeromonas hydrophila from drinking water samples (USEPA 2001). This 

method employs ampicillin dextrin agar (ADA) which uses 10 mg/L ampicillin and 2 mg/L 

vancomycin as selective agents. While this medium is capable of isolating aeromonads in 

addition to A. hydrophila, A. trota and some strains of A. caviae are inhibited by ampicillin and 

will not be detected by this method. 

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Several new or modified culture media have been introduced since the review of Moyer 

(1996). Neyts et al. (2000a) modified bile salts irgasan brilliant green agar for enumeration of 

Aeromonas spp. in foods by increasing pH to 8.7, replacing xylose with soluble starch as the 

carbon source and decreasing the concentration of bile salts. The resulting mBIBG medium 

suppressed growth of the Enterobacteriaceae except for Citrobacter freundii. While 

Pseudomonas spp. grew, they were easily differentiated from Aeromonas spp by differences in 

colony morphology. Quantitative recovery of aeromonads from foods with densities from 2-7 

log10 CFU/g was possible in the presence of 5-6 log10 CFU/g of background flora. 

Aeromonads have been shown to interfere with methods for detecting and enumerating 

coliform bacteria. Geissler et al. (2000) used LMX® broth (Merck), Chromocult Coliform® (CC) 

agar and Chromocult Coliform® agar plus cefsulodin (CC-CFS) with standard methods for 

multiple-tube fermentation (MTF) enumeration of coliforms and E. coli in marine waters. βgalactosidase positive non-coliform bacteria (Aeromonas spp. and Vibrio spp.) contributed to the 

numbers obtained for coliform tests when methods are used that include chromogenic and 

fluorogenic media. False positive tests due to Aeromonas spp. occurred with LMX® and CC, 

since most Aeromonas spp. are β-galactosidase positive. Holler et al. (1995) found a high 

percentage of Aeromonas spp. in laurylsulfate-tryptose broth with 4-methylumbelliferyl-β-Dglucuronide ( LSTB-MUG) and Ley et al. (1993) observed that Aeromonas spp. comprised 76% 

of β- galactosidase positive colonies isolated on X-Gal. Fricker and Fricker (1996) reported that 

most of the false positive result for coliforms on LMX® were due to Aeromonas spp. Of the 

media evaluated, CC-CFS was most effective in inhibiting aeromonads that may interfere with 

coliform tests. 

Methods for identification of aeromonads based upon phenotypic characterization of 

strains using biochemicals resulted in confusion due to extreme phenotypic diversity between 

and within genomospecies. The schema of Abbott et al. (2003) and Martin-Carnahan and Joseph 

(2005) make it possible to identify most isolates to named species. Atypical strains are frequently 

isolated from environmental samples, and accurate identification of these strains is problematic. 

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Aeromonas: Human Health Criteria Document 

While there are more than 17 genomospecies of Aeromonas, only 7 are recognized as 

human pathogens (Carnahan et al., 1991b; Janda and Abbott 1998). Aeromonads associated with 

human diarrhea include A. hydrophila, A. caviae, A. veronii biovar sobria, A. veronii biovar 

veronii, A. schubertii, A. jandaei, and A. trota (Carnahan 1993). To assist clinical laboratories 

with identification of Aeromonas spp., Carnahan et al. (1991b) devised Aerokey II, which 

provided a reliable and accurate system for identification of most of the currently recognized 

strains of clinical significance. In a two year evaluation of Aerokey II, it successfully identified 

95.3% of clinical strains isolated in 2000 and 98.2% of clinical strains isolated in 2001 (Sinha et 

al., 2004). A. caviae was the predominant species, followed by A. hydrophila, A. veronii biovar 

sobria, A. trota, A. veronii biovar veronii, A. schubertii and A. jandaei. Four strains could not be 

identified. 

In an alternative approach, Carson et al. (2001) developed a phenotypic identification 

system miniaturized for use in 96 well microtiter plates. A computer assisted identification 

program was developed based upon this identification system which produced a matrix of 

substrate reaction probabilities for known aeromonads. Carnahan and Joseph (1991) found that 

colistin resistance could be used as an additional phenotypic marker for identification of 

aeromonads. The A. veronii group shows 2.5% resistant to colistin with the exception of A. 

jandaei, which is 100% colistin resistant. A. hydrophila group is 85.8% resistant, while A. caviae 

group is 2.1% resistant. When colistin was included in a 14 panel test format, 96.2% of strains 

could be identified to phenospecies and 93.6% of strains could be identified to genomospecies 

(Fosse et al. 2003a). 

Huys et al. (1995) used whole cell fatty acid analysis (FAME) by gas-liquid 

chromatography to correctly classify 73.5% of water isolates to the correct DNA hybridization 

group. While this method offers the advantages of an instrumental method with autosampling for 

unattended operation, the reliability of identifications does not compare to newer genomic 

methods. 

Antigen-antibody methods for identification of Aeromonas spp. have not been widely 

used. Shin et al. (2000) developed a latex agglutination test for the rapid identification of 

EPA/OW/OST/HECD 22 
Aeromonas: Human Health Criteria Document 

Aeromonas hydrophila isolates from various sources. Delamare et al. (2002a) developed a 

monoclonal antibody ELISA test for rapid detection of Aeromonas spp. in human feces. The 

limited response of the monoclonal antibody to other aeromonads causing diarrhea in humans 

limits the utility of the test in clinical diagnosis of Aeromonas spp. associated gastrointestinal 

disease. 

2.4.1 Isolation and Enumeration from Environmental Samples

 Isolation of Aeromonas spp. from environmental samples provides a challenge because of 

the presence of competing bacteria and the possibility of sample matrix interference with sample 

preparation and culture methods. The use of dilution schemes and enrichment media facilitate 

isolation of aeromonads heavily contaminated samples such as sewage sludge and sewage 

effluents. Kersters et al. (1996a) compared several culture media for isolation and enumeration 

of aeromonads from water samples and concluded that ampicillin dextrin agar (ADA) produced 

the best overall results. Water and food samples are usually processed using a multiple tube 

technique such as the multiple tube fermentation test (MFT) used for coliform analyses but 

employing alkaline peptone water for determining the most probable number (MPN). Non-turbid 

water samples may be processed using the membrane filtration (MF) method, where samples are 

filtered through a membrane with a pore size of 0.45 Fm, and the membrane is either placed into 

APW enrichment broth or placed onto the surface of selective and differential culture media such 

as ampicillin dextrin agar (ADA) and incubated aerobically at 35o C for 24-48 hr. EPA Method 

1605 employing MF and ADA-V has been validated for detection and enumeration of A. 

hydrophila from drinking water (USEPA 2001). 

The use of a presence-absence screening test is proposed and PA Broth, Nutrient Broth 

and Buffered Peptone Water were compared for recovery of coliforms, aeromonads, 

pseudomonads, and S. aureus (Warburton 2000). Several plating media were evaluated for 

detection of target bacteria. A combination of PA broth and buffered peptone water provided 

best recovery of target organisms when used in conjunction with plating on differential/selective 

media. Use of the double enrichment method reduces costs associated with individual tests for 

each regulated indicator organism. 

EPA/OW/OST/HECD 23 
Aeromonas: Human Health Criteria Document 

Several culture enrichment and culture media have been evaluated for isolation of 

aeromomads from foods (Moyer, 1996; Palumbo, 1996). Starch ampicillin agar (SAA) and bile 

salts inositol brilliant green agar (BIBG) with prior enrichment in APW or tryptose broth 

containing ampicillin (TSB-30) (ampicillin 30 mg/L) are recommended, together with 

commercially available media such as Aeromonas Medium (Ryan’s Medium). Starch glutamate 

ampicillin penicillin (SGAP-10) medium was used to isolate aeromonads from sewage sludge. 

This medium is highly selective, and it has been used to isolate aeromonads from foods and other 

challenging matrices. Samples are prepared in dilution, inoculated into culture media with or 

without enrichment, and incubated aerobically at 35o C for 24-48 hr. Colonies are screened by 

performing a spot oxidase test and identified using biochemical methods or commerciallyavailable bacterial identification kits. 

Merino et al. (1993c) developed an antibody capture method using microtiter plates and 

enzyme-linked immunosorbent assay for detection of A. hydrophila O:11 in food samples. This 

assay was highly specific for the target antigen, and other species or serotypes of A. hydrophila 

were not detected. 

2.4.2 Isolation from Clinical Specimens 

Aeromonas species grow readily in blood culture media and on 5% sheep blood agar used 

in clinical laboratories for isolation of human pathogens for normally sterile body sites. Isolation 

of aeromonads from contaminated specimens such as feces require the use of selective and 

differential plating media such as McConkey agar, cefsulodin irgasan novobiocin (CIN) agar, or 

blood ampicillin agar (10 mg/L ampicillin). To facilitate recovery of aeromonads from heavily 

contaminated specimens such as feces, enrichment broths such as alkaline peptone water are 

incubated overnight and subcultured to blood ampicillin agar and CIN agar. Culture plates are 

incubated aerobically at 35o C for 24-48 hr. Aeromonas spp. produce characteristic colonies, with 

or without hemolysis on blood agar, and colonies may be quickly screened using the spot oxidase 

test. Oxidase positive colonies are further screened using tube biochemicals or by inoculation of 

a cell suspension into one of the commercially available bacterial identification kits. 

EPA/OW/OST/HECD 24 
Aeromonas: Human Health Criteria Document 

2.4.3 Culture Identification Methods 

Commercial systems for bacterial identification are notoriously inaccurate for 

identification of Aeromonas spp., since they do not incorporate the key substrates necessary for 

correct identification (Abbott et al., 1998; Vivas et al., 2000; Soler et al., 2003a; Park et al., 

2003). Bascomb et al. (1997) found the MicroScan system to be more reliable than its 

competitors because it relies upon more substrates for bacterial identifications. Contrary to these 

findings, Soler et al. (2003a) reported that the MicroScan/WalkAway system correctly identified 

only 14.8% of isolates, while the BBL Crystal Enteric/Nonfermenter system correctly identified 

20.3% of isolates originally assigned species by using 16S rRNA gene restriction fragment 

length polymorphism analysis. Most misidentifications placed Aeromonas spp. in the genus 

Vibrio, though one strain of A. veronii was identified as Burkholderia cepacia by the BBL 

Crystal system. MicroScan was able to identify Aeromonas spp. to group, but was not able to 

differentiate species within groups. BBL Crystal frequently assigned an incorrect species within 

the genus Aeromonas. 

Vivas et al. (2000) compared MicroScan Walk/Away in conjunction with the MicroScan 

Combo Negative type 1S panels with conventional biochemical methods for identification of 85 

environmental, clinical and reference strains of Aeromonas spp. Using the MicroScan Combo 

Negative type1S substrate panel, 67 of 88 (78.8%) of strains were correctly identified, 4 of 88 

(4.7%) of strains were incorrectly identified, and 10 of 88 (11.8%) of strains represented rare 

biotypes that could not be identified. 

Differences in the reliability of identifications between Soler et al. (2003a) and Vivas et 

al. (2000) result from the use of different reference methods, conventional biochemicals in the 

case of Vivas et al. (2000) and 16S RFLP in the case of Soler et al. (2003b). Neither reference 

method is 100% accurate in assigning the correct species identification. Incubation temperature 

appears to alter the physiological activity of aeromonads, complicating the use of phenotypic 

characteristics for identification and classification of isolates (Merino et al., 1995). 

EPA/OW/OST/HECD 25 
Aeromonas: Human Health Criteria Document 

Commercial kits have difficulty in correct identification of Aeromonas species because 

they do not include some of the key substrates required for differentiation of species. Even more 

problematic is the inaccurate identification of Aeromonas species as members of other genera. 

For example, Aeromonas veronii biovar sobria is identified as Vibrio algiolyticus by the Vitek 

system (Park et al., 2003). The API 20NE kit correctly identified this isolate as A. sobria (later 

recognized as A. veronii biovar sobria). Other examples of misidentification are shown in Exhibit 

2.5. 

Exhibit 2.5. Misidentification of Aeromonads by Commercial Kits 

Correct Identification Misidentified As Commercial Kit 

A. caviae V. fluvialis Not specified 

A. hydrophila V. alginolyticus API 24; Vitek GNI+ 

A. schubertii V. damsela Vitek GNI 

A. veronii biovar sobria V. alginolyticus Vitek GNI+ 

A. veronii biovar sobria V. cholerae API 20E

A. veronii Burkholderia cepacia BBL Crystal 

A. caviae V. fluvialis MicroScan 

A. veronii V. fluvialis 

A. media A. hydrophila 

V. fluvialis 

A. jandaei A. hydrophila 

V. fluvialis 

A. bestiarum A. hydrophila 

A. salmonicida A. hydrophila 

V. fluvialis

From Park et al., 2003; Soler et al., 2003a 

Because of the difficulty and expense of identifying Aeromonas spp. in clinical 

laboratories, Janda and Abbott (1998) suggest that it may be sufficient to continue to report 

aeromonads by group, i.e. A. hydrophila group, A. caviae group, and A. veronii biovar sobria 

group. This approach would result in misidentification of less than 15% of aeromonads important 

in clinical medicine and have little impact on patient treatment. By adding a few additional 

biochemical tests to a basic system, differentiation of A. jandaei and A. schubertii would be 

possible, thus covering the clinically relevant species. Epidemiological studies, taxonomic 

studies, and published reports require identification of isolates to genomospecies. 

EPA/OW/OST/HECD 26 
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Abbott et al. (2003) characterized a culture collection of 193 strains representing the 15 

species recognized at the time of the study for 63 phenotypic traits. The results of this study were 

used to design a schema for identification of aeromonads, first to group, then to species within 

each group. Tests recommended for performance in clinical and reference laboratories are shown 

in Exhibit 2.6. 

Exhibit 2.6. Recommended Tests for Identification of Aeromonas 

glycerol 

esculin 

citrate 

gelatin 

indole 

Voges-Proskauer 

susceptibility 

lactose gluconate 

potassium cyanide 

) 

) 

lipase 

Laboratory Type Carbohydrate Tests Tube/Slant Tests Plate Tests 

Clinical Laboratories L-arabinose 

D-glucose (gas) 

D-mannitol 

sucrose 

ampicillin susceptibility 

cephalothin 

Reference Laboratories 

(additional tests to those 

performed in clinical 

laboratories) 

sucrose 

cellobiose 

L-rhamnose 

D-sorbitol 

D-mannose 

lactulose 

glucose-1-phosphate 

glucose-6-phosphate 

(KCN

DL-lactate 

urocanic acid 

pyrazinamidase (PZA

elastase 

stapholysin 

From Abbott et al., 2003 

By using selected substrates recommended by Abbott et al. (2003), it is possible to 

identify clinical isolates of Aeromonas spp. to one of the three major complexes of significance 

in human disease, as shown in Exhibit 2.7. 

Exhibit 2.7. Biochemical Tests for Aeromonas spp. Group Identification (% Positive) 

A. hydrophila group A. caviae group A. sobria group 

Test (A. hydrophila, A. bestiarum, (A. caviae, A. media, (A. veronii, A. jandaei, 

A. salmonicida) A. eucrenophila) A. schubertii, A. trota) 

Esculin 87 (92, 81, 85) 71 (76, 55, 78) 0 

Voges­ 74 (88, 63, 62) 0 54 (88, 87 17, 0) 

Proskauer 

Glucose (gas) 81 (92, 69, 77) 16 (0, 0, 78) 87 (92, 100, 0, 69) 

L-arabinose 93 (84, 100, 100) 96 (100, 100, 78) 4 (12, 0, 0, 0) 

From Abbott et al., 2003 

The biochemical reactions necessary to separate individual species within complexes are 

shown in Exhibits 2.8 through 2.10. 

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Aeromonas: Human Health Criteria Document 

Exhibit 2.8. Biochemical Tests for Separation of A. hydrophila Group (% Positive) 

Test A. hydrophila A. bestiarum A. salmonicida

citrate 92 38 85 

DL-lactate 84 0 0 

urocanic acid 16 94 100 

gluconate 64 13 0 

cellobiose 4 38 69 

lactose 64 13 92 

L-rhamnose 24 69 0 

D-sorbitol 0 0 85 

From Abbott et al., 2003 

Exhibit 2.9. Biochemical Tests for Separation of A. caviae Group (% Positive) 

Test A. caviae A. media A. eucrenophila 

citrate 88 82 0 

DL-lactate 96 56 0 

urocanic acid 100 100 0 

glucose (gas) 0 0 78 

pyrazinamidase 88 18 100 

glucose-1-phosphate 4 100 100 

glucose-6-phosphate 4 100 100 

lactulose 68 55 0 

D-mannose 32 100 100 

From Abbott et al., 2003 

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Aeromonas: Human Health Criteria Document 

Exhibit 2.10. Biochemical Tests for Separation of A. sobria Group (% Positive) 

Test A. veronii A. jandaei A. schubertii A. trota 

indole 100 100 17 100 

Voges-Proskauer 92 87 17 0 

lipase (corn oil) 92 100 100 0 

D-glucose (gas) 92 100 0 69 

cellobiose 20 20 0 100 

glycerol 100 100 0 94 

D-mannitol 100 100 0 69 

sucrose 100 0 0 19 

ampicillin resistence (10 :g/mL) 100 93 92 6 

From Abbott et al., 2003 

2.4.4 Molecular Detection Methods 

Direct detection of aeromonads by polymerase chain reaction (PCR) or use of genetic 

probes has advanced rapidly in the past decade. PCR methods have been developed for detection 

of Aeromonas spp. in a variety of matrices as shown in Exhibit 2.11. 

Exhibit 2.11. PCR Methods for Detection of Aeromonas species 

Organism Matrix Type PCR Target Detection Detection Reference 

Gene Method Limit 

A. hydrophila raw milk semi-nested 

multiplex 

aerA gel 2 log10 

CFU/mL 

Ozbas et al., 

2000 

A. hydrophila marine 

water 

multiplex aerA gel 2 log10 CFU Kong et al., 

2002 

A. hydrophila water immunocapture 

16S 

ribosome 

gel 10 CFU Peng et al., 

2002 

A. trota 10-15 CFU Khan et al., 

1999 

A. caviae flies culture 

enrichment 

16S 

ribosome 

gel ND Nayduch et 

al., 2001 

A. hydrophila fish culture aerA and gel 1-10 CFU/g Gonzalezenrichment hlyA Rodriguez et 

al., 2002 

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Aeromonas: Human Health Criteria Document 

Exhibit 2.11. PCR Methods for Detection of Aeromonas species, continued 

Organism Matrix Type PCR Target Detection Detection Reference 

Gene Method Limit 

A. hydrophila feces duplex real­ aerA SYBR green 4-5 log10 Fukushima et 

time without al., 2003 

enrichment 

A. hydrophila various universal 

primer 

16S 

ribosome 

denaturing 

gradient gel 

electroND Ji et al., 2004 

phoresis 

PCR methods may be more sensitive than culture for complex matrices with dense 

background flora that interferes with culture detection, and for organisms for which culture 

methods are inadequate or unavailable. Ozbas et al. (2000) developed a PCR method for detection 

of A. hydrophila in raw milk. The detection limit was 2 log10 CFU/g and the detection rate was 

23% for PCR and 14% for culture. 

The use of selective agents, particularly antibiotics, in culture media may inhibit some 

strains of the target bacteria. Kannan et al. (2001) developed a PCR method for direct detection in 

feces targeting the aerolysin toxin gene (aer-1 and aer-2 primer pairs). The sensitivity was 100% 

and specificity was 98%, but the detection limit not given, though it compared favorably with 

culture. This method detected A. trota strains that are missed using culture media containing 

ampicillin. 

Nucleic acid amplification methods targeting virulence genes are used for detection of 

pathogenic bacteria and to differentiate pathogenic from non-pathogenic strains (Kingombe et al., 

1999; Chacon et al., 2003; Sen and Rodgers 2004). Wang et al. (2003) developed a multiplex 

PCR method for detection of hemolysin and aerolysin genes in A. hydrophila and A. sobria, and 

used it to screen 121 clinical and 7 reference strains for ahh1, asa1, and aerA genes. Five 

genotypes were identified, Type 1 contained only the ahh1 gene, Type 2 contained only the asa1 

gene, Type 3 contained both ahh1 and asa1 genes, Type 4 contained ahh1 and aerA gene, and 

Type 5 containing no hemolysin genes. Phenotypic expression of toxins was evaluated in Vero 

cell culture cytotoxicity assay and the presence and number of hemolysin genes corresponded 

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Aeromonas: Human Health Criteria Document 

with cytotoxin titers. The range of virulence of aeromonads is thought to result from the variety of 

genotypes present in the environment. Both phenotypic and genotypic heterogeneity are common 

among aeromonads. 

Stine et al. (2003) constructed microarrays of DNA probes to study the population 

dynamics of microbial communities, and used one such microarray to study population 

interactions of marine bacteria in coastal waters where aeromonads make up a significant 

percentage of the microbial flora. Microarray technology provides a powerful tool for studying 

bacterial populations in biofilms. In a novel application of microarray technology, Galindo et al. 

(2003) and Galindo et al. (2004a) used microarrays to identify A. hydrophila cytotoxic 

enterotoxin-inducing genes in macrophages, thus demonstrating the potential of microarrays in 

elucidating intracellular mechanisms of pathogenesis. Microarrays and proteomics have also been 

used to examine the effects of cytotoxic enterotoxin on human epithelial cells (Galindo et al. 

2005). 

In another novel application of probe technology, Asfie et al. (2003) used fluorescent in 

situ hybridization (FISH) to characterize the fecal microflora of goldfish. Tetramethylrhodamine 

labeled rRNA targeted oligonucleotides were used as group specific probes (AER66 for 

Aeromonas). Aeromonas spp. made up 18.9% to 34.5% of total cell density in fecal samples, at 

counts from 2.5x108

 to 2.3x1010 cells/g. 

No single phenotypic or genotypic identification system has been found that will reliably 

classify all strains of Aeromonas spp. Ciapini et al. (2002) used phenotyping by Biolog system 

and amplified 16S rDNA restriction analysis (ARDRA) to identify aeromonads. Discrepancies 

were noted between biolog results and ARDRA. Chromosomal DNA restriction analysis results 

did not agree with ARDRA profiles. These data suggest the need for a polyphasic approach to 

defining the taxonomic status of species in the genus Aeromonas. Phenotypic (biochemicals), 

phylogenic (16S rDNA sequencing) and genomic DNA-DNA hybridization) data are required for 

accurate classification. New identification systems such as MLST (Stackebrandt et al. 2002) and 

matrix-assisted laser desorption/ionization mass spectormetry (MALDI-MS) (Donohue et al. 

2005) offer promise of rapid and specific species identification. 

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2.4.5 Typing Methods 

Typing methods are used in a variety of ways: 

$ Taxonomic studies of genetic relatedness 

$ Analysis of microbial populations in complex matrices 

$ Analysis of genetic relationships between clinical and environmental strains of the 

same species 

Attempts to epidemiologically relate clinical and environmental isolates required typing 

methods such as isoenzyme analysis or molecular genetic methods such as rDNA restriction 

patterns (ribotyping) (Moyer et al., 1992a), restriction fragment length polymorphism analysis 

(RFLP) (Borrell et al., 1997), pulsed-field gel electrophoresis (PFGE) (Borchardt et al., 2003), or 

polymerase chain reaction (PCR) (Khan and Cerniglia 1997). 

The marked heterogeneity and incongruity between phenotypic and genomic identification 

of aeromonads lead to the application of various typing methods for epidemiological 

investigations. Altwegg (1996) reviewed typing systems and concluded that phenotypic methods 

such as biotyping, phage typing, serotyping, antibiograms, and protein typing did not provide 

sufficient discrimination among strains to be useful. Among the genotyping methods, plasmid 

analysis was equally useless. Restriction enzyme analysis methods such as ribotyping and pulsedfield gel electrophoresis provided discrimination at the strain level, and ribotyping results had 

both epidemiological significance and sufficient taxonomic value to permit assignment of strains 

to DNA-DNA hybridization groups. The recent widespread application of PFGE in microbiology 

laboratories lead to recognition that genomic DNA of aeromonads may be characterized by PFGE 

(Garcia et al., 2000), and PFGE is replacing ribotyping as the preferred method for genomic 

typing of aeromonads (Bonadonna et al., 2002). 

Considerable progress in typing methods has been made since the review of Altwegg 

(1996). Minana-Galbis et al. (2002) was able to identify 91% of clinical and environmental 

isolates from Spain in a numerical taxonomy schema using 16 phenotypic tests. By introducing 

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non-traditional substrates, Borrell et al. (1998) reported that phenotypic identification was 

successful to the genomospecies level for 93% of 983 environmental and clinical strains of 

Aeromonas. Citrate and sorbitol enabled separation of the A. hydrophila complex. Of human 

isolates, A. veronii biovar sobria and A. caviae were most common intestinal aeromonads, while 

A. hydrophila isolates were from extraintestinal sources. A. veronii biovar sobria was found in 

lakes, reservoirs, and treated drinking water, and A. caviae was common in seawater and milk 

products. A. hydrophila was second most common freshwater isolate. The tests used were based 

upon publications by Altwegg et al. (1990) and Abbott et al. (1992). All substrates were incubated 

at 36o " 1o C except sorbitol and citrate, which were incubated at 30o C according to Altwegg et 

al. (1990). Some strains (7% in this study) cannot be identified using phenotypic characteristics. 

These findings are similar to those reported by Hanninen and Siitonen (1995). Molecular methods 

are still necessary for clarification of the taxonomic standing of unknown strains. 

Several investigators are using restriction fragment length polymorphism of the intergenic 

spacer region between the 16S and 23S rRNA gene for genotyping aeromonads in investigations 

of water treatment plant colonization and foodborne outbreaks (Martinez-Murcia et al., 2000; 

Solar et al., 2003b). A comparison of three genomic typing methods, RFLP of the 16S-23S 

intergenic spacer region (ISR), enterobacterial repetitive intergenic consensus sequence PCR 

(ERIC-PCR), and repetitive extragenic palindromic (REP-PCR) on 26 isolates of A. popoffii from 

different geographical regions suggested that all three methods were required to obtain strong 

geographical discrimination among aeromonads (Solar et al., 2003b). 

Szczuka and Kaznowksi (2004) analyzed 120 isolated from water and human stools from 

patients with gastroenteritis by random amplified polymorphic DNA PCR (RAPD), REP-PCR, 

and ERIC-PCR. No determinant clone was associated with human diarrhea. There was no genetic 

similarity between clinical and environmental strains isolated from the same or different 

geographical areas. Some clones colonized a specific ecosystem, e.g. drinking water. RAPD and 

ERIC-PCR had the same discriminatory power and provided the same utility for epidemiological 

investigations and population genetic analysis, while REP-PCR was less effective for 

differentiating aeromonads. These techniques were used to show that the hospital environment 

was not the source of aeromonads in hospitalized children with diarrhea since no clonal 

EPA/OW/OST/HECD 33 
Aeromonas: Human Health Criteria Document 

relationship could be demonstrated between stool and environmental isolates. Aeromonads 

exhibit extreme genetic diversity, and it is difficult to link clinical and environmental isolates 

epidemiologically using typing methods. Borchardt et al. (2003) compared clinical and well water 

isolates by PFGE and found them to be genetically unrelated, suggesting that gastrointestinal 

infection were not linked with groundwater exposure. 

Alavandi et al. (2001) used RAPD-PCR and SDS-PAGE whole cell profiles to examine 

the relationship between Aeromonas isolates from children with diarrhea and their water sources. 

Water isolates clustered in two major groups while clinical isolates were scattered across the 

dendogram. RAPD-PCR could distinguish environmental from clinical isolates but it could not 

distinguish phenospecies or genomospecies of aeromonads. Whole cell protein fingerprinting and 

cluster analysis could neither differentiate isolated from clinical or environmental sources or 

differentiate phenospecies of the genus Aeromonas. 

Delamare et al. (2002b) used SDS-PAGE showed promise for species differentiation but 

lacked discrimination to differentiate strains within a species, while RAPD offered strain 

differentiation but was not useful in species identification. These results differ from those of 

Alavandi et al. (2001) where SDS-PAGE analysis could not differentiate phenospecies of 

Aeromonas. 

Another promising method was described by Donohue et al. (2005), consisting of matrixassisted laser desorption/ionization mass spectrometry (MALDI-MS). This method is based upon 

protein fingerprinting using whole cell preparations. 

2.5 Summary 

Aeromonas spp. are oxidase-positive, gram-negative bacteria that are widely distributed in 

the aquatic environment. Aeromonads have been isolated from diseased animals and fish for over 

100 years, but they have been recognized as human pathogens only in past 50 years, and there is 

still controversy concerning their relationship to enterotoxin production and resulting 

gastrointestinal disease (Albert et al 2000). Aeromonads may produce a variety of extracellular 

EPA/OW/OST/HECD 34 
Aeromonas: Human Health Criteria Document 

products that confer virulence in some strains. The taxonomy of the genus Aeromonas was 

codified with publication of the Second Edition of Bergey’s Manual of Systematic Bacteriology 

(Martin-Carnahan and Joseph 2005), however some recognized species have not been named and 

new species continue to be described. No universal selective/differential medium has been 

developed for isolation of aeromonads (Merino et al., 1995). The available media used for 

isolation of aeromonads suffer from insufficient differentiation and selectivity (allowing 

overgrowth and differential ambiguity) or over-selectivity (suppression of ampicillin-sensitive 

strains). EPA Method 1605 was validated recently for detection and enumeration of A. hydrophila 

from drinking water. Molecular systems such as PCR have been developed for detection of 

aeromonads, however unless culture enrichment methods precede PCR amplification, the 

detection limit is too low for practical use with environmental samples. While ribotyping, PFGE, 

or other typing methods are useful in epidemiological investigations, the results must be 

interpreted cautiously because of the extreme biological diversity inherent among aeromonads. 

3.0 Occurrence 

Mesophilic, motile aeromonads are ubiquitous (everywhere) and autochthonous (naturally 

occurring) in the aquatic environment. They inhabit freshwater, esturine waters, and wastewater, 

and have been found in chlorinated and unchlorinated drinking water (Havelaar et al., 1992). 

They are also found in soil and the feces of aquatic and terrestrial animals and humans. They 

occur in fish and other seafoods, vegetables, raw milk, and processed food products. Aeromonas 

spp. occur in the human gastrointestinal tract both in the presence and absence of disease, but the 

presence of Aeromonas spp. in other body sites is usually associated with infection and disease. 

Aeromonads cause disease in poikilothermophiles (cold blooded animals) such as frogs, eels, and 

fish, where they are an economic liability to the aquaculture industry. Aeromonads have been 

found in association with marine copepods and plankton, where they are present at cell densities 

from 4 CFU/mL to 1.3x103

 CFU/100mL in seawater and from 1.5x101 CFU/100mL to 6x102 

CFU/100mL on plankton. 

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3.1 Worldwide Occurrence 

Aeromonas spp. are found worldwide in surface water, ground water, non-chlorinated 

drinking water, chlorinated drinking water, and bottled mineral water (Holmes et al., 1996). 

Aeromonads are found in a wide variety of foods (Palumbo, 1996). They are found in the 

intestinal tract of humans and animals, raw sewage, sewage effluents, activated sludge, and 

sewage-contaminated waters (Holmes et al., 1996). 

Aeromonads reach population densities of 106-108 CFU/mL in raw sewage and 103-105 

CFU/mL remain in sewage effluents after treatment (Holmes et al., 1996). Their occurrence in the 

environment is not dependent upon fecal pollution; however, they reach higher numbers in 

nutrient-rich waters contaminated by sewage. They may reach 3-5 log10 CFU/mL in surface 

waters during summer months. They are not common in groundwater, though they may colonize 

poorly constructed wells. 

3.1.1 Occurrence in Human Population 

 Humans carry Aeromonas spp. in their gastrointestinal tract both in the presence and 

absence of disease. The rates of fecal carriage in asymptomatic persons in developed countries 

range from 0% to 4.0% (Millership et al., 1983; Agger et al., 1985; Quan et al., 1986; 

Svenungsson et al., 2000 ), while the isolation rate from persons with diarrheal illness ranges 

from 0.8 to 7.4% (Agger et al., 1985; Moyer 1987; Albert et al. 2000). In Southeast Asia, 

asymptomatic carriage rates as high as 27.5% and recovery rates from patients with diarrhea as 

high as 34% have been reported (Pazzaglia et al., 1990). Among western Peace Corp workers in 

Thailand, aeromonads were recovered from 8.5% of healthy persons and 30.8% of persons with 

diarrhea (Echeverria et al., 1981). 

Recovery rates among children with diarrhea vary geographically: 0.62 to 4% in Malaysia 

(Lee and Puthucheary 2001; Lee and Puthucheary 2002), 0.75% in Nigeria (Kehinde et al., 2001), 

2% in Sweden (Svenungsson et al., 2000), 2.3% in Taiwan (Juan et al., 2000), 4.8% in 

Switzerland (Essers et al., 2000), and 6.8% in Greece (Maltezou et al., 2001). Sinha et al. (2004) 

EPA/OW/OST/HECD 36 
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reported aeromonads in 6.5% of all patients in India, and Chan et al. (2003) reported aeromonads 

in 6.9% of adult patients with acute diarrhea in Hong Kong. Other investigators report isolation 

rates from symptomatic patients from 0.04% to 21% (Kuijper and Peeters 1991; Dumontet et al., 

2003; Maraki et al., 2003). Pazzaglia et al. (1990) reported that 23.1% of newborns in Peru 

demonstrated transitory gastrointestinal colonization with Aeromonas spp. during the first days of 

life. The isolation rates for human fecal specimens vary widely, as geographical areas, patient 

populations, food habits, level of sanitation, and culture methods influence the recovery rates 

(Dumontet et al., 2003). 

Saad et al. (1995) observed that the frequency of recovery of Aeromonas spp. from stools 

corresponded to the warm summer months when Aeromonas growth reached their maximum and 

postulated that fresh vegetables may be a source. No corresponding increase in the number of 

aeromonads in water was evident, suggesting food as the primary contributor to human carriage 

of Aeromonas spp. The relationship between presence of Aeromonas spp. in human fecal 

specimens and clinical manifestations of disease continues to challenge epidemiologists. 

Kannan et al. (2001) reported that Aeromonas spp. isolated from clinical specimens in 

India include A. hydrophila (59.3%), A. caviae (18.7%), A. veronii (10.9%), A. schubertii (4.6%), 

A. jandaei (3.1%), and A. trota (3.1%). Fecal carriage rates of 6.6% of symptomatic 10-year olds 

have been reported (Komathi et al., 1998). Seventeen of 2,565 stool samples (0.66%) were 

positive for Aeromonas spp. (Borchardt et al., 2003). Agger et al. (1985) reported Aeromonas spp. 

in 1.1% of stools in Wisconsin and Moyer (1987) reported a fecal isolation rate of 7.1% in Iowa. 

The choice of culture methods especially the use of alkaline peptone water or other enrichment 

methods prior to plating fecal samples markedly affects the detection rate both within and 

between patient populations. 

The clinical significance of isolates in these surveys is not clear, even when all patients in 

the surveyed population had diarrhea. Enteropathogenicity is influenced by growth temperature, 

where strains of O:34 grown at 20o exhibit enhanced virulence over strains grown at 37o C 

(Merino et al., 1992). Strains isolated at 35-37o C, the typical incubation temperature used in 

clinical laboratories may produce false negative tests for virulence factors, making the 

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retrospective assessment of clinical significance impossible. 

3.1.2 Occurrence in Animal Population 

Aeromonas spp. have been recognized as animal pathogens since they were first isolated 

from diseased frogs and fish (Gosling 1996b). They are now recognized to cause disease in birds 

and domestic animals (Gray 1984; Shane and Gifford 1985). 

Fish 

A. salmonicida and A. hydrophila cause hemorrhagic disease, ulcerative disease, 

furunculosis, red sore disease and septicemia in fish (Austin and Adams 1996). Lehane and 

Rawlin (2000) reviewed zoonoses acquired from fish and reported that aeromonads caused 

cellulitis, myositis, and septicemia following injuries from handling fish, working in aquaculture, 

or keeping fish as pets. 

Reptiles and Amphibians 

A. hydrophila has been isolated from diseased turtles, alligators, snakes, and frogs 

(Gosling 1996b). 

Birds 

Aeromonads have been isolated from feces of wild and pet birds. Aeromonads may cause 

septicemia in poultry (Saif and Busch 1974). Okrend et al., 1987 demonstrated that chicken meat 

is contaminated with Aeromonas spp., and Hudson and De Lacy (1991) showed that 3 of 36 ready 

to eat products involving chicken meat contained Aeromonas spp. 

Domestic Livestock 

Gray (1984) isolated A. hydrophila from feces of normal horses (7 of 110, 6.4%), pigs (11 

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of 115, 9.6%), sheep (10 of 111, 9.0%), and cows (26 of 123, 21.1%). Most positive cultures were 

recovered only after enrichment in alkaline peptone water. The total fecal carriage rate in animals 

is slightly higher than the fecal carriage rate of normal humans, which is < 1 to 7% for most 

studies, although some studies report higher rates (Pitaragnsi et al., 1982). Populations in animals 

probably reflect the presence of aeromonads in their feed and water. 

In their study of Aeromonas spp. in the feces of domestic animals, Figura and Marri 

(1985) isolated A. hydrophila more frequently than A. caviae. They did not detect A. sobria in 

tongue swabs or cecal contents of pigs. Stern et al. (1987) isolated aeromonads from 1 of 32 cows 

and 3 of 21 turkeys, but none were isolated from 22 pigs or 24 sheep. Gray and Stickler (1989) 

reported finding predominantly A. hydrophila in cow feces and A. caviae in pig feces. Diet and 

water sources influenced recovery of aeromonads from feces of domestic animals. Aeromonas 

spp. have been isolated from feces, bedding, and drinking water of health cows and pigs 

(Hathcock et al., 1999). They survive in soil for months (Brandi et al., 1996). Both healthy and 

diseased animals shed aeromonads in feces (Hathcock et al., 1999). 

Household Pets 

Ghenghesh et al. (1999a) isolated Aeromonas spp. from rectal swabs of 13 of 120 dogs 

and 1 of 15 cats in Libya by using alkaline peptone water (pH 8.6) enrichment and blood agar 

containing ampicillin at a concentration of 15 mg/L. Nine of 14 isolates were examined for 

hemolytic activity using human erythrocytes, and 6 strains were found positive. These qualitative 

studies provide no estimate of the number of aeromonads shed in the feces of dogs and cats, and 

therefore the public health significance of these findings is unknown. 

Feral Animals 

Apart from fish and amphibians, little is known of the occurrence of Aeromonas disease in 

feral animals. Aeromonas may be an opportunistic pathogen of stressed animals (Gosling, 1996b). 

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Invertebrates 

A. caviae has been cultured from common houseflies captured on two farms and in a 

restaurant in South Carolina (Nayduch et al., 2001). APW enrichment was followed by PCR 

detection (APW-PCR). A. caviae was found in 272 of 349 (78%) of dairy farm flies, 54 of 99 

(55%) of pig farm flies, and 77 of 200 (39%) restaurant flies. Feces from cows (58%) and pigs 

(100%) on the farms were positive for A. caviae. Nayduch et al. (2002) proposed that houseflies 

could serve as vectors for transmission of Aeromonas spp. since the bacteria multiplied in the gut 

and persisted for several days. Fly to fly transmission was demonstrated and transmission of A. 

caviae from fly to food was observed. Though A. culicicola has been isolated from mosquitoes, 

there is no epidemiological evidence of transmission to humans (Pidiyar et al. 2002). 

Gu and Mitchell (2002) isolated A. media, A. veronii, A. salmonicida, and other bacteria 

from zebra mussels. A. media and A. salmonicida were shown to be pathogenic for zebra mussels, 

with high morbidity in 5 days when challenged with an inoculum of 7 log10 CFU per mussel. 

Pathogenicity of A. media was not temperature dependent, though longer incubation times were 

required to produce a lethal effect. Mussels harbor bacterial populations as high as 5 log10 CFU 

per mussel and pseudomonads are the predominate flora of healthy mussels. The microflora of 

mussels may pose a public health concern, especially when aeromonads are the predominant 

component of the microflora of mussels fouling drinking water intakes and pipes (Austin and 

Adams 1996; Gosling 1996b; Palumbo 1996). The use of aeromonads for biological control of 

zebra mussels warrants further investigation. 

Aeromonads have been found in association with marine copepods and plankton, where 

they are present at cell densities from 4 to 1.3x103

 CFU/100mL in seawater and from 1.5x101 

CFU/100mL to 6x102 CFU/100mL in plankton (Dumontet et al 1996; 2000). 

The use of medicinal leeches to treat vascular infiltration in surgical wounds has been 

recognized as a risk factor for Aeromonas infections since 1983, and there are numerous reports 

of cellulitis and septicemia resulting from leech therapy (Mercer et al. 1987; Snower et al. 1987). 

The symbiotic association between A. veronii biovar sobria and medicinal leeches has been 

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reported by Graf (1999), who proposed medicinal leeches as a model for studying pathogenesis of 

Aeromonas infection. 

3.1.3 Occurrence in Water 

Surface Waters 

Maalej et al. (2002) studied the seasonal occurrence of aeromonads in urban effluents and 

the costal marine environment. In urban sewage effluents, presence of aeromonads exhibited a 

seasonal cyclic distribution similar to fecal coliforms, with the highest numbers (29x106 

CFU/100mL) in winter months and the lowest levels in summer months. In coastal waters, 

aeromonads reached highest levels (56 CFU/100mL) in summer months. Lowest levels of 

aeromonads occurred under conditions of maximal solar irradiation and minimum turbidity. The 

lack of correlation between fecal indicator bacteria and aeromonads suggests that the former 

group of organisms is not predictive of the presence of aeromonads in ambient waters. 

Bonadonna et al. (2002) studied the occurrence of bacteria of anthropomorphic (human) 

origin and those of autochthonous (natural) origin using model systems for prediction of public 

health risk to marine bathers. The resulting model used salinity, total coliforms, fecal coliforms, 

E. coli, and location as predictive variables for presence of aeromonads. Presence of E. coli and 

fecal coliforms were associated with lower Aeromonas counts, while predominance of total 

coliform was associated with higher Aeromonas counts. Fecal coliforms and increased salinity 

was associated with higher Aeromonas counts. Aeromonas counts ranged from < 100 

CFU/100mL to > 105

 CFU/100mL. The complexity of the association between anthropomorphic 

and autochthonous bacteria confounds development of a predictive model for estimating public 

health risk of recreational exposure to marine waters. 

Maalej et al. (2003) studied seasonal dynamics on Aeromonas populations in urban 

effluent and natural seawater. Effluents contained 1.48x105

 CFU/100mL to 2.2x108 CFU/100mL, 

while seawater densities were from below the detection limit to 7.9x103

 CFU/100mL. The 

seasonal density of aeromonas was inversely related to the seasonal density of fecal coliforms. 

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During cold months, aeromonads increased in effluent and decreased in seawater, while during 

warm months, aeromonads decreased in effluent and increased in seawater. Inactivation of 

aeromonads in seawater is thought to result from lower water turbidity with increased solar 

penetration and lower levels of available organic nutrients compared to urban effluents. 

Ivanova et al. (2001) isolated A. veronii biovar sobria and A. popoffii from surface 

drinking water reservoir in Russia in the absence of apparent sources of fecal pollution. A. 

popoffii was also described by Huys et al. (1997) and it is common in freshwater and seawater 

(Soler et al., 2002). A. popoffii has been isolated from humans only once, though strains may 

produce virulence factors such as aerolysin, serine protease, DNAse, and lipase (Hua et al 2004). 

Ground Waters 

Anoxic groundwater may support growth of aeromonads, and aeration has proven 

successful in reducing the organic nutrients that supported growth. 

Massa et al. (2001) studied occurrence of aeromonads in natural mineral water and well 

water in Italy. Aeromonads were not detected in the 60 natural mineral waters examined. 

Aeromonads were found in 5 of 20 well samples with counts ranging from 26-1,609 CFU/250 

mL. Presence of aeromonads did not correlate with fecal indicators in 2 of 5 wells where they 

were detected. Putative virulence factors were demonstrated in well water isolates. The clinical 

and epidemiological significance of these findings is unknown. 

Villari et al. (2003) examined 103 isolates obtained over 3 years from natural mineral 

water and surface streams within the watershed of the wells from which the mineral water 

samples were collected. Evidence of clonal identity was found in the A. caviae isolates and among 

A. hydrophila strains in the mineral water samples using PFGE. Aeromonads from surface waters 

did not show clonal identity. Biofilm was thought to be responsible for the clonal nature of well 

water isolates. 

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Drinking Water 

Aeromonas spp. have been isolated from chlorinated drinking water supplies in several 

countries (Hazen et al., 1978; Burke et al., 1984a; van der Kooij 1988; Fernandez et al., 2000; 

Figueras et al., 2005). Aeromonads are typically below 10 CFU/mL in drinking water distribution 

systems. Aeromonads grow in distribution systems (Havelaar et al., 1990; van der Kooij 1991). 

They occur in distribution system biofilms where they may be protected from disinfection (van 

der Kooij et al., 1995; Holmes and Nicolls 1995). A drinking water standard of 200 CFU/100 mL 

at 25o C has been established in the Netherlands (Havelaar et al 1990). Multiple strains are 

frequently found in water sources (Kuhn et al., 1997; Sen and Rodgers 2004). 

Gavriel et al. (1998) studied a drinking water distribution system in Scotland for the 

presence of Aeromonas spp. and their relationship to chlorine concentration, pH, temperature, and 

rainfall. They isolated aeromonads from chlorinated reservoirs and suggested a relationship 

between rainfall and increased recovery, probably due to increased organic load caused by 

formation of chloramines. 

The presence of aeromonads in distribution system water indicates neither fecal pollution 

nor treatment failure; however, a large number of aeromonads present in distribution water 

suggests that water conditions support growth. Aeromonads in biofilm may resist disinfection. 

Aeromonas spp. have been found at a frequency of 1-27% of drinking water supplies 

examined (Rusin et al., 1997). They are found in all aqueous environments except thermal 

springs, hypersaline lakes, and extremely polluted waters (Janda and Abbott 1999). Aeromonads 

have been isolated from ground pork, chicken, beef, milk, produce, shellfish, fish, and fish eggs. 

Most of the foodborne outbreaks attributed to Aeromonas spp. resulted from ingestion of fish or 

shellfish. Because of the prevalence of aeromonads in foods, water appears to be an incidental 

source of colonization of the human gastrointestinal tract. The only incidence data reported for 

drinking water exposures show a predicted lifetime exposure case rate of 0.07 cases per 100,000 

population (Rusin et al., 1997). 

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A. popoffii has been isolated from drinking water (Huys et al., 1997) and A. culicicola was 

isolated from drinking water by Figueras et al. (2005). The longer survival rate of A. caviae 

compared to A. hydrophila may explain its frequent isolation from drinking water (Hanninen and 

Siitonen 1995) and its high concentrations in treated sewage (Holmes et al., 1996). 

Simmons et al. (2001) studied aeromonads in 125 roof-collected rainwater systems in New 

Zealand and found that 22 of them exceeded regulatory levels of indicator bacteria. Aeromonas 

spp. were isolated from 20 of 125 (16%) supplies and there was a positive association between 

presence of aeromonads and presence of fecal indicators. Households reporting gastrointestinal 

illness were more likely to have aeromonads in their water. 

Aeromonas spp. were isolated form 11.4% of water samples supplied to a slaughterhouse 

and 25.7% of water samples drained from carcasses (Bizani and Brandelli 2001). Water was 

shown to contaminate meats and may be a factor in food processing. 

Bottled Water 

Aeromonas spp. have been cultured from mineral water by Gonzales et al. (1987), 

Quevedo-Sarmiento et al. (1986), Slade et al. (1986), Manaia et al. (1990), Havelaar et al. (1990), 

Warburton et al. (1998), and Tsai and Yu (1997). Isolations rates as high as 35.5% and cell 

concentrations greater than 3 log10 CFU/mL have been reported. 

Croci et al. (2001) studied the behavior of aeromonads in bottled mineral waters under 

various conditions of temperature and nutrient levels. They demonstrated growth in polyethylene 

bottles at 10o C reaching peak cell densities of 4.47 log10 CFU/100mL in 28 days. It took 60 days 

to reach this cell density at 20o C. 

Bottled water standards in Canada require testing for heterotrophic plate count (HPC), 

most probable number (MPN) for coliforms and E. coli, and membrane filtration (MF) for 

Aeromonas and Pseudomonas. Warburton (2000) proposed using enrichment methods to reduce 

the cost associated with testing bottled water. In the U.S., bottled water is classified as a food and 

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therefore it is under FDA jurisdiction. No regulatory standards exist, although industry guidelines 

have been proposed. Of 3,460 samples tested, 0.6% contained aeromonads, 1.2% contained 

pseudomonads, 3.7% contained total coliforms, and 2.1% contained fecal coliforms. 

Wastewaters 

Aeromonads are widespread in wastewater treatment processes (Poffe and Op de Beeck 

1991; Kampfer et al., 1996). Aeromonas spp. were not detected in aerosols generated by 

wastewater treatment plants (Brandi et al., 2000). Nsabimana et al. (2000) studied microbial 

populations in activated sludge over time using phenotyping and ribotyping. The initial variety of 

hybridization groups was reduced and replaced by dominant hybridization groups. Over the 

course of a year, Aeromonas populations shifted as shown in Exhibit 3.1. 

Exhibit 3.1. Aeromonas Population Shift in Activated Sludge 

Initial Populations 

54.5% 47.1% 

22.7% 41.2% 

A. sobria 18.2% 5.4% 

4.5% A. sobria 5.8% 

Altered Populations 

A. caviae group A. enucrenophila 

A. hydrophila group A. caviae group 

 group A. hydrophila group 

A. eucrenophila group 

From Nsabimana et al., 2000 

Albrechtsen (2002) evaluated microbial quality of rainwater and graywater systems used 

for flushing toilets in Denmark. Rainwater systems introduced bacteria not found in toilets 

flushed with treated water. Graywater was associated with odor and high coliform counts. 

Aeromonas counts in stored rainwater were < 10 to 30 CFU/mL. Toilet bowls flushed with 

rainwater contained < 10 to 4,400 CFU/mL, while toilet bowls flushed with treated water 

contained 10 to 8,800 CFU/mL. 

Irrigation Waters 

Irrigation water contains aeromonads at concentrations between 2 to 4 log10 CFU/mL 

(Pianetti et al., 2004). Irrigation waters may contaminate fruits and vegetables. 

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Marine and Estuarine Waters

 Several investigators have reported the presence of A. hydrophila in seawater (Echeverria 

et al., 1983; Brandi et al., 1996). Additionally, A. caviae has been cultured from seawater at 

densities of 2 log10 CFU/mL, whereas in fecally contaminated water, wastewater, and sewage, A. 

caviae may reach densities of 7-8 log10 CFU/mL. 

3.1.4 Occurrence in Food 

Isonhood and Drake (2002) reviewed Aeromonas spp. in foods. While aeromonads have 

been isolated from fish, shellfish, meats, dairy products, and fresh vegetables, few foodborne 

outbreaks have been reported. A growing body of epidemiological evidence supports the 

possibility of aeromonads causing foodborne gastroenteritis. While a plethora of putative 

virulence factors has been postulated and demonstrated in food isolates, the exact role and 

mechanism of aeromonads in causing diarrheal illness has not been elucidated. Evidence suggests 

that a high infective dose is necessary to produce gastrointestinal disease in a susceptible host, 

and the fact that aeromonads may survive and grow at refrigerator temperatures provides a 

reservoir of bacteria that may achieve an infective dose when foods are mishandled. 

A variety of foods have been shown to harbor aeromonads (Yadav and Verma 1998). 

Ibrahim and MacRae (1991) reported aeromonads present in beef (60%), lamb (58%), pork 

(74%), and milk (26%) samples. Krovacek et al. (1992) found aeromonads in 43% of random 

samples from retail food outlets in Sweden. Walker and Brooks (1993) reported aeromonads in 

fish and fresh salads, and aeromonds have been isolated from lamb (Sierra et al., 1995), oysters 

(Tsai and Chen 1996), cheese and raw milk (Melas et al., 1999), and fish and seafood (Hanninen 

et al., 1997a). Szabo et al. (2000) isolated Aeromonas spp. from 70 of 120 samples of lettuce in 

Australia. Aeromonads are found in ready to eat foods, including seafoods (Hudson and Avery 

1994; Tsai and Chen 1996). Studies published before 1990 relied upon phenotypic identification, 

while several studies published after that time identified isolates to hybridization group. While 

hybridization groups containing virulence factors are found in environmental samples and foods, 

aeromonads only cause gastroenteritis when their presence exceeds an infective dose for a 

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susceptible host (Neyts et al., 2000b). Strain variability and undetermined host susceptibility 

factors have made it impossible to determine a nominal infective dose, however anecdotal 

evidence suggests that the infective dose is highly strain and host dependent, and probably 

exceeds 6 log10 CFU/g. Such high doses are unlikely to be ingested in drinking water, since 

ambient water concentrations rarely exceed 4 log10 CFU/mL except in sewage-polluted waters. 

Exhibit 3.2. Occurrence of Aeromonas spp. in Foods 

Food Cell Density (log10 CFU/g) Reference 

vegetables < 2 to > 6 McMahon and Wilson 2001 

fish 2-4 Pin et al., 1995 

seafood < 2 to > 5 Hanninen et al., 1997a 

luncheon meats 3-4 Gobat and Jemmi 1993 

cheese 3-5 Pin et al., 1995 

meats (beef, pork, lamb) 2-4 Pin et al., 1995 

poultry 2-4 Pin et al., 1995 

milk (pasteurized) 4-5 Pin et al., 1995 

Agarwal et al. (2000) isolated aeromonads from fish (22%), snails (6.25%), and quail eggs 

(18%), buffalo milk (2.8%), and goat meat 8.9% – all foods of animal origin in India. 

These findings are consistent with those of Tsai and Chen (1996), who found 22.2% of fish 

samples contained aeromonads, and Glunder and Siegmann (1993), who reported finding 

aeromonads in birds and poultry eggs. Abbey and Etang (1988) reported finding aeromonads in 

28-29% of snails in Nigeria. 

Neyts et al. (2000b) cultured 68 food samples quantitatively to determine the presence of 

mesophilic Aeromonas spp. Aeromonads were found in 26% of vegetable samples, 70% of meat 

and poultry samples, and 72% of fish and shrimp samples at numbers from < 2 log10 CFU/g to > 5 

log10 CFU/g. 

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3.1.5 Occurrence in Other Media 

Soil 

There are few studies of Aeromonas spp. in soil apart from the contribution of water. Gray 

and Stickler (1989) reported the presence of aeromonads in pasture soil, probably as a 

contribution from manure. 

Air 

Aerosolization of waters occurs in wastewater treatment. Fannin et al. (1985) studied 

airborne counts of aeromonads in an activated sludge wastewater treatment plant, finding them in 

airborne particles before and after treatment. Aeromonads were insignificant compared to the 

numbers of the Enterobacteriaceae that were present. 

Surfaces 

Aeromonads forms biofilms on surfaces and may pose a threat of contamination in food 

processing. Bal’a et al. (1998) found that heat and chlorine were effective against biofilm on 

stainless steel surfaces, however older biofilm was more resistant to heat and less established 

biofilm. Eight-day old biofilm was destroyed by heating to 60o C and by exposure to 75 mg/L 

chlorine for 1 min. 

3.2 Environmental Factors Affecting Survival 

Environmental survival of bacteria is dependent upon many physical and biological 

factors. Temperature, pH, ionic strength, sunlight (UV irradiation), moisture, available nutrient, 

presence of suspended solids, cell-specific protection mechanisms, and the presence of toxic 

substances and predators all interact to determine survival times. 

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3.2.1 Survival in Water 

Aeromonas spp. have their natural habitat in water and grow over a wide temperature 

range. Because Aeromonas spp. grow between 0o C and 45o C, with a temperature optimum of 22o 

C to 32o C, there are few environmental habitats where they are not found. Both high (Tsai and 

Yu 1997; Warburton 2000; Croci et al., 2001) and low (Kersters et al., 1996b) survival rates have 

been reported. Nutrient availability, temperature, and water activity most affected growth rates. 

Growth was optimal at 30o C at pH 7 and a water activity of 0.99 (Sautour et al., 2003). Imbert 

and Gancel (2004) studied the effect of temperature downshift on protein synthesis of A. 

hydrophila. While a few proteins were under-expressed, two-dimensional electrophoresis 

revealed that numerous new proteins appeared with a decrease in temperature and some others 

were over-expressed. Cold shock proteins distinct from those produced by E. coli were 

recognized. Additional studies are required to elucidate the nature of heat and cold shock proteins 

produced by aeromonads. 

Aeromonads grew in filtered autoclaved tapwater but were inhibited by the presence of 

heterotrophic bacteria, suggesting that competition for nutrients is a selective factor limiting the 

growth of aeromonads (Mary et al., 2001). Environmental strains had no competitive advantage 

over clinical isolates. Survival rates vary by strain, with HG-4 > HG-5B > HG-17 > HG-1. Cell 

density increased from 1-2 log10 CFU/mL to 6 log10 CFU/mL in 3-5 days in both filteredautoclaved and unfiltered-autoclaved tapwater. Cells suspended in unfiltered water that had not 

been autoclaved declined from initial concentrations of 4 log10 CFU/mL to 1 log10 CFU/mL 

within 14 days. Cells suspended in filtered water that had not been autoclaved showed strainspecific differences in survival with decay rates from 4 to 1 log10 CFU/mL on day 1 (A. popoffii), 

day 3 (A. media), day 7 (A. hydrophila) and day 14 (A. caviae). 

The upper growth limit is approximately 6x105 CFU/mL in nutrient-poor water (Miettinen 

et al., 1997; Tsai and Yu 1997; Kersters et al., 1996b; Mary et al., 2001). Phosphorous (Miettinen 

et al., 1997) and assimilable organic carbon (AOC) (van der Kooij 1992) are considered to be the 

primary factors regulating growth. A. hydrophila (HG-1) grows less well than other Aeromonas 

spp. in nutrient poor water (van der Kooij and Hijnen 1988; Mary et al., 2001). 

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Studies in low nutrient waters such as bottled mineral water show that Aeromonas spp. 

remain viable for extended periods of time. A. hydrophila survives in distilled bottled water 

between 30-60 days, and for > 223 days in spring water (Warburton 2000). A. hydrophila 

increased by 1 log10 during the first 24 hr. then declined 3 log10 CFU in 90 days and 6 log10 CFU 

in 150 days (Messi et al., 2002). Experiments using mixed cultures reduced the survival time of 

aeromonads. van der Kooij and Hijnen (1988) have shown that microgram per liter concentrations 

of amino acids and long-chain fatty acids promote growth of aeromonads in water distribution 

systems. Aeromonads can also metabolize a wide variety of biopolymers, including proteins, 

carbohydrates and lipids, which are attacked by aeromonads wide array of extracellular enzymes. 

Photooxidation from visible and ultraviolet light has an inimical effect on aeromonads. A. 

salmonicida populations were reduced by 99.9% after 2 hrs of exposure to sunlight (Liltved and 

Landfald 2000). The presence of humic acids enhanced the effectiveness of photoinactivation, 

while the presence of sodium chloride at 0.9% increased survivability. The efficacy of UV 

irradiation in conjunction with photoinactivation was used to characterize the parameters needed 

for inlet and effluent water in aquaculture. Benchokroun et al. (2003) examined the effects of 

solar radiation on Aeromonas spp. in an algal pond. Inactivation was enhanced by increasing 

oxygenation or pH, while addition of catalase or sodium pyruvate prevented die-off, suggesting 

that damage to cells was through photooxidation. A. sobria was slightly more resistant to 

photooxidation which may explain its relative presence at high density in waste stabilization 

ponds. 

Aeromonads grow best between pH 7-9 (Vivekanandhan et al., 2003). Sautour et al. 

(2003) reported that variation in pH had little effect upon survival over a range of pH 5-9, and this 

is consistent with the growth range reported by Popoff and Lallier (1984). Aeromonas spp. are 

sensitive to acid conditions below pH 3.5; however, they exhibit an acid stress response in that 

when they are acclimated at pH 5, the kill time at pH 3.5 is extended. Treatment with proteininhibiting antibiotics prior to exposure to low pH eliminated the acid stress response, suggesting 

that protein synthesis is an important part of the acid stress response. 

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Delamare et al. (2000) examined salt tolerance among aeromonads. All strains challenged 

were able to grow in the presence of 0.34 M sodium chloride, and all but A. veronii biovar sobria 

(referred to as A. ichthiosmia), A. sobria, and A. salmonicida subspecies salmonicida were able to 

grow in the presence of 0.51 M sodium chloride. A. hydrophila, A. caviae, A. encheleia, A. trota, 

A. euchrenophila, and A. media tolerated 0.68 molar salt concentration; A. caviae and A. trota 

grew at 0.85 M salt concentration, and A. trota tolerated 1.02 molar salt concentration. 

3.2.2 Survival in Food 

Aeromonads have been shown to grow in foods held at refrigerator temperatures 

(Palumbo and Buchanan 1988; Majeed and MacRae 1991; McMahon 2000). A. hydrophila counts 

increased from 3-5 log10 CFU to 6-7 log10 CFU in ground meat at 7o C over 7 days (Vaid and 

Garg 2002). Counts reached 8 log10 CFU in one day at 25o C. Background microflora were not 

inhibitory for aeromonads. While members of the Enterobacteriaceae are inhibited, growth of A. 

caviae was stimulated from 4 log10/g to 8 log10 CFU/g at ambient temperatures and from 5 log10 

CFU/g to 7 log10 CFU/g at refrigerator temperature in salad materials packaged under 

superatmospheric oxygen (Allende et al., 2002). 

The Food Micromodel (UK) and the Pathogen Modeling Program (US) were developed to 

model growth of bacteria in foods using variables of temperature, sodium chloride concentration, 

sodium nitrate (US model only), and pH. 

 Growth temperature is an important feature in differentiation of clinical and environmental 

strains. Approximately half of clinical isolates show some growth at 4-5o C, all food isolates grow 

at this temperature (Knochel 1990). While most clinical strains grow at 42o C, only a few isolates 

from vegetables stored at 5o C grew at elevated temperature. Knochel (1990) found that some 

aeromonads isolated from cold water did not grow at 37o C. The growth temperature range for 

aeromonads is from 4 to 44o C, but individual strains typically have a restricted growth range 

according to their ecological niche, and growth of a strain at both extremes of the range are rare 

(Kirov et al., 1993b). Aeromonads are considered heat sensitive with respect to other foodborne 

pathogens. 

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Aeromonads tolerate high pH well and this feature has been exploited by using alkaline 

peptone water at pH 8.6 for sample enrichment. Aeromonads grow at pH 5.8 or higher, and may 

survive at pH 4.6 or higher according to computer modeling using Food Micromodel. Speciesspecific acid tolerance is know to occur, since A. caviae grown on glucose or other simple sugars 

produces sufficient acetic acid to auto-sterilize a broth culture within 48 hr. in weakly buffered 

systems. This metabolic activity has been called the suicide phenomenon (Namdari and Cabelli 

1989). 

Aeromonads do not tolerate high salt concentrations. Knochel (1990) reported that a few 

strains tolerated 6% NaCl, but generally aeromonads do not tolerate concentrations above 5% 

NaCl. Growth in a competitive environment may not occur at salt concentrations between 3-4%, 

and a few isolates are sensitive to 2% NaCl. Vivekanandhan et al. (2003) examined the effects of 

salt concentration on A. hydrophila. Salt concentrations above 2% inhibited growth somewhat. 

Some growth occurred at 4% NaCl concentration; however, no growth occurred at 5% though 

cells remained viable. These results may explain the distribution of aeromonads in seawater and 

have implication in food preservation. 

Modified atmospheres are increasingly being used in food packaging. Pin et al. (2004) 

studied the response of A. hydrophila to various combinations of pH, temperature, and CO2 and 

O2 concentrations. The results were used to develop and validate a predictive model for growth 

and death estimates under modified atmospheres at refrigerator temperature. Reduced oxygen 

levels do not exert a detrimental effect on survival and growth of aeromonads, and they may be 

isolated from vacuum packed foods (Berrang et al., 1989). Expression of proteases and 

hemolysins was inhibited, particularly at low temperature (McMahon 2000). High amounts of 

carbon dioxide (94-99%) markedly affected growth and viability (Golden et al., 1989). 

Devlieghere et al. (2000) reported that the use of carbon dioxide in the package atmosphere in 

combination with reduced water activity effectively inhibited growth. Gas-packed cured cooked 

meat products did not support growth of Aeromonas spp stored at < 7o C. 

Irradiation is effective at doses of 125 to 150 kilorads, which is sufficient to eliminate 

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viable cells from foods. Polyphosphates together with sodium chloride, sorbates, and smoke are 

inhibitory to aeromonads (Gram 1991; Palumbo et al., 1995). Velazquez et al. (2001) studied the 

antibacterial effects of phosphates, common food additives in meat products. A. hydrophila was 

totally inhibited at concentrations between 0.5 and 3%. 0.5% sodium acid pyrophosphate 

exhibited the greatest inhibition as it possesses both bactericidal and bacteriolytic properties. 

Mary et al. (2003) studied the tolerance of A. hydrophila to ethanol, sodium chloride, 

drying, and temperature. Starved cells developed increased tolerance to salt and ethanol challenge 

but not to heat. These findings suggest that less rigorous food processing and preservation 

methods may not eradicate aeromonads from foods. Uyttendaele et al. (2004) examined the effect 

of lactic acid, chlorinated water, and thyme essential oil solution for control of Aeromonas spp. in 

fresh vegetables. Lactic acid at 1% or 2%, and 0.5% or 1% thyme essential oil solutions were able 

to control growth of aeromonads, while waters with chlorine concentrations of 0.1 to 0.5 mg/L 

were not. 

Usual food preservative processes (pH < 5, sodium chloride > 3.5%,) are sufficient to 

suppress growth of aeromonads with the exception of refrigeration, as refrigeration alone does not 

suppress growth of aeromonads (Palumbo et al., 1995). 

3.2.3 Viable but Non-culturable (VNC) State 

Starvation (nutrient deprivation) of bacteria has been reported to induce physiological 

changes that reduce the ability to detect them using culture methods. Nutritional depletion is 

correlated with detachment of Aeromonas cells adsorbed to surfaces (Sawyer and Hermanowicz 

2000). 

Maalej et al. (2004) studied survival of A. hydrophila in natural filtered seawater. 

Populations declined below the detection limit at both 5o C and 23o C in 3-5 weeks. Cells grown 

at 5o C were more resistant to stress than cells grown at higher temperature. A temperature shift 

from 5o C to 23o C did not result in cell resuscitation. Cells lost respiratory activity before they 

lost membrane integrity. The shift to VNC state is associated with formation of hydrogen 

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peroxide sensitive cells populations. Catalase or sodium pyruvate may permit cell damaged by 

photooxidation to be recovered, but this was not true for A. hydrophila in seawater experiments. 

A. hydrophila has been shown by exposure to hydrogen peroxide to resist loss of viability due to 

oxidative stress by exposure to sub-lethal levels of oxidants which induce polypeptide synthesis in 

a stress response similar to other bacteria (Landre et al., 2000). 

Sun et al. (2000) claims to have induced A. hydrophila into a VNC state by incubation at 

4o C for 45 days. Cells were resuscitated using liquid media and solid media containing catalase 

or sodium pyruvate. Wai et al. (2000) also reported induction of VNC aeromonads with recovery 

on media through addition of catalase or sodium pyruvate. Contrary to these reports, Rahman et 

al. (2001) reported that induction of the VNC state was not reversible in aeromonads. Mary et al. 

(2002) reported that A. hydrophila declined to non-detectable levels in nutrient-poor filter 

sterilized distilled water at 4o C within 7 weeks, while the number of cells with intact membranes 

by the Live/Dead® method decreased by 1 log10 CFU. Cells could not be resuscitated by an 

increase in temperature to 25o C, and neither catalase or sodium pyruvate improved recovery. 

Whether or not aeromonads exist in a reversible VNC state remains to be determined. 

3.2.4 Preservation in Stock Cultures 

While freezing at -70o C is common for culture preservation, survival after freezing has 

not been systematically studied with foods. Palumbo and Buchanan (1988) reported poor survival 

of stock cultures lyophilized in skim milk, while Marino et al. (1995) reported excellent survival 

rates for up to two years for cultures lyophilized in a mixture of glycerol-skim milk-tryptone soy 

broth. The recovery rate of aeromonads lyophilized in serum-inositol cryopreservative has been 

variable, with good recovery for 2-3 years, but poor recovery after 10 years or more. 

3.3 Summary 

Aeromonas spp. are found worldwide in surface water (Hazen et al., 1978; Kersters et al., 

1995), ground water (Kersters et al., 1995), non-chlorinated drinking water (Gavriel et al., 1998), 

chlorinated drinking water (LeChevallier et al., 1983; Gavriel et al., 1998), and bottled mineral 

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water (Tsai and Yu 1997). Aeromonads are found in a wide variety of foods (Kirov 1993a). They 

are found in the intestinal tract of humans and animals (Janda and Abbott 1998), raw sewage 

(Holmes et al., 1996), sewage effluents (Holmes et al., 1996), activated sludge (Kampfer et al., 

1996), and sewage-contaminated waters (Boussaid et al., 1991). Aeromonads have been shown to 

grow in foods held at refrigerator temperatures (Palumbo and Buchanan 1988; Majeed and 

MacRae 1991; McMahon 2000). However, while aeromonads have been isolated from fish, 

shellfish, meats, dairy products, and fresh vegetables, few foodborne outbreaks have been 

reported. 

4.0 Health Effect in Humans 

 Some Aeromonas spp. are opportunistic pathogens of humans, causing a wide variety of 

extra-intestinal infections and occasionally associated with gastrointestinal disease. Aeromonas 

infections occur in four broad groups of patients: 

a. Persons with impared immune function or serious underlying disease, especially 

cirrhosis or hematologic malignancy. 

b. Persons with hospital-acquired postoperative infections or infections associated with 

health care. 

c. Previously healthy persons with community-acquired infection following trauma 

and/or exposure to contaminated water. 

d. Previously healthy persons, especially children, who ingest contaminated foods and 

subsequently develop gastrointestinal illness. 

The frequency of Aeromonas infections remained stable over the past 15 years, so that 

Aeromonas infection should not be considered an “emerging” problem. 

The first comprehensive review of human infections caused by Aeromonas spp. was 

undertaken by von Graevenitz and Mensch (1968). Many comprehensive reviews have followed 

(Janda and Abbott 1996; Chopra and Houston 1999; Figueras 2005). Aeromonas spp. reportedly 

cause cellulitis, abscess, wound infection, necrotizing fasciitis, myonecrosis, pneumonia, 

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empyema, septicemia, septic arthritis, osteomyelitis, endocarditis, meningitis, gastroenteritis, 

appendicitis, peritonitis, acute suppurative cholangitis, and corneal ulcer. 

4.1 Clinical Symptoms 

Clinical symptoms of Aeromonas infection depend upon the site and severity of infection. 

Wound infections frequently result in cellulitis and rarely necrotizing fasciitis. Septicemia may 

follow wound infection, or may be secondary to systemic diseases such as cancer, cirrhosis, 

diabetes, biliary disease, or diseases resulting in gastrointestinal perforation. Dissemination may 

result in meningitis or endocarditis. Pneumonia is rare and it is usually associated with aspiration, 

such as in near drowning. Gastroenteritis symptoms range from mild self-limiting to dysentery or 

cholera-like illness. 

Patients present with a spectrum of disease symptoms from mild self-limiting diarrhea to 

acute, severe diarrhea with abdominal cramps, vomiting, and fever. Bloody stools occur with 

some strains. Adults have chronic diarrhea and abdominal cramps, whereas children 12 years or 

younger are likely to have more acute and severe illness. A. caviae and A. hydrophila have been 

associated with chronic diarrhea lasting up to one year (del Val et al., 1990). A syndrome 

resembling ulcerative colitis has been observed by endoscopy, and segmented colitis has also 

been reported (Farraye et al., 1989). Colonoscopy reveals exudates, superficial ulcerations, 

erythema, and friability of the mucosa, as well as loss of vascular pattern and overlying mucus. 

4.1.1 Mechanism of Virulence 

Virulence of aeromonads is multifactorial and incompletely understood despite decades of 

intense investigation (Trower et al., 2000). Many putative virulence factors have been described, 

including toxins, enterotoxins, proteases, hemolysins, lipases, adhesins, agglutinins, hydrolytic 

enzymes, outer membrane proteins, S-layer, flagella, and pili. Janda (2002) reviewed the many 

virulence factors produced by Aeromonas spp., and readers are referred to this review for details 

of virulence mehanisms that are beyond the scope of this document. 

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Only one virulence factor, A-layer of A. salmonicida, had unequivocally been linked to 

disease causation in 1981, but much progress has been made in understanding virulence 

mechanisms since then (Kay et al., 1981). No animal model has been found for testing other 

aeromonads, though streptomycin-treated mouse, clindamycin-treated rat, and suckling mouse 

models have been used to study virulence factors (Sanderson et al., 1996). Pazzaglia et al. (1990) 

reported that the removable intestinal tie adult rabbit diarrhea (RITARD) model was the preferred 

animal model for studying diarrheal disease. Sanyal et al. (1975) demonstrated multiplication of 

aeromonads within the gut of experimental animals; however, these experiments did not result in 

identification of an animal model. Several in vitro cell lines have been used to study attachment, 

invasion and toxic effects of Aeromonas virulence factors. Vero, HEp-2, INT 407, mouse Y1 

adrenal cells, Caco-2 have been used with varying success (Thornley et al., 1997). Studies using 

purified ileal and colonic enterocytes and human mucosal slices are currently being evaluated. 

Structural factors of bacteria promote attachment (pili, flagella) colonization (adhesins, 

outer membrane proteins) and protect cells from host response (S-layer, lipopolysaccharide 

(LPS), capsule). Long wavy flexible fimbriae and afimbrial adhesions are associated with 

colonization of A. hydrophila. Kirov et al. (1995a) and Kirov et al. (1996) demonstrated that 

removal of surface structures reduces adherence in HEp-2 cells by up to 80%. A. caviae and A. 

veronii biovar sobria were found to adhere better than A. hydrophila. Most adherence studies 

were done with clinical isolates and little is known about adherence of environmental strains. 

Carrello et al. (1988) reported differences in pili of clinical and environmental strains. Adhesion 

to HEp-2 cells correlated with clinical strains possessing low numbers of thin flexible long L/W 

type pili, while environmental strains expressed a larger number of short rigid pili termed the S/R 

type. 

Nonfilamentous adhesins include S-layer, LPS, and outer membrane proteins. Collagenbinding protein is found extracelluarly and in loose association with cells, and it is thought to 

have adherence properties. Surface array proteins are thought to have antiphagocytic properties 

(Sara and Sleytr 2000). The role of LPS is not clear, but it is thought to be associated with 

colonization. Tso and Dooley (1995) suggested that it might confer resistance to complementmediated lethal effects. Martinez et al. (1995) found capsules in pathogenic A. hydrophila 

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serotypes O:11 and O:34. Gavin et al. (2003a) reviewed molecular mechanisms of Aeromonas 

pathogenesis. Aeromonads possess all of the requirements of pathogenic bacteria. Attachment and 

entry into host cells is facilitated through production of flagella, pili, and adhesins. Multiplication 

in host tissue is aided by production of siderophores and outer membrane proteins, while 

resistance to host defenses is conferred by production of capsule, S-layer, lipopolysaccharide and 

porins. Enterotoxins, proteases, phospholipases, and hemolysins effect damage to host cells 

leading to cell death. 

Several exotoxins and enzymes are elaborated by Aeromonas spp. with putative virulence 

properties (Chopra et al 1986). The cytotoxic group of extracellular products includes hemolysin, 

aerolysin, and phospholipase. The enterotoxin group is comprised of heat-labile enterotoxin, heatstable enterotoxin, non-cholera toxin cross reactive, cholera toxin cross reactive, aerolysin (Asaotoxin, β hemolysin), and non-channel forming hemolysin (HlyA). The protease group contains 

thermostable metalloprotease (38 kDa), thermolabile serine protease (68kDa), thermostable serine 

protease (22 kDa), and zinc protease (19 kDa). The hydrolase group includes DNAse, gelatinase, 

acetylcholinesterase, amylase, lipase, and chitinase. Other non-enzyme proteins such as histonelike protein, multidrug-resistance protein, and collagen-binding protein are thought to play a role 

in virulence. 

Thornley et al. (1997) reviewed the virulence genes of Aeromonas spp. A summary of 

virulence factors is shown in Exhibit 4.1. 

Exhibit 4.1. Virulence Factors of Aeromonas species 

Cell-Associated Virulence Factors Extracellular Virulence Factors 

Pili (fimbriae) Hemolysin 

Flagella Enterotoxin 

Outer membrane proteins Cytotoxin 

A or S layer Protease 

Lipopolysaccharide Glycerophospholipid cholesterol acetyltransferase (GCAT) 

Capsule Other hydrolytic enzymes 

From Thornley et al., 1997 

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4.1.1.1 Cell-Associated Virulence Factors

Invasins 

Several studies have shown invasive ability of Aeromonas to equal that of Campylobacter 

(Nishikawa et al., 1994; Shaw et al., 1995). While intracellular bacteria have been demonstrated 

by electron microscopy, no gene or product has been identified specifically with invasion. 

Adhesins 

Cell properties promoting adhesion of aeromonads to host cells were recognized early in 

the studies of the pathogenesis (Gosling 1996a). Ascencio et al. (1998) reported cell surface 

extracts containing active mucin-binding components from 22-95 kDa from Aeromonas spp. 

Adhesions to HEp-2, Caco-2 and INT407 cells has been reported by several investigators 

(Nishikawa et al., 1994; Bartkova and Ciznar 1994; Kirov et al., 1995b). A correlation between 

high level HEp-2 cell adherence and enteropathogenicity has been reported (Kirov and Sanderson 

1995). Some investigators have reported weak adherence of A. caviae to HEp-2 cells (Carrello et 

al., 1988). 

Outer Membrane Proteins 

The literature contains several conflicting reports concerning attachment mechanisms, but 

it is generally accepted that outer membrane proteins (OMP) mediate bacterial adherence to host 

cells. Recent studies suggest that non-pilar adhesins play a major role in adhesion of A. 

hydrophila and A. veronii biovar sobria. Nishikawa et al. (1994) suggested the role of an outer 

membrane protein in binding of aeromonads to Caco-2 cells. The role of OMPs has been 

proposed for attachment to HEp-2, HeLa, Chineses hamster ovary (CHO) and Vero cells 

(Bartkova and Ciznar 1994). Some OMP have hemagglutination activity, while other OMPs are 

thought to have pore-forming capability. 

Rocha de Souza et al. (2001) identified a 43 kDa OMP from A. caviae that promoted 

adhesion to HEp-2 cells, and demonstrated that expression of the 43 kDa OMP is greater in cells 

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grown at 22o C than those grown at 37o C. This characteristic has been observed with A. veronii 

biovar sobria (Kirov et al., 1995a). All A. caviae strains of Rocha de Souza et al. (2001) exhibited 

the aggregative adherence pattern and the aggregates were able to adhere to HEp-2 cells, forming 

a stacked-brick appearance. Microbeads coated with the protein adhered to HEp-2 cells and 

antibody to the 43 kDa protein blocked adherence of the protein-coated beads. Rocha de Souza et 

al. (2003) studied interaction of adherence and invasion properties of A. caviae using Caco-2 cells 

and observed that the 43 kDa OMP facilitated cell binding. TEM did not demonstrate fimbrial 

structures on cell surfaces of highly-adherent A. caviae strains. These data suggest that OMPs 

mediate adherence in A. caviae instead of pili. Merino et al. (1992) suggested that LPS and OMP 

might be other important adhesins of Aeromonas strains to HEp-2 cells and the findings of Rocha 

de Souza et al. (2001) support the role of OMPs in adherence. Fang et al. (2004) cloned and 

characterized the major 43 kDa adhesin (Aha1) of A. hydrophila. The amino acid sequences 

showed a high degree of similarity to OMPs of A. hydrophila. 

A 43 kDa OMP exhibiting hemagglutinin activity produced by A. hydrophila was 

inhibited by L-fucose (Atkinson et al., 1987). This OMP was shown to have pore-forming activity 

using an artificial planar bilayer membrane (Quinn et al., 1993). None of the A. caviae strains of 

Rocha de Souza et al. (2001) demonstrated hemagglutinin activity to human erythrocytes, nor did 

they show L-fucose sensitive adherence to HEp-2 cells. 

Guzman-Murillo et al. (2000) showed that Omp48 is associated with adherence properties 

of A. veronii. Vazquez-Juarez et al. (2003) cloned and sequenced the gene encoding Omp48, a 

major outer membrane protein of A. veronii. BLAST analysis revealed over 50% identity with the 

LamB porin of other pathogenic gram-negative bacteria. Vazquez-Juarez et al. (2004) identified 

mucin and lactoferrin binding proteins in A. veronii. The 48 kDa protein was identified as Omp48 

which is similar to E. coli OMP LamB, a porin mediating maltose transport across the outer 

membrane. Omp48 binds collagen and fibronectin in addition to having mucin and lactoferrin 

binding activity. OMPs exhibit immunoprotective attributes, and specific antibody inhibited 

adhesion to HeLa cells Vazquez-Juarez et al. (2004). 

Contrasting results were obtained by Kirov et al. (1998), who reported that Type IV pili 

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were present in A. caviae strains isolated from clinical sources in Australia, and they suggested 

that pili were the principal adhesins. 

S-Layer Proteins 

S-layer (also termed A-layer) has been extensively studied and it has been linked to 

virulence by providing protection against bactericidal effects of the immune response, protecting 

cells from proteolysis, facilitating association with macrophages, binds collagen, and plays a role 

in colonization (Gavin et al., 2003b). The S-layer has a paracrystalline array made up of protein 

units measuring 49-51 kDa, and made up of two domains. Virulence is markedly reduced when 

the S-layer is lost. S-layer caries binding sites for fibronectin, laminin and other extracellular 

proteins, and provides protection against proteases and phagocytic cells (Kay and Trust 1991). 

The protein is produced under control of the ashA gene (Thomas and Trust 1995). S-layers of A. 

hydrophila and A. veronii biovar sobria are morphologically similar but antigenically diverse 

(Kostrzynska et al., 1992). S-layer producing Aeromonas spp. are associated with extraintestinal 

infection of humans (Kokka et al., 1992; Janda et al., 1994). Generally, the S-layers of A. veronii 

biovar sobria are less associated with virulence than the S-layer of A. salmonicida. 

Sara and Sleytr (2000) reviewed the structural and functional nature of S-layers, which are 

formed of monomolecular crystalline arrays of protein subunits. The ahs gene encodes S-layer of 

A. hydrophila and the vapA gene encodes S-layer in A. salmonicida. S-layers are thought to 

protect against bacterial parasites such as Bdellovibrio bacteriovorus, but they are ineffective 

against protozoa. S-layer of A. salmonicida is thought to contribute to virulence by rendering cells 

resistant to bactericidal activity of complement in immune and nonimmune sera. A-layer plays a 

role in uptake of porphyrins and shows unique immunoglobulin and extracellular matrix proteinbinding capacity. 

Lipopolysaccharide 

Lipopolysaccharide plays a role in adhesion to epithelial cells (Merino et al., 1996), 

resistance to nonimmune serum (Merino et al., 1991), and virulence (Aguilar et al., 1997). It also 

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comprises the major structural element of the gram-negative cell wall that is responsible for 

somatic antigenic specificity (O-antigen). Serotyping of aeromonads is based upon O-antigen of 

the LPS. LPS is thought to be a colonization factor, as its presence facilitates adherence to HEp-2 

cells (Francki and Chang 1994). Loss of LPS reduced adhesion by 61%. Tso and Dooley (1995) 

suggested that O-polysaccharides were not found in a high percentage of clinical isolates and 

were not required for virulence. They also suggested that LPS was required for colonization of 

cells by non-invasive or enterotoxigenic strains, but would be required by invasive strains. The 

structure and function of O-polysaccharides of A. hydrophila O:34 were determined by Knirel et 

al. (2002). Knirel et al. (2004) elucidated the structure of R-type lipopolysaccharide of A. 

hydrophila in an effort to understand the nature of rough strains compared to strains with intact 

O-antigen determinants. Recently published work on the LPS of A. hydrophila serotype O:34 

suggests that the UDP N-acetylgalactosamine-4-epimerase gene is essential for mesophilic 

A.hydrophila serotype O:34 virululence (Canals et al 2006). 

Capsule 

Few investigations have examined the role of capsule polysaccharide as a virulence factor 

since most motile strains are not encapsulated. Capsules have been shown in A. hydrophila 

serotypes O:11 and O:34 when they are grown in glucose-rich media (Martinez et al., 1995). 

Preliminary work suggests that capsule may play a role in septicemia, as non-encapsulated strains 

are less virulent. Zhang et al. (2002) report that Aeromonas capsule material has the capability of 

protecting cells from complement-mediated serum killing activity. Aguilar et al. (1999) also 

reports serum resistance properties of capsule polysaccharide. 

Flagella

 Flagellation in Aeromonas is usually monotrichous and polar. However, lateral flagella 

occur in some strains and some strains are nonmotile. Peritrichous flagella are unsheathed and 

they are associated with swarming movement across solid media surfaces (Kirov et al., 2002). 

Flagella allow Aeromonas to reach target cells where they colonize (Barnett et al., 1997). 

Flagellar production is under the control of over 40 genes (Macnab 1996). Lateral flagella 

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facilitate biofilm formation (Gavin et al., 2002) and persistence during infection (Costerton et al., 

1987). Polar sheathed flagella are expressed constitutively in liquid media, and peritrichous lateral 

flagella are produced in matrices which do not permit motility by single polar flagella such as 

growth on solid culture media (Gavin et al., 2003b). Merino et al. (2001a) suggest that the mgtE 

gene encodes a Mg2+ and Co2+ transport system that have a role in swarming ability and 

consequently, adherence and biofilm formation. 

Gryllos et al. (2001) studied the flm gene locus of Aeromonas spp. relative to adherence 

and concluded that flagella and the LPS O-antigen are involved in adherence to human epithelial 

cells. FlmA and flmB are widely distributed among aeromonads and they are involved in flagellar 

assembly. In A. caviae, neuA, flmD and neuB are located on the same operon with flmA and flmB 

and they are involved in O-antigen biosynthesis and flagellar assembly. Nine lateral flagella genes 

have been identified in A. hydrophila. Five polar flagellar genes have been found in A. caviae and 

polar flagella are required for optimal adherence of A. caviae to HEp-2 cells (Rabaan et al., 2001). 

Merino et al. (2003) described nine polar flagellar genes in A. salmonicida but found no lateral 

flagella. Production of lateral flagella would confer no advantage to A. salmonicida, since it is 

adapted to fish. Genes controlling polar flagella were required for expression of lateral flagella 

(Altarriba et al., 2003). 

Kirov (2003) reviewed expression of lateral flagella and their multi-functional role in 

pathogenesis. Lateral flagella enable swarming motility on surfaces, and function as adhesins 

which contribute to microcolony formation and biofilm formation on surfaces. Polar flagellar 

genes flaA, flaB, flaG, flaH, and flaJ have been identified. Lateral flagellar genes identified are 

lafA1 and lafA2, which code for flagellin subunits, plus lafB, lafC, laf,E, lafF, lafS, lafT, lafU, 

lafX, and fliU, most of which have known gene function. FlaA and flaB, lafA1, and lafA2 genes 

have been sequenced. Only laf-positive strain swarm. Lateral flagella promote auto-agglutination 

and function as adhesions. The role of lateral flagella in intestinal colonization is not fully 

understood; however, swarming has been associated with urinary tract colonization in Proteus 

spp. 

Kirov et al. (2004) reported that both polar and lateral flagella are enterocyte adhesins that 

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contribute to biofilm formation on surfaces. Approximately 60% of mesophilic species produce 

lateral flagella. Strains that loose polar flagella are virtually nonadherent to cell lines, while 

strains lacking lateral flagella have a reduced capacity for cell binding. Flagellar mutants were 

shown to have decreased binding capacity by more than 80%. 

Gavin et al. (2002) reported that lateral flagella are essential for adherence to epithelial 

cells and biofilm formation. Mutations in lateral flagellar genes did not affect polar flagellation; 

however, mutation of genes controlling polar flagella inhibit lateral flagellar expression. Gavin et 

al. (2003b) report that lateral flagella are required for increased cell adherence, invasion and 

biofilm formation. Aeromonads expend considerable resources of cell energy in flagellar 

regulation, and the overlapping mechanisms controlling polar and lateral flagella is not fully 

understood. 

Pili (Fimbriae) 

Attachment to host cells is mediated by extracellular appendages of bacteria such as pili, 

and these filamentous structures were described as potential colonization factors in A. hydrophila 

and A. veronii biovar sobria (Hokama and Iwanaga 1991). Kirov (1993b) reported that pili were 

important adhesive factors for mucosal surface attachment and described filamentous and 

nonfilamentous adhesins. Two morphotypes of pili have been observed in Aeromonas spp., short 

rigid pili (S/R type) and long wavy flexible pili (L/W type). 

S/R pili are present in 95% of strains regardless of source (Kirov et al., 1995a). S/R pili 

facilitate autoagglutination but not hemagglutination. Contrary to Kirov et al. (1995a), Ho et al. 

(1990) found they were more common in environmental strains, and were not thought to be 

colonizing factors. S/R pili have diameters of 7-10 nm. They have been purified from A. 

hydrophila and the 17kDa pillin protein shows homology in 28 of 40 residues with E. coli Type I 

and the Pap-pyelonephritis-associated pilin. 

L/W pili cause hemagglutination, are associated with outer membrane proteins, and are 

found on clinical isolates Kirov et al., 1995b). L/W pili, bundle-forming pili and filamentous 

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networks showed maximal expression when grown at 22o C. (Kirov et al., 1995a; Kirov et al., 

1995b). The L/W pili have a diameter of 4-7 nm and can form bundles or filamentous networks 

(Kirov et al., 1995a). Removal of these pili blocks adherence and reaction of removed pili with 

cell receptors competitively block the adhesion of the parent bacterial strain (Iwanaga and 

Hokkama 1992). Aeromonas pili share sequence similarities with Type IV pilins but they are 

immunologically and electrophoretically distinct. 

Kirov and Sanderson (1996) reported the first purification of bundle-forming pili from A. 

veronii biovar sobria. The N-terminus sequence of 21 kD A. sobria flexible pilus was most similar 

to the mannose-sensitive hemagglutinin of V. cholerae with 22 of 26 amino acids being identical 

(Kirov and Sanderson 1996). Bundle-forming pili were most closely related to Type IV-A class. 

The fxp gene of A. hydrophila has been sequenced. It is found on a plasmid and encodes the 4 

kDa mini pilin of Ho et al. (1990). The fxp gene was not found in other A. hydrophila strains so 

its role in pathogenesis is doubtful. 

Aeromonas spp. from cases of gastroenteritis may exhibit Type IV pili (Tap) (Barnett and 

Kirov 1999) or bundle-forming pili (Bfp) (Kirov and Sanderson 1996; Kirov et al., 1999). Barnett 

et al. (1997) also reported the presence of two distinct families of Type IV bundle-forming pili 

and Tap pili in Aeromonas strains from patients with gastroenteritis. 

Kirov et al. (2000) investigated the role of Type IV pili (Tap) in the pathogenesis of 

gastrointestinal disease. While Type IV bundle-forming pili (Bfp) have been linked to intestinal 

colonization (Kirov et al., 1999), less is known about Tap pili, though the presence of Tap pili is 

thought to promote biofilm formation (Bechet and Blondeau 2003). Mutations in genes 

controlling Tap pili did not affect adhesion to Henle 407 intestinal cells or HEp-2 cells. Tap 

mutants produced disease in rabbits (ileal loop model) and suckling mice (oral challenge model). 

Tap and Bpf differ in amino acid sequence and molecular weight. Bpf are significant colonization 

factors. The role of Tap pili requires further study. 

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4.1.1.2 Extracellular Virulence Factors

Enzymes 

Aeromonas spp. produce a wide variety of extracellular enzymes, some of which are 

thought to contribute to pathogenesis. Pemberton et al. (1997) reviewed the secreted enzymes of 

Aeromonas spp. and their genetic control and biological function. Examples of some of these 

enzymes and their activity are shown in Exhibit 4.2. 

Exhibit 4.2. Extracellular Enzymes Secreted by Aeromonas spp. 

Enzyme Activity Abbreviation Size (kDa) Source 

Aminopeptidase prolyl Pap 48 A. veronii biovar 

sobria 

Amylase "-amylase AmyA 49 A. hydrophila 

"-amylase AmyA 48 A. hydrophila 

"-amylase AmyB 72 A. hydrophila 

$-lactamase metallo-$-lactamase CphA 28 A. hydrophila 

metallo-$-lactamase CphA2 28 A. hydrophila 

metallo-$-lactamase ImiS 28 A. veronii biovar 

sobria 

cephalosporinase CepS 38 A. veronii biovar 

sobria 

penicillinase AmpS 27 A. veronii biovar 

sobria 

Cell elongating toxin 70 A. hydrophila 

Chitinase Group A ChiA 94 A. caviae 

Group B ChiIII 53 A. hydrophila 

Group B ORF-3 57 10S-24 

Group C ChiIII 5 10S-24 

Enterotoxin cytolytic Ahh1 52 A. hydrophila 

cytotonic 35 A. hydrophila 

Hemolysin "-hemolysin Ahh5 54 A. hydrophila 

"-hemolysin Asa1 54 A. veronii biovar sobria 

aerolysin AerA 57 A. hydrophila 

aerolysin Ahh3 54 A. hydrophila 

aerolysin 54 A. salmonicida 

aerolysin 63 A. salmonicida 

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Exhibit 4.2. Extracellular Enzymes Secreted by Aeromonas spp., continued 

Enzyme Activity Abbreviation Size (kDa) Source 

Hemolysin aerolysin 65 A. salmonicida 

Lipase acetlycholine esterase 15-5 A. hydrophila 

glycerophospho-lipid- GCAT 31 A. hydrophila 

chloesterol 

acyltransferase

glycerophospho-lipid- GCAT 26 A. salmonicida 

chloesterol 

acyltransferase

lipase H3 72 A. hydrophila 

phospholipase Apl1 73 A. hydrophila 

Nuclease DNAse Dns 25 A. hydrophila 

DNAse NucH 110 A. hydrophila 

RNase RNaseA 24 A. hydrophila 

Porin carbohydrate reactive CROMP 40 A. hydrophila 

carbohydrate reactive CROMP 43 A. hydrophila 

maltoporin LamB 49 A. salmonicida 

porin 28 A. salmonicida 

porin OmpA1 36 A. salmonicida 

porin OmpA2 35 A. salmonicida 

protein I 47 A. hydrophila 

protein II 39 A. hydrophila 

Porin protein III 36 A. hydrophila 

protein IV 27 A. hydrophila 

Protease fibrinolytic protease 87.5 A. salmonicida 

metallo-caseinase AsaP1 20 A. salmonicida 

metallo-protease P2 20 A. salmonicida 

serine protease P1 70 A. salmonicida 

serine protease AspA 67 A. salmonicida 

zinc protease Ahp 19 A. hydrophila 

Siderophore receptor ferric siderophore FstA 86 A. salmonicida 

t

Superoxide dismutase iron 50 A. salmonicida 

manganese 46 A. salmonicida 

Xylanase xylanase I XynA 23 A. caviae

xylanase IV 41 A. caviae

Amended from Pemberton et al., 1997 

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Proteases 

The role of proteases as virulence factors has not been fully elucidated (Gonzalez-Serrano 

et al., 2002). Differences in protease elaboration have been attributed to differences in strain 

origin, incubation temperature, and culture media (Mateos et al., 1993). Proteases of A. 

hydrophila and A. veronii biovar sobria are broadly shown to consist of two distinct types, a 

thermostable metalloprotease (TSMP) that is sensitive to EDTA, and a thermolabile serine 

protease (TLSP) that is sensitive to phenylmethylsulphonyl fluoride (PMSF) (Leung and 

Stevenson 1988). Both types of proteases are produced by most strains, though a small number of 

strains produce only TSMP. Very few strains produce only TLSP. A. caviae protease has been 

investigated by Karunakaran and Devi (1995), who found evidence for production of a serine 

protease, while most work with A. caviae supports production of thermostable metalloproteases. 

Serine protease has been shown to activate proaerolysin (Howard and Buckley 1985; Abrami et 

al., 1998a) and glycerophospholipid-cholesterol acyl-transferase (GCAT) (Vipond et al., 1998); 

however, Gonzalez-Serrano et al. (2002) found no relationship between protease activity and 

expression of hemolysins. McMahon (2000) reported that proteases and hemolysins of A. 

hydrophila has lower activity when cells were grown at sub-optimal temperature. 

Elastase 

Elastase was described in aeromonads and pseudomonads in 1972 (Scharmann 1972), and 

Nord et al. (1975) evaluated elastase production for characterization of aeromonads and 

pseudomonads. Elastase production has been recommended as a phenotypic marker for 

characterization of Aeromonas spp. (Carnahan et al., 1991b). Hasan et al. (1992) developed a 

sensitive method for elastase detection using a bilayer plate assay. Recently, elastase has attracted 

interest as a potential virulence factor (Cascon et al., 2000a; 2000b). Kawakami et al. (2000) 

cloned the apk gene encoding the extracellular metallo-protease of A. caviae which exhibits 

elastase activity, and Keller et al. (2004) demonstrated that a metallo-protease of A. hydrophila 

possessed elastase activity. The role of elastase in pathogenesis has not been fully explored. 

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Lipases 

Lipase genes such as plc, lipH3, pla and lip have been identified as potential virulence 

factors, and mutations in these genes reduce lethal effects in mice and fish (Merino et al., 1999). 

Nucleases 

Nucleases have been described, but their role in pathogenesis has not been verified 

(Pemberton et al., 1997). 

Glycerophospholipid Cholesterol Acyl-Transferase (GCAT)

 A. salmonicida produces glycerophospholipid cholesterol acyl-transferase (GCAT) as a 

major virulence factor resulting in furunculosis in fish. A. hydrophila secretes a similar enzyme 

that hydrolyzes both neutral and phospholipids, however the role of GCAT in human 

pathogenicity is unknown (Brumlik and Buckley 1996). GCAT has lipase or phospholipase 

activity and could cause erythrocyte lysis by digesting their plasma membrane (Pemberton et al., 

1997). 

 Superoxide dismutase 

Leclere et al. (2004) found that the manganese superoxide dismutase of A. hydrophila has 

the equivalent function as the copper/zinc (Cu/Zn) superoxide dismutase of E. coli. It is located in 

the periplasmic space and may confer a virulence advantage on the bacteria. 

Toxins 

β-hemolysin 

Two hemolytic toxins have been described in aeromonads, hemolysin encoded by the hlyA 

gene and aerolysin controlled by the aerA gene (Heuzenroeder et al., 1999). AerA is a channel 

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forming β-hemolysin promoting channel formation leading to cell death. HylA is a non-channel 

forming hemolysin similar to the V. cholerae hemolysin. 

Epple et al. (2004) investigated the effect of aeromonads and their secretory products on 

ion secretion and barrier function of human intestinal cells HT-29/B6. Chloride ion secretion 

activity followed exposure of cells with an aerolysin-like β-hemolysin, probably as the result of 

channel insertion into the apical membrane and activation of protein kinase C. Xia et al. (2004) 

cloned the β-hemolysin gene from A. hydrophila isolated from freshwater fish in China. The 

cloned β-hemolysin sequences were used in a PCR assay to survey environmental isolates to 

detect potential pathogenic A. hydrophila strains. Such screens have economic importance in 

aquaculture. 

Cytotoxic Toxins 

The major hemolysin produced by aeromonads is called aerolysin, though it is know by 

several other names (cytotoxic enterotoxin, Asao toxin, and cholera toxin cross-reactive cytolytic 

enterotoxin). Aerolysin is produced by some strains of A. hydrophila, A. veronii biovar sobria and 

A. caviae (Singh and Sanyal 1992). The aerA gene that encodes the structural protein is present in 

all Aeromonas strains and the conditions that result in expression are being elucidated. Singh and 

Sanyal (1992) reported that environmental strains could be induced to produce aerolysin with 

repeated passage (1-3 times) in rabbit ileal loops. The implication is that all environmental strains 

are potentially enterotoxigenic when passed from host to host, but environmentally adapted 

strains are not pathogenic when acquired directly from the environment. Toxin is expressed in the 

latter stages of logarithmic growth. Production of aerolysin in A. caviae is enhanced by iron 

deficiency and repressed by glucose (Singh and Sanyal 1992). 

Aerolysin is a 50-52 kDa heat labile protein that causes fluid accumulation in rabbit ileal 

loops and lyses a wide range of cells including CHO and rabbit erythrocytes (Chopra et al., 1993). 

It belongs to a class of pore-forming cytotoxins that disrupt cell membranes and is highly lethal 

for rats and mice. Aerolysin from A. hydrophila and A. veronii biovar sobria share properties with 

the cardiotoxic thermostable hemolysin of V. parahaemolyticus. The structure has been 

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determined by X-ray crystalography and the monomer protein is divided into four domains 

(Parker et al., 1994). Antibodies specific for cholera toxin react with aerolysin, but there is no 

sequence homology and cholera antisera do not neutralize aerolysin activity. Aerolysin crosses 

the bacterial cell membrane as an inactive proaerolysin, which binds to the receptor glycophorin 

on the membrane of erythrocytes. The toxin is then activated by proteolysis, either by bacterial or 

host proteases, and forms a transmembrane channel. The mechanism of action has been elucidated 

using erythrocytes and little is know about the toxins effect on intestinal cells, though antisera to 

aerolysin neutralized the toxin effects in rabbit ileal loops (Ferguson et al. 1997). The role of 

aerolysin in gastroenteritis is controversial. 

Cytotonic toxins 

At least four non-cytoxic but cytotonic factors have been described in Aeromonas spp. 

(Gosling et al., 1993). They elongate CHO cells, round Y1 adrenal cells and increase the 

intracellular levels of cAMP (McCardell et al., 1995). A 15 kDa heat labile protein from A. 

hydrophila causes fluid accumulation in rabbit ileal loops and increases the intracellular 

concentration of cAMP in Y-1 adrenal cells. A toxin of similar size and characteristics has been 

purified from A. veronii biovar sobria (Gosling et al., 1993). A 44 kDa toxin was isolated from A. 

hydrophila by Chopra et al. (1993) with similar properties, and the ability to cause an increase in 

prostaglandin E2 (PGE2) levels in CHO cells. Another 70 kDa toxin has been described in A. 

hydrophila that caused elongation of CHO cells but does not increase intracellular levels of PGE2 

or cAMP. None of these toxins cross-react with antisera specific for cholera toxin. A heat-stable 

toxin that cross-reacts with cholera toxin has been reported in A. hydrophila, A. veronii biovar 

sobria, and A. caviae. Several cytotonic enterotoxins have been cloned, including a heat-stable 

and a heat-labile cAMP-stimulating toxin from A. hydrophila (Chopra et al., 1993). Further 

characterization of heat-labile and heat-stable enterotoxins of aeromonads was published by 

Chopra et al. (1996) and Sha et al. (2002). Chakraborty et al. (1984) cloned a heat-stable toxin of 

undetermined size from A. hydrophila. The relationship between these cloned products is not 

known. 

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Few A. caviae strains have been reported to produce cytotonic enterotoxins (Namdari and 

Bottone 1990a). The mechanism of pathogenicity is obscure, though cholera toxin has been 

shown to cross-react with some variants of aerolysin (Honda et al., 1985). The role of A. caviae as 

a cause of gastroenteritis is becoming established as investigators have correlated non-cytotonic 

enterotoxin producing strains of A. caviae with gastroenteritis. Aeromonads may represent a 

paradox. Although they may be capable of producing enterotoxins, the enterotoxins may play 

little role in pathogenicity of some strains (Wilcox et al., 1994). 

Aerolysin 

Fivaz et al. (2001a) reviewed aerolysin of A. hydrophila. Aerolysin was identified by 

Bernheimer et al. (1975) and later purified by Buckley et al. (1981). Chakraborty et al. (1987b) 

confirmed that aerolysin was a characteristic virulence factor of Aeromonas spp. Aerolysin is 

secreted as a 52 kDa protein called proaerolysin because it is inactive until it is proteolytically 

activated. The active toxin binds to the target host cell and oligomerizes into a ring-like structure 

that is able to insert into the cell membrane and form aqueous trans-membrane pores. Membrane 

perforation triggers a number of intracellular events leading to cell death. 

Aerolysin, a pore-forming toxin of Aeromonas spp., is a known virulence factor (Krause et 

al., 1998). The toxin is secreted in an inactive precursor form (Parker et al., 1996) which becomes 

active with cleavage of a C-terminal peptide. The toxin is thought to bind to specific receptors 

located on host target cells (Nelson et al., 1997; Abrami et al., 1998b). Binding is thought to 

concentrate the toxin and facilitate polymerization into heptameric complexes that penetrate the 

cell membrane and form water-filled channels leading to cell lysis (Abrami et al., 1998b; Krause 

et al., 1998; Rossjohn et al., 1998). 

Krause et al. (1998) reported that aerolysin-induced chemotaxis and release of calcium 

ions (Ca2+) from intracellular stores was mediated via the activation of a toxin-sensitive Gprotein. Proaerolysin and aerolysin were found to increase cytosolic Ca2+ in a dose-dependent 

manner, which suggested that protoxin and toxin bind to receptors on the target cell which 

express proteases that convert the protoxin to active toxin, that is then inserted through the host 

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cell membrane through the heptamerization pores. Abrami et al. (2000a; 2000b) showed that 

heptimer formation occurred following attachment of GPI-anchoring proteins to permit aerolysin 

to bond to its receptor. These pores modify potassium ion (K+

) and sodium ion (Na+) gradients, 

membrane potential, and cytosolic Ca2+ levels. Abrami et al. (2003) studied the interaction of 

aerolysin and GPI-anchoring proteins to form pores in the plasma membrane of host cells. Fujii et 

al. (2003) studied the mechanism of action of aerolysin-like hemolysins of A. sobria for 

stimulating T84 cells to product cAMP. Song et al. (2004) showed that aerolysin is activated by a 

metalloprotease in A. veronii biovar sobria. 

Barry et al. (2001) found that proaerolysin, the pro-form of the channel-forming protein 

aerolysin, is secreted as a dimer. There is controversy about whether the protein is active in its 

dimer or monomer form, as suggested by Fivaz et al. (1999). Barry et al. (2001) showed by 

several means that the protein remains in dimeric form at low concentration of the protein and 

that the dimeric protoxin is capable of receptor binding. 

Diep et al. (1999) reported that aerolysin contains two receptor-binding sites. One is 

located on the large lobe for glycosylphosphatidylinositol-anchored proteins; and the other is on a 

site in the small lobe that binds surface carbohydrate determinants. The different binding 

properties of Act compared with aerolysins of some strains suggests that separate proteins are 

involved and several isomers of aerolysin may exist. 

Fivaz et al. (2001b) concluded that the mechanism of cell death is far more complicated 

that simple disruption of the cytoplasmic membrane with resulting cell lysis. Laohachai et al. 

(2003) reviewed the role of bacterial toxins that induce changes in membrane transport leading to 

diarrheal disease. While progress is being made in understanding the mechanism of aerolysin 

activity, the actual mechanism is complex and incompletely understood. 

Entertoxins 

Aeromonas spp. may produce a variety of virulence factors including enterotoxin 

(Krovacek et al., 1991; Majeed and MacRae 1991). Three cytotoxic and cytotonic enterotoxins 

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have been described in Aeromonas spp., Aeromonas cytotoxic enterotoxin (Act), Aeromonas heatlabile (56o C) cytotonic enterotoxin (Alt), and Aeromonas heat-stable cytotonic enterotoxin (Ast). 

Ast and Alt are not related to cholera toxin (Chopra and Houston 1999; Chopra et al. 2000). 

Asao et al. (1984) purified a hemolysin from A. hydrophila with cytotoxic and enterotoxin 

activities. Rose et al. (1989) and Ferguson et al. (1997) purified a 52 kDa single peptide chain 

with cytotoxic properties that is now recognized as Act. 

Act is a single-chain polypeptide 52 kDa in length and is related to aerolysin, with 

hemolytic, cytotoxic and enterotoxic activity. Act activity has been demonstrated in LD50 studies 

and by the inability of Act isogenic mutants to induce a fluid-secretory response in mouse ligated 

ileal loops (Xu et al., 1998). 

Act is a cytotoxic enterotoxin that has hemolytic, cytotoxin, and enterotoxic properties. 

Act causes fluid secretion and tissue damage in mouse ileal loops. Act has no homology with CT 

coding gene ctx, however it cross-reacts with antibody to CT. Potomski et al. (1987) isolated a 

similar 63 kDa toxin from A. sobria, but it did not cross-react with CT. The aerolysin gene from 

A. trota and A bestiarum (originally identified as A. hydrophila), was cloned, and these gene 

products are related to the hemolysin of Asao (Chakraborty et al., 1986; Howard and Buckley 

1986). Sha et al. (2004) showed that the gidA gene regulates act production. Presence of the gidA 

gene markedly increased the median lethal dose (LD50) for mice inoculated by interperitoneal 

injection. Addition of iron or glucose repressed gene expression. 

The act gene is known to stimulate proinflammatory cytokine and eicosanoid cascades in 

macrophages in the rat intestinal epithelial cell line ICE-6, leading to tissue damage and fluid 

secretory response (Chopra et al., 2000). There is good correlation between the cytotonic 

enterotoxins Alt and Ast and elongation of Chinese hamster ovary cells and production of c-AMP, 

which is typical enterotoxic activity (Chopra et al., 1994). Previous data show that Act is the 

major enterotoxin contributing to fluid secretory response, followed by alt and ast in A. 

hydrophila (Sha et al., 2002). Sinha et al., 2004 found that the majority of the A. veronii biovar 

sobria strains contained act. Presence of all three genes is rare (Albert et al., 2000; Sinha et al., 

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2004). The presence of these three enterotoxin genes in various combinations may increase or 

decrease expression of specific enterotoxin genes and mediate the severity of diarrhea (Sha et al., 

2002). 

Act has multiple biological functions such as lysis of red blood cells, and destruction of 

cell culture lines, which evokes a fluid secretory response in ligated intestinal loop models and 

lethality for mice. Chopra et al. (2000) demonstrated that Act increased cAMP production in 

macrophages along with PGE2 and promoted fluid secretion in animal models. Act upregulates 

the production of proinflammatory cytokines and antiapoptotic protein Bcl-2 and activates 

arachidonic acid (AA) metabolism in macrophages and monocytes. 

Alt is a cytotoxic enterotoxin which is heat labile at 56o C. It consists of a single 44 kDa 

polypeptide chain that is unrelated to cholera toxin (Albert et al., 2000). Alt elevates cyclic AMP 

and prostaglandin levels in Chinese hamster ovary and intestinal epithelial cells and caused fluid 

accumulation in rat ileal loops. 

Ast is a cytotoxic enterotoxin consisting of a single 44 kDa polypeptide chain that is heat 

stable at 56o C. Chopra et al. (1994) identified ast gene products of 32 kDa and 67 kDa which 

evoked fluid secretion in rat small intestine and cAMP levels in mucosal cells. McCardell et al. 

(1995) purified an Ast-like toxin, is not related to cholera toxin, and does not increase cAMP, 

CGMP or PGE2 levels in CHO cells, but it does cause fluid accumulation in infant mice. 

Falcon et al. (2001) reported that cytotoxic enterotoxin of A. hydrophila induced apoptosis 

in HT29 human intestinal cells. Toxin exposure resulted in cell detachment, cytoplasmic 

disorganization, and blebbing. The cytotonic enterotoxins Ast and Alt cause fluid accumulation in 

ligated ileal loops in animal models and probably have an undescribed role in causing diarrhea in 

humans (Sha et al., 2002). Galindo et al. (2004b) described apoptosis as an effect of cytotoxic 

enterotoxin activity in murine macrophages and human epithelial cells 

Chopra and Houston (1999) reviewed enterotoxins of Aeromonas spp. associated with 

production of gastrointestinal disease. Since publication of this review, TagA, a lipoprotein 

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controlled by the toxR gene, has been identified in Aeromonas. This gene is also found in E.coli 

O157:H7 were it mediates serum resistance. The regulation of genes associated with cytotoxic 

enterotoxins is under continuing investigation (Sha et al., 2002). 

4.1.1.3 Type II Secretion Systems

Aerolysin is released from A. hydrophila and A. salmonicida by a Type II secretion system 

as a soluble precursor referred to as pro-aerolysin (Abrami et al., 2000b; Burr et al., 2001). It 

undergoes a monomer to dimer transition at high concentrations. Aerolysin shares high sequence 

similarity with the alpha toxin of Clostridium septicum and has a similar mode of action. 

Proaerolysin binds to GPI-anchored proteins expressed on cell surfaces. Heptemerization results 

in a hole in the plasma membrane of receptor cells with disruption of membrane polarization and 

chloride ion leakage. Transit across the outer membrane may be rate limiting and there is 

evidence for periplasmic accumulation of the protein. Secrete of toxins via the Type II pathway is 

under the control of at least two genes, exeAB (gspAB) and exeC-N (gspC-N) (Ast et al., 2002). 

4.1.1.4 Type III Secretion Systems

Burr et al. (2002) found genes in A. salmonicida which are associated with Type III 

secretion systems in other bacteria, suggesting that Type III secretion systems contribute to 

virulence of aeromonads. Burr et al. (2003) presented evidence that A. salmonicida exoenzyme T 

(AexT) is translocated via a Type III secretion system. AexT plays a role in cytotoxicity to fish 

cells. 

Chacon et al. (2004) identified genes in 50% of clinical strains of Aeromonas that encode 

for a putative Type III secretion system. The genes were found more frequently in A. hydrophila 

and A. veronii strains than in A. caviae. Type III secretion systems play an essential role in 

pathogenicity as they facilitate the delivery of toxins directly into host cells. The presence of Type 

III secretion systems suggests that pathogenicity islands may be present in Aeromonas spp. A 

cagA-like gene of H. pylori was found in Aeromonas spp. isolated from water samples in India 

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(Datta et al., 2003). Nucleotide sequencing demonstrated 97-98% relatedness, lending support for 

the presence of pathogenicity islands in Aeromonas spp. 

Yu et al. (2004) located the gene cluster responsible for a Type III secretion system and 

presented evidence that it is required for A. hydrophila pathogenesis. Several proteins associated 

with Type III secretion systems in aeromonds have homologs in Pseudomonas spp. and Yersinia 

spp. Characterization of these proteins has value in development of vaccines and for enhanced 

understanding of the pathogenesis of disease caused by aeromonads. 

Sha et al. (2005) used isogenic mutants to demonstate reduced Act production and 

associated virulence for mice. Production of DNA adenine methyltransferase encoded by the 

damAhSSU gene is necessary for viability and over-production of Dam is associated with reduced 

virulence for mice (Erova et al. 2006). These recent studies on the relationships between Type III 

secretion systems, cytotoxic enterotoxin and Dam are providing new information on the complex 

inter-relationship among virulence factors of Aeromonas spp. 

4.1.1.5 Siderophores

During infection, microbes must acquire iron from the host, and this is accomplished by 

production of siderophores; thus, siderophores are considered to be virulence factors. 

Siderophores are iron-specific ligands of low molecular mass. The ferric siderophore gene fstA of 

A. salmonicida has significant sequence similarity with the fstA gene of several known pathogens 

(Pemberton et al., 1997). 

Motile aeromonads produce either of the phenolate siderophores, enterobactin or 

amonabactin. Amonabactin is unique to Aeromonas spp.; whereas, enterobactin is produced by 

many enteric bacteria. A. hydrophila expresses primarily amonabactin; while A. veronii biovar 

sobria expresses mostly enterobactin. Amonabactin is thought to drive an iron-delivery system 

that is able to trap Fe-transferrin in serum. Enterobactin is believed to be inhibited by albumin so 

it is inactive in serum. Most isolates have a siderophore-independent chelation system that can 

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acquire iron from haem-containing molecules. Both amonabactin and enterobactin genes have 

been cloned and they are genetically distinct from those of E. coli (Byers et al., 1991). 

4.1.1.6 Quorum Sensing

Quorum sensing is a mechanism for controlling gene expression in response to an 

expanding bacterial population. It has been reported in Aeromonas spp., and it is a subject of 

intensive investigation in many bacteria (Swift et al., 1997). The quorum sensing signal generator 

and response regulator were cloned from A. hydrophila and A. salmonicida and termed ahyRI and 

asaRI, respectively. The iciA gene is downstream and a similar gene in E. coli is known to inhibit 

chromosome replication, which suggests that cell division in Aeromonas is linked to quorum 

sensing. Expression of the LuxR protein of Aeromonas may function to regulate production of 

virulence factors in addition to regulate cell division as a manifestation of quorum sensing. 

Sha et al. (2005) observed that lactone production was decreased in act and aopB mutants 

of A. hydrophila, suggesting a relationship between reduction in virulence and a reduction in 

quorum sensing ability. The role of quorum sensing in regulation of Type III secretion systems is 

under current investigation. 

4.1.1.7 Opsonins

Merino et al. (2001b) characterized genes responsible for opsonophagocytosis resistance 

in A. hydrophila O:34. Expression of a filamentous phenotype is associated with cell aggregation 

and it is hypothesized that filaments may interfere with phagocytosis directly or by encouraging 

cell aggregation. 

4.1.1.8 Glycosylation

Power and Jennings (2003) reviewed the genetics of glycosylation in gram-negative 

bacteria, including A. caviae, where the virulence role of glycosylation of proteins associated with 

virulence such as pili, adhesins, and flagella is unresolved. 

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4.1.1.9 Suicide Phenomenon

 A. caviae, and some strains of A. hydrophila and A. veronii biovar sobria, grown in broth 

cultures in media containing 0.5% glucose are known to undergo auto-sterilization after 24 hr. 

This effect has been referred to as the “suicide phenomenon” (Namdari and Cabelli 1989). Studies 

have shown that acetate accumulation rapidly lowers pH resulting in cell death. Ballal et al. 

(2001) observed that non-suicidal strains were lethal for mice while suicidal strains were not; 

thus, suggesting a relationship between the suicide phenomenon and virulence. 

4.1.1.10 Vacuolation

Guimaraes et al. (2002) reported that a strain of A. hydrophila isolated from a patient with 

diarrhea did not produce cytotonic enterotoxin in mice and only small quantities of aerolysin; 

however, it produced vacuolating activity in Caco-2 cells. Other effects on cells besides 

enterotoxins, such as vacuolation, may be operative in mild infections. 

4.1.1.11 Pathogenicity Islands

 A cagA-like gene of H. pylori was found in Aeromonas spp. isolated from water samples 

in India (Datta et al., 2003). Nucleotide sequencing demonstrated 97-98% relatedness. The gene 

was unstable and tended to be lost with repeated subculture in the laboratory. The mechanism of 

acquiring this gene and the reason for its instability on subculture is unknown. 

The distribution of some of the recognized virulence genes in Aeromonas spp. is shown in 

Exhibit 4.3. 

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74 

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Exhibit 4.3. Distribution of Virulence Genes in Aeromonas spp. Isolated from Patients with 

Watery Diarrhea 

Toxin gene (%)

Species No. Strains 

alt act ast hlyA aer 

A. caviae 72 (97.3) 1 (1.4) 1 (1.4) 3 (4.1) 3 (4.1)

25 12 (48.0) 21 (84.0) 2 (8.0) 6 (24.0) 1 (4.0)) 

biovar 

sobria 

A. veronii 

A. 35 31(88.6) 14 (40.0) 0 31 (88.6) 10 (28.6) 

hydrophila 

A. veronii 9 7 (77.7) 7 (77.7) 0 0 0 

biovar 

veronii 

A. trota 10 5 (50.0) 0 1 (10.0) 1 (10.0) 0 

A. schubertii 4 3 (75.0) 2 (50.0) 0 1 (25.0) 0 

A. jandaei 1 1 (100%) 0 0 0 0 

Unidentified 6 5 (83.3) 3 (50.0) 0 4 (66.7) 0 

From Sinha et al., 2004 

4.1.2 Gastrointestinal Infection 

The role of aeromonads as causes of diarrheal disease is problematic and frustrating. 

While some stains of Aeromonas undoubtedly cause diarrhea, the ability to unequivocally 

demonstrate cause and effects continues to elude investigators, who must rely upon clinical and 

epidemiological associations rather than conclusive evidence. This dilemma results from the 

inability to find an animal model system that replicates the pathogenesis of gastroenteritis in 

humans. 

While the production of extra-intestinal disease in humans is incontrovertible, the role of 

mesophilic Aeromonas spp. as agents causing gastroenteritis is controversial. Much of the 

controversy results from the inconclusive human volunteer feeding study of Morgan et al. (1985), 

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where ingestion of Aeromonas strains in concentrations as high as 9 log10 CFU/mL failed to 

produce disease. Because no animal model has been identified to fulfill Koch’s postulates for 

gastrointestinal disease in humans, the role of aeromonads as agents of gastroenteritis has been 

extrapolated from anecdotal case reports, case-control studies, and a handful of outbreaks 

epidemiologically associated with food or water ingestion (Kirov 2001). 

Aeromonads produce multiple virulence factors typically associated with gastrointestinal 

disease in other bacteria, but the direct relationship between most of these virulence factors and 

gastrointestinal disease has not been proven. No epidemiological studies have indisputably linked 

aeromonads with outbreaks of diarrheal disease. The strongest evidence to support the thesis that 

some strains of Aeromonas cause diarrhea is found in individual case reports (Moyer and Larew 

1988; Bloom and Bottone 1990). 

Though the role of aeromonads as agents of gastroenteritis remains controversial, several 

microbiological, epidemiological, and clinical/immunological investigations indicate that some 

strains of Aeromonas are enteric pathogens (Altwegg and Geiss 1989; Joseph, 1996). 

Gastroenteritis has been linked with but not necessarily caused by Aeromonas spp. worldwide 

(Altwegg and Geiss 1989). The association is strongest in children under the age of 2 years, adults 

over 50 years of age, and the immunocompromised (Agger 1986; Moyer 1987; San Joaquin and 

Pickett 1988). A summer peak for isolation of aeromonads from stools corresponds with their 

increased presence in the environment (Burke et al., 1984a; Agger et al., 1985; Nishikawa and 

Kishi 1988). While strains possess virulence properties such as the ability to produce enterotoxin, 

cytoxtoxin, hemolysins, adhesins, invasins, and an array of hydrolytic enzymes, not all strains 

possessing these properties cause disease in humans, and the host factors predisposing to 

colonization and disease are unknown (Kirov 1993b). While A. hydrophila and A. veronii biovar 

sobria are generally recognized as agents of gastroenteritis, the causal role of A. caviae was 

considered controversial despite the fact that a causal role was first proposed by Fritsch et al. 

(1975) thirty years ago. Today, most investigators acknowledge A. caviae as the cause of 

gastrointestinal disease (Rautelin et al., 1995). Aeromonas spp. have been established as an 

enteric pathogen (Albert et al., 2000), but the mechanisms of pathogenicity remain elusive. A. 

caviae adheres to mucosal epithelial cells (Krzyminska et al., 2001). A. hydrophila and A. sobria 

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produce a number of virulence factors that result in diarrhea, including enterotoxins, cytotoxins, 

hemolysins, aerolysins, proteases and hemagglutinatins (Albert et al., 2000). 

Aeromonads may be present in the gastrointestinal tract of humans, and most 

epidemiological studies show higher numbers in stools of patients with gastroenteritis than in 

asymptomatic individuals. Acute self-limiting diarrhea occurs in children, and chronic 

gastroenteritis or enterocolitis may occur in children and the elderly. The presentation of 

gastroenteritis caused by aeromonads includes various combinations of fever, vomiting, and 

increased fecal leucocytes or erythrocytes (Janda and Abbott 1996). 

Exhibit 4.4 lists representative case reports where Aeromonas spp. are strongly associated 

with gastrointestinal disease. 

Exhibit 4.4. Documented Cases of Gastroenteritis Caused by Aeromonas 

Patient Symptoms Serological Titer Biopsy Organism 

(age/gender) 

10/F typhoid fever yes not done A. hydrophila 

67/F cholera yes not done A. hydrophila 

35/M dysentery not done yes A. hydrophila 

67/F cholera yes not done A. veronii biovar sobria 

67/M enteritis yes (convalescent only) not done A. hydrophila 

30/F dysentery yes (convalescent only) not done A. media 

80/M enteritis yes (convalescent only) not done A. veronii

24/F ulcerative colitis yes (acute only) yes A. veronii

 Janda and Abbott 1998 

The few epidemiological studies that link Aeromonas spp. to gastroenteritis are shown in 

Exhibit 4.5. 

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Exhibit 4.5. Cases of Gastroenteritis Epidemiologically Linked to Aeromonas 

Patient 

(age/gender) 

Symptoms Typing 

Method 

Source Organism Reference 

3 mo./M chronic 

diarrhea 

ribotyping drinking water, 

rice cereal 

A. caviae, A. 

veronia biovar 

sobria 

Moyer and 

Larew 1988 

38 yr/M gastroenteritis ribotyping shrimp A. hydrophila Altwegg et al., 

1991a 

2 yr/M gastroenteritis ribotyping well water A. caviae Moyer 2002 

Khan and Cerniglia (1997) consider Aeromonas species to be enteric pathogens. Certain 

strains of aeromonads ingested at a high inoculum levels (probably > 8 log10 CFU) may produce 

diarrheal disease in susceptible hosts (Janda and Abbott 1998). Buchanan (1984) suggested that 

aeromonads may be associated with up to 13% of gastroenteritis cases in the U.S. O:11 and O:34 

serotypes are common in gastroenteritis (Thomas et al., 1990; Kokka et al., 1991b, Merino et al., 

1993a; Merino et al., 1993b). According to Kirov (2001), the majority of aeromonads associated 

with gastroenteritis are A. veronii biovar sobria (HG-8/10), A. hydrophila (HG-1), and A. caviae 

(HG-4), though A. veronii biovar veronii (HG-8/10), A. trota (HG-13), and A. jandaei (HG-9) 

occur occasionally. Gastroenteritis attributed to A. sobria was characterized by acute watery 

diarrhea, vomiting abdominal pain, and fever (Taher et al., 2000). A fatal case of gastroenteritis 

resulted from eating contaminated mozzarella cheese (Montagna et al., 1998). Goldsweig and 

Pacheco (2001) reviewed infection colitis caused by Aeromonas spp. Picard et al. (1984) 

suggested the intestinal tract as the source of invasive infections, but Outin et al. (1984) observed 

that the source of gastrointestinal infection with Aeromonas is rarely proven. 

While some investigators refuse to accept aeromonads as a cause of diarrheal disease, the 

available evidence supports the conclusion that some strains do cause either acute or chronic 

gastrointestinal illness in susceptible hosts. 

4.1.3 Skin and Soft Tissue Infection 

Skin and soft tissue infection caused by Aeromonas spp. resulting in cellulitis and 

bacteremia were reviewed by Gold and Salit (1993). Skin and soft tissue infections may follow 

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traumatic injury in environments where soil and water may contaminate the wound. Voss et al. 

(1992) reported that 82% of Aeromonas wound infections resulted from penetrating injury and 

43% of these infections resulted from environmental exposures to Aeromonas spp. Swimming, 

diving, boating, and fishing are all aquatic recreational activities placing persons at risk of 

Aeromonas infection. A review of 32 foot injuries revealed that at least one-third of cases resulted 

from introduction of aeromonads in soil-contaminated glass, nails, or sticks (Wakabongo 1995). 

Infections from severe trauma associated with automobile accidents or other accidents resulting in 

crushing injury, compound fractures, or severe burns may lead to osteomylitis, myonecrosis, or 

gangrene. Campbell (2001) reviewed infectious complications of lawn mower injuries and 

identified Aeromonas spp. as a cause of resulting wound infections, and Larka et al. (2003) 

observed that Aeromonas infection following foot trauma may not always be associated with an 

aquatic environment. 

Necrotizing fasciitis is a rapidly advancing form of cellulitis characterized by muscle 

necrosis. Minnaganti et al. (2000) reported necrotizing fasciitis caused by A. hydrophila in 

patients with suppressed immune systems, burns, and trauma in aquatic settings. The case of an 

85-year old man with no history of trauma suggests that sepsis from an intestinal source resulted 

in soft tissue infection and subsequent necrotizing fasciitis. Angel et al. (2002) reported a case of 

necrotizing fasciitis caused by A. hydrophila following a water moccasin bite. Cheng et al., 

(2004) reported a fatal case of nosocomial necrotizing fasciitis caused by A. hydrophila in a 44­

year old man who underwent valve replacement surgery. 

Bite wounds are associated with Aeromonas infection, but it is not known whether the 

organism originated from the oral flora of the animal, or was introduced into the puncture wound 

from the environment. Wound infections have been reported following bear, alligators (Raynor et 

al., 1983), piranha (Revord et al., 1988), and snake (Angel et al., 2002) bites. 

Burn infections caused by aeromonads are rare events – only 29 cases have been reported 

in English language literature (Kienzle et al., 2000). These authors reported 5 cases of A. 

hydrophila or A. caviae infections from burns associated with explosions (4 of 5) and a campfire 

accident (1 of 5). In 4 of 5 cases, water was used to quench the fire or as a first aid treatment. 

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Wilcox et al. (2000) reported A. hydrophila infection in 73-year old victim of an electrical burn 

on the hand. 

Trauma is most closely associated with Aeromonas wound infections. While the typical 

presentation in persons with normal immune status is cellulitis, more serious infections and sepsis 

may occur as shown in Exhibit 4.6. Patients developing myonecrosis have a mortality rate 

exceeding 90%. 

Exhibit 4.6. Circumstances Promoting Aeromonas Wound Infections 

Injury Event Location Source Immune Presentation 

Status 

laceration, water sports, community water, soil normal cellulitis, 

abrasion seafood abscess, 

shucking ulceration, 

septicemia 

puncture occupational community soil, water normal cellulitis, 

exposure abscess, 

ulceration 

crush motor vehicle community soil, water normal pyomyositis, 

accidents myonecrosis, 

osteomyelitis, 

gangrene 

burn occupational community water, soil normal cellulitis, 

septicemia 

invasive intraabdominal nosocomial feces, fomites impaired purulent 

medical surgery, exudate, 

procedure catheterization abscess 

Modified from Janda and Abbott 1998 

The significance of aeromonads as a cause of skin and soft-tissue infections was made 

abundantly clear as a result of the tsunami that devastate Southeast Asia in December 2004 

(Hiransuthikul et al. 2005; Lim 2005). Among 777 patients hospitalized for injuries suffered as a 

result of the tsunami, 515 had skin and soft-tissue infections and 145 isolates for 305 patients 

were either A. hydrophila or A. veronii biovar sobria. Many of these infections developed because 

traumatic wounds were not cleaned properly or because of delay in obtaining medical care. 

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4.1.4 Respiratory Tract Infections 

Pneumonia and lung abscess in adults has been reported (Goncalves et al., 1992). 

Pneumonia may also occur in children (Sirinavin et al., 1984), and Kao et al. (2003) reported a 

case of bacteremic pneumonia caused by A. hydrophila in a previously healthy 5-year old child. 

Predisposing conditions were present in 11 of 15 (73%) of cases. Reported predisposing 

conditions include alcohol abuse (20%), neurologic disease (20%), cardiovascular disease (27%), 

chronic renal failure (7%), chronic obstructive lung disease (20%), traffic accidents (7%), and 

malignancy (7%). Murata et al., 2001 reported fulminant pneumonia caused by A. hydrophila in a 

patient undergoing hemodialysis with chronic renal failure and cirrhosis. The source of infection 

was not determined. 

Respiratory infections occur in the immunocompetent persons who involuntarily aspirate 

surface water while swimming or as the result of an accident (Mukhopadhyay et al., 2003). 

Miyake et al. (2000) reported Aeromonas pneumonia from near-drowning experiences. 

Respiratory infections may also occur in persons with underlying diseases placing them at risk for 

Aeromonas bacteremia originating from an intestinal source as shown in Exhibit 4.7. 

Exhibit 4.7. Respiratory Tract Infections Caused by Aeromonas 

Patient 

(age/gender) 

Underlying 

Disease 

Infection Specimen Source 

38/? None Lung abscess Bronchial lavage Fresh water 

24/M None pulmonary edema, 

ARDS 

Endotracheal 

aspirate, blood 

Water 

41/F Chronic renal failure Parapharyngeal softtissue infection 

oral secreations, 

blood 

Denture cup 

water 

24/M None Pneumonia blood, lung Seawater 

8/F β-thalessemia Epiglottitis Epiglottis Unknown 

59/M None Lung abscess Sputum, bronchial 

wash 

Unknown 

69/M Diabetes melitus, 

cirrhosis, ethanol 

Pneumonia, sepsis Blood, Bronchial 

wash 

GI tract 

43/M Ethanol abuse, 

cocaine abuse 

Pneumonia Blood, sputum River water 

50/M Cirrhosis Empyema Blood, plural fluid GI tract 

From Janda and Abbott 1998 

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 Isolation of Aeromonas spp from respiratory specimens must be interpreted together with 

clinical findings, since the upper respiratory and nasopharyngeal tracts may be transitorily 

colonized. 

4.1.5 Meningitis 

Meningitis is a rare complication of extraintestinal infections with Aeromonas spp. Of 

eight cases reviewed by Parras et al. (1993), four cases occurred in children and two cases had 

underlying hematologic disorders (sickle cell anemia and β-thalessemia). 

4.1.6 Endocarditis 

Brouqui and Raoult (2001) reviewed endocarditis and found only two cases caused by 

Aeromonas, both in patients with cirrhosis. 

4.1.7 Osteomyelitis 

Osteomyelitis has been reported following compound fractures or crushing trauma where 

wounds were contaminated by soil or water (Janda and Abbott 1998). 

4.1.8 Hepatobiliary Infections 

Liver disease is a recognized predisposing factor leading to Aeromonas infection resulting 

from septicemia (Clark and Chenoweth 2003). Underlying hepatitis B infection and cirrhosis are 

predisposing factors for liver disease. Liver abscess, supperative cholangitis, and empyema may 

occur following septicemia in patients with underlying hepatobiliary disease. A. veronii biovar 

sobria and A. veronii biovar veronii are typically involved in these infections; while A. caviae has 

been reported as the cause of cholecystitis (Kumar et al., 2000a). 

Chan et al. (2000) reviewed 30 cases of acute suppurative chlolangitis caused by 

Aeromonas spp. Septicemia occurred in 4 of 30 patients with Aeromonas infection. Most patients 

(83%) had bile duct stones, 13% had chloangiocarcinoma and 7% had pancreatic cancer. Previous 

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instrumentation, e.g. catheterization, or cystoscopy, predisposed patients (83%) to Aeromonas 

infection. 

4.1.9 Bacteremia/Septicemia 

Bacteremia resulting from Aeromonas infection was reviewed by Ko and Chuang (1995), 

and Ko et al. (2000). A. hydrophila sepsis is associated with gastrointestinal disease, liver 

cirrhosis, diabetes, malignancy, pancreatitis, trauma, cardiac anomalies, and respiratory disease. 

Sepsis is accompanied by fever, hypotension, jaundice, and chills, and complications of 

Aeromonas infection may include intravascular coagulation, purpura fulminans, and ecthyma 

gangrenosum (Ko et al., 2000). Disseminated infection progresses rapidly and has a high fatality 

rate (Shiina et al., 2004). 

Llopis et al. (2004) reviewed Aeromonas bacteremia in 75 elderly patients to determine 

the clinical and epidemiological characteristics. Bacteremias were mostly in males (72%) 

suffering from chronic liver disease (36%) or neoplasm (33%). The origin of infection was from 

the abdomen in 52% of cases. Nosocomial infections constituted 29% cases, and 48% of cases 

occurred in the elderly, and 28% were polymicrobic infections. Aeromonas spp. were isolated 

from blood, ascitic fluid, bile, urine, surgical wound infection, and liver abscess. A. hydrophila 

was most frequently isolated (68%), followed by A. veronii biovar sobria, and A. caviae. 

4.1.10 Peritonitis

Peritonitis sometimes occurs as a secondary infection following colonization of the 

intestinal tract, and is also associated with peritoneal dialysis or intestinal perforation (Ramos et 

al., 1996). Most infections occur in patients with chronic liver disease, where the case-fatality rate 

approaches 60%. Tsoufa et al. (2000) described a case of peritonitis caused by A. veronii biovar 

sobria that occurred following intestinal perforation resulting from cancer surgery. 

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4.1.11 Hemorrhagic Uremic Syndrome 

Bogdanovic et al. (1991) reported a case of hemolytic uremic syndrome (HUS) caused by 

A. hydrophila in a 23-month old child that occurred six days following an episode of bloody 

diarrhea. In a follow-up report, Robson et al. (1992) reported that 2 of 82 cases of HUS were due 

to A. hydrophila. This report provides additional evidence that this organism produces a cytolytic 

hemolysin (aerolysin) and/or cytotoxic enterotoxin (Act), which behaves in a similar fashion to 

the Shiga-like toxin of E. coli O157:H7, while remaining genetically and antigenically distinct. 

4.1.12 Ocular Infections 

Ocular infections caused by Aeromonas spp. are rare, and they usually result from waterrelated trauma (Carta et al., 1994). A. hydrophila, A. sobria and A. caviae have all been implicated 

as the cause of ocular infections. Most ocular infections follow eye trauma and water or soil 

contamination (Carta et al., 1994). Puri et al. (2003) reported a case of corneal ulcer in a 58-yearold patient caused by A. sobria where neither prior trauma nor water/soil exposure could be 

identified. Pinna et al. (2004) report a case of keratitis caused by A. caviae in a patient who wore 

soft contact lenses. Contamination of the contact lens case probably resulted in infection. 

The causal role of Aeromonas spp. in ocular disease must be evaluated in conjunction with 

clinical presentation, since aeromonads have been cultured from eye swabs of persons without 

eye disease (Smith 1980). 

Exhibit 4.8. Ocular Infections Caused by Aeromonas 

Disease Exposure Reference 

Keratitis contact lenses 

trauma 

Carta et al., 1994 

Pinna et al., 2004 

Carta et al., 1994 

Endophthalmitis trauma Carta et al., 1994 

Conjunctivitis unknown Smith 1980 

Corneal ulcer unknown Puri et al., 2003 

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4.1.13 Septic Arthritis 

Septic arthritis caused by Aeromonas spp. is relatively rare (Chmel and Armstrong 1976; 

Steinfeld et al. 1998; Roux et al., 2000). 

4.2 Predisposing Factors 

Predisposing factors identified with Aeromonas infection of the gastrointestinal tract 

include hospitalization, antimicrobial therapy, neutralization or underproduction of gastric acid, 

hepatic disease, and underlying enteric conditions such as gastric or colon cancer, colon surgery, 

gastrointestinal bleeding, or idiopathic inflammatory bowel disease (Joseph 1996). Formula-fed 

infants and children with disruption of normal bowel flora for antibiotic therapy may become 

colonized with aeromonads resulting in chronic diarrheal illness (Moyer 1987; Moyer and Larew 

1988; Namdari and Bottone 1991). Aeromonas spp. have been isolated from chlorinated and nonchlorinated drinking water supplies (LeChevallier et al., 1983; Burke et al., 1984a, 1984b; 

Havelaar et al., 1992), and drinking untreated ground water has been identified as a risk factor 

(Moyer 1987). Selective infection among persons with common exposure have been attributed to 

differences in host susceptibility, especially among children, thought the nature of these host 

determinants has not been elucidated (Demarta et al., 2000). 

Predisposing factors for extra-intestinal Aeromonas infection include trauma, cirrhosis, 

diabetes, alcoholism, severe malnutrition, renal failure, and severe peripheral vascular disease 

(Altwegg and Geiss 1989; Furusu et al., 1997). The pathogenicity of aeromonads differs based 

upon the genospecies and phenospecies involved and strain differrences (Janda and Kokka 1991). 

Based upon intraperitoneal infection of mice, the relative order of virulence for highest to lowest 

was A. jandaei, A. hydrophila, A. schubertii, A. veronii biovar veronii, A. veronii biovar sobria, 

HG-11 (unnamed), A. bestiarum, HG-3 (A. hydrophila -like), A. caviae, A. eucrenophila, A. 

media, and A. sobria. 

4.3 Chronic Conditions/Sequelae 

Underlying diseases associated with Aeromonas infection include liver dysfunction 

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(cirrhosis), blood dyscrasias (leukemias, lymphomas, myelomas), biliary disease, and diabetes 

(Janda and Duffy 1988). Complications associated with infections caused by Aeromonas include 

hemolytic uremic syndrome, septicemia, meningitis, peritonitis, wound infections, respiratory 

tract infections, and ocular infections. 

4.4 Dose Response 

The single human feeding study failed to determine the infectious dose (Morgan et al., 

1985). Empirical evidence from case reports and food borne outbreaks suggests that the infectious 

dose is very large (> 6-8 log10 CFU/g) and that only select strains have the ability to produce 

gastrointestinal disease; however, it is possible that some strains may have a lower infective dose 

in sensitive sub-populations. 

4.5 Immunity/Vaccines 

Jiang et al. (1991) first demonstrated a secretory immunoglobulin A (sIgA) response 

following Aeromonas infection, and Aeromonas exotoxins induce both IgM and IgG response. 

Crivelli et al. (2001) characterized the sIgA response to Aeromonas exoproteins in patients with 

naturally acquired infection. Fecal antibodies (sIgA response) could be used to assess the 

progression of intestinal infection for some strains of Aeromonas where IgM and IgG response 

was delayed or nonexistent. While no reference strains tested induced sIgA, 11 of 14 patient 

isolates of A. bestiarum induced sIgA production, and 2 of 8 strains of A. caviae failed to induce 

sIgA. Ahh1 and aerA gene products were shown to induce antibodies, which supports the 

conclusion that at least some strains of Aeromonas are enteropathogenic. 

4.6 Therapeutic Measures 

Zong et al., 2002 reviewed clinical aspects of Aeromonas infection and therapeutic 

options. Aggressive antibiotic therapy is indicated for disseminated infections. Gastrointestinal 

infections are typically self-limiting, and antibiotic therapy is required only in prolonged cases in 

immunocompromised hosts. Rehydration therapy may be required in infants with watery diarrhea. 

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Surgical intervention may be necessary in cases of nectotizing fasciitis. In addition, cellutis may 

require debridment, and abscesses may require draining. 

4.6.1 Antibiotic Treatment 

The role of antibiotics in treatment of gastrointestinal Aeromonas infections is 

controversial, since most patients regain health without treatment. Antimicrobials are indicated 

for only severe and unresponsive cases of Aeromonas gastroenteritis (Phavichitr and Catto-Smith 

2003). Antimicrobials should be considered for chronic gastrointestinal disease or extra-intestinal 

infection (Ghenghesh et al., 1999b). For adult patients with severe gastrointestinal illness such as 

colitis, quinolones, chloramphenical, trimethoprim-sulfamethoxazole, and tetracyclines are 

effective (Farraye et al., 1989). However, Ko et al. (1996) reported resistance in strains from 

Taiwan to tetracycline, trimethoprim-sulfamethoxazole, ceftriaxone, cefotaxime, cefixime and 

tobramycin. Prophylactic antimicrobial therapy is recommended for patients undergoing leech 

therapy (Snower et al., 1989). 

In addition to selection of antibiotic therapy in the clinical setting, antibiotic sensitivity 

patterns are sometimes useful as phenotypic characteristics for species identification, especially 

for clinical isolates (Overman and Janda 1999). Aeromonas spp. are characteristically resistant to 

ampicillin (94.9%), with variable resistance to cephalexin (76.3%), trimethoprim (37.3%), 

tetracycline (11.9%), cefuroxime (5.1%), and ceftazidime (1.7%). All strains tested were 

susceptible to gentamicin, chloramphenicol, and ciprofloxacin (Murphy et al., 1995). In another 

study, antibiotic susceptibility tests were performed on 164 strains, and resistance to ciprofloxacin 

(12-22%), nalidixic acid (54-62%), and norfloxacin (14-19%) were recorded (Sinha et al., 2004). 

Wide annual fluctuations in resistance during two consecutive years occurred with furazolidone 

(7% vs. 74%) and streptomycin (45% vs. 72%). These data support antibiotic susceptibility 

testing of isolates from extra-intestinal source. 

Susceptibility results show discrepancies depending upon methods used (Overman 1980). 

Disc diffusion and broth dilution tests for amoxicillin-clavulanic acid, sulfonamide, trimethoprim, 

and trimethoprim-sulfamethoxazole are unreliable (Koehler and Ashdown 1993). Overman and 

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Janda (1999) published antimicrobial susceptibility patterns for A. jandaei, A. trota, A. schubertii, 

and A. veronii biovar veronii, using the Microscan system and found that the antibiograms were 

useful in establishing the identification of these four species when isolates originated from clinical 

specimens. In the U.S., more than 90% of strains are susceptible to third-generation 

celphalosporins (cefotaxime, ceftriaxone, ceftazidime and cefoperazone) and aminoglycosides 

(gentamicin, tobramycin, amikacin, sisomicin, netilmicin, kanamycin, and neomycin). Nearly all 

aeromonads are susceptible to quinolones (ciprofloxacin, norfloxacin, ofloxacin, levofloxacin, 

sparfloxacin, moxifloxacin and gatifloxacin (Zong et al., 2002). Most U.S. strains are susceptible 

to chloramphenicol, tetracycline, minocycline, doxycycline, and nitrofurantoine, but resistant to 

clindamycin, vancomycin, and erythromycin. Imipenem is effective for treatment of Aeromonas 

infection (Lupiola-Gomez et al., 2003). 

Aeromonas spp. are typically sensitive to tetracycline, aminoglycosides, trimethoprimsulfamethoxazole, third-generation cephalosporins, and quinolones (Koehler and Ashdown 1993; 

Janda and Abbott 1996). Ko et al. (1996) reported increasing resistance to tetracyclines, 

trimethoprim-sulfamethoxazole, some extended-spectrum cephalosporins (ceftriaxone, 

cefotaxime, and cefixime), and tobramycin in strains from Taiwan. Huys et al. (2001) found 

oxytetracycline resistant strains in water from fish farms and in hospital sewage. 

Aeromonas species exhibit differences in their susceptibility to antibiotics. Clinical 

isolates of A. caviae were more susceptible to ticarcillin than A. veronii and A. hydrophila (Vila et 

al., 2002). Use of clavulanic acid with amoxicillin enhanced antibacterial activity but tazobactam 

did not enhance the effect of piperacillin. A. veronii (79%) were more susceptible to cefazolin 

than A. caviae (53%) or A. hydrophila (40%). Gentamicin and amikacin were more active than 

tobramycin. Tobramycin resistance and gentamicin susceptibility has been observed in Australia, 

Taiwan and the U.S. Fluoroquinolones have been the first choice treatment for Aeromonas 

infections. Strains resistance to nalidixic acid and susceptibility to ciprofloxacin are known to 

have a mutation in the gyrA gene; thus, naladixic acid resistant strains should not be treated with 

fluoroquinolones. Resistance to nalidixic acid is a function of a mutation of the gyrA, gyrB, parC, 

and parE genes which make up the quinolone resistance-determining regions (Goni-Urriza et al., 

2002). Gyrase and topoisomerase IV are the primary and secondary targets for quinolones, 

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respectively. 

Ko et al. (2001) demonstrated that minocycline and cefotaxime administered together 

produced a synergistic effect against A. hydrophila using a murine model. Ciprofloxacin was as 

effective as cefotaxime-minocycline in vitro and in a murine model, suggesting that clinical 

studies are warranted (Ko et al., 2003). 

Two metal ion binding sites are conserved in the metallo-β-lactamase produced by A. 

hydrophila (Valladares et al., 2000). Class B metalloproteins require bivalent transition-metal 

ions for activity, and Zn2+ is required for activation. Quiroga et al. (2000) found that cefotetan 

was efficacious against Aeromonas spp. because it behaved as a transitory inactivator of metalloβ-lactamase activity. Walsh et al. (2002) used Etest strips for detection of metallo-β-lactamases in 

clinical isolates. The test is based upon reduction of imipenem or ceftazidime in the presence of 

EDTA or 2-mercaptopropionic acid. Fosse et al. (2003b) demonstrated the β-lactamase classes 

produced by Aeromonas spp., and proposed they had taxonomic value as an adjunct in species 

identification. 

Edwards et al. (2001) reported antibiotic resistance in Aeromonas spp. isolated from a 

eutrophic lake in England. Goni-Urriza et al. (2000a; 2000b) reported antibiotic resistance to 

ampicillin (99%), ticarcillin (87%), cephalothin (93%), cefoxitin (56%), streptomycin (65%), 

sulfamethoxazole (90%), trimethoprim (42%), pipemidic acid (67%), oxolinic acid (67%), 

naladixic acid (59%), and tetracycline (14%) in Aeromonas spp. isolated from European rivers. 

Susceptibility to fluoroquinolones varied from 54 to 98%. Most strains were susceptible to 

ciprofloxacin, colistin, fosfomycin, cotrimoxazole, chloramphenicol, gentamicin, tobramycin, 

cefotaxime, and imipenem (Goni-Urriza et al., 2000b). Urban wastewater effluents are thought to 

contribute to the increasing rate of antibiotic resistance in environmental aeromonads (GoniUrriza et al., 2000a). 

Oxytetracycline is used in aquaculture and the development of resistant strains of 

Aeromonas spp. has both commercial and public health implications (Adams et al., 1998). 

Because resistance is plasmid mediated, it is easily transferred among strains, and has been 

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demonstrated in isolates from hospital sewage and a German hospital patient, in addition to fish 

farming environments. Dissemination of resistant strains has occurred in Norway, Scotland, 

England, and Germany (Rhodes et al., 2000). Bruun et al. (2003) demonstrated transfer of large 

plasmids conferring oxytetracycline resistance among environmental aeromonads and between 

Aeromonas spp. and E. coli. 

A. popoffii has been isolated from freshwater and seawater, but not from humans, though 

they are known to produce virulence factors and exhibit antibiotic resistance (Soler et al., 2002). 

Third-generation cephalosporins and quinolones are most active against A. popoffii in laboratory 

studies. 

4.6.2 Other Therapeutic Strategies 

Rehydration therapy is sufficient intervention in most pediatric cases of gastroenteritis 

caused by Aeromonas spp. (San Joaquin and Pickett 1988). Draining obstructions and antibiotic 

therapy cleared infection in patients with acute suppurative cholangitis (Chan et al., 2000). 

Debridment or more intensive surgical intervention may be necessary to arrest progression of soft 

tissue infections. 

4.7 Sensitive Sub-populations 

Aeromonads cause disease in elderly and immunocompromised patients (San Joacquin 

and Pickett 1988; Merino et al., 1995), as well as in previously healthy hosts (Goncalves et al., 

1992). Dumontet et al. (2003) recommends including Aeromonas in routine stool culture for 

patients under 10 years of age, the elderly and immunocompromised. 

4.7.1 Elderly 

Echeverria et al. (1981) reported a strong association between Aeromonas and 

gastroenteritis in adults older than 60 years and in travelers. Cryan et al. (1990) reported a case of 

diarrhea in a 70-year-old man caused by A. sobria presumably acquired from drinking water. An 

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organism with the same biotype and antibiogram was isolated from the group water system that 

supplied his house. Because no other persons exposed to the contaminated water source became 

ill, age is thought to be a predisposing factor resulting in illness. Champsaur et al. (1982) reported 

the case of a 67-year-old Thai woman with cholera-like illness caused by A. sobria. The source of 

infection was not determined, but either food or water could have been the source. 

Persons with underlying diseases are susceptible to Aeromonas infections, and elderly 

patients are at increased risk of acquiring Aeromonas bacteremia (Ko and Chuang 1995). 

Thomsen and Kristiansen (2001) reported 3 cases of bacteremia caused by A. sobria where 2 of 3 

patients were 75-years old or older. Leukemia, HIV infection, and colorectal cancer were 

underlying factors. Gelbart et al. (1985) reported a case of cholera-like diarrhea in a 62-year-old 

male caused by A. sobria. The patient has a history of chronic alcoholism with organic brain 

syndrome, but no underlying immunocompromising disease. The nature of exposure leading to 

illness was not determined. 

Elderly patients with diabetes are subject to opportunistic infections caused by Aeromonas 

spp. Furusu et al. (1997) reported necrotizing fasciitis and gas gangrene in a diabetic patient on 

hemodialysis. Grobusch et al. (2001) reported a case of cellulitis and septicemia caused by A. 

hydrophila in a 68-year-old patient with multiple underlying medical conditions including 

diabetes. The patient soaked his legs in a bucket of tapwater and infrequently changed the water. 

Moawad and Zeiderman (2002) reported a case of surgical wound infection in an 83-year-old 

diabetic patient cause by A. hydrophila. Sarma (2002) reported a case of A. jandaei cellusitis and 

bacteremia in a 75-year old patient with diabetes following a snake bite. Roux et al. (2000) 

reported a case of septic arthritis in an elderly diabetic. 

4.7.2 Children 

A strong association between gastroenteritis and Aeromonas species has been shown in 

children (Agger et al., 1985; Moyer 1987; San Joaquin and Pickett 1988; Namdari and Bottone 

1990b). Diarrheal illness seems to be more common in children under the age of three years (Sack 

et al., 1988), while children under the age of two years with multiple underlying medical 

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complications are at risk of sepsis from aeromonads in their intestinal tract (Janda and Abbott 

1998). Acute gastrointestinal disease in children usually resolves within 7 days, and it is 

characterized by watery diarrhea (100%), fever (70%) and vomiting (30%). Bloody stools occur 

in up to 30% of cases for A. hydrophila and A. sobria, but not with A. caviae (Lee and 

Puthucheary 2001). Chronic disease with soft stools persisting for months to years have been 

reported (Moyer et al., 1992a). 

de la Morena et al. (1993) reported an outbreak of diarrhea associated with Aeromonas 

spp. in daycare centers. In this outbreak, 6 of 25 children had Aeromonas caviae and/or A. 

hydrophila as the only pathogen, and 5 of 6 of these children had diarrhea. The source of infection 

was not determined; however, the authors discounted person-to-person transmission. Person-toperson transmission of A. caviae from an infant to an adult was shown by ribotyping (Moyer and 

Larew 1988). 

Kafetzis et al. (2001a) isolated Aeromonas spp. from 7 of 294 (2.5%) of children (median 

age 1 year) with acute diarrhea, and observed a seasonal peak incidence in summer months in 

Greece. 

Saxena et al. (2002) cultured stools of healthy and diseased children and adults and found 

higher counts of Aeromonas spp. in healthy subjects than in those with disease. The only culture 

method for aeromonads was MacConkey agar, which may have affected the results. 

Sirinavin et al. (1984) reviewed septicemia in infants and children, where disease 

manifestations resemble those found in adults. Septicemia, gastroenteritis, peritonitis, skin and 

wound infections, septic arthritis, osteomyelitis, ocular infections, myositis, urinary tract 

infections, pneumonia, meningitis, and hemolytic uremic syndrome have been reported in 

children (Janda and Abbott 1998). 

4.7.3 Immunocompromised 

Persons with compromised immune systems associated with malignant hematologic 

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disease, mainly leukemia, other malignancies, hepatobiliary disease such as cirrhosis, hepatitis, 

hepatic abscess, and biliary obstruction are at risk of opportunistic infections caused by 

aeromonads (Altwegg and Geiss 1989; Furusu et al., 1997). A. hydrophila and A. veronii biovar 

sobria are more pathogenic than other aeromonads for immunocompromised populations (Janda 

et al., 1994). 

Wolff et al. (1980) reviewed bacteremic Aeromonas infections in ambulatory 

immunocompromised hosts. Patients with malignancy, hepatobiliary disease, diabetes, 

pancreatitis, trauma, cardiac anomalies, gastrointestinal disorders, anemia, and respiratory 

problems are subject to developing septicemia caused by aeromonads (Janda and Abbott 1996). 

Kafarakis et al. (2001b) reported septicemia in an immunosuppressed female patient. The source 

of infection in these patients is thought to be endogenous resulting from gastrointestinal 

colonization. Mortality rates among these groups ranges from 25-50%. 

Lau et al. (2000) reviewed underlying factors predisposing patients to bacteremia and 

reported Aeromonas infections in 14 of 42 patients with neoplasm, 11 of 42 patients with 

cirrhosis, 10 of 42 patients with biliary tract disease, and 7 of 42 patients with other illnesses. In 

this study, 33 of 42 (79%) of cases were community acquired. Alcoholism and chronic hepatitis B 

infections were frequent predisposing factors for Aeromonas infection (Kim et al., 2001; Lau et 

al., 2000). Neutropenic patients (Orlando et al., 2001) and patients with underlying conditions 

such as E-β thalassemia (Cigni et al., 2003), ovarian cancer (Kafarakis et al., 2001b), and 

hematological malignancies or solid tumors (Martino et al., 2000) are at increased risk of 

infection. Flynn and Knepp (1987) reported a case of septicemia in an immunocompromised 

patient who apparently acquired his infection while shucking seafood. 

The mortality rate for immunocompromised patients with soft tissue infection and 

septicemia ranges from 56 to 75% despite aggressive surgical intervention and antibiotic therapy 

(Altwegg and Geiss 1989; Gold and Salit 1993). 

Reports of gastroenteritis in HIV patients have been limited (Dumontet et al., 2003). 

Interestingly, Suthienkul et al. (2001) reported that Aeromonas diarrhea was present in 4.6% of 

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AIDS patients and 8.3% of non-AIDS patients in Thailand. 

4.8 Summary 

Aeromonas spp. cause a variety of extraintestinal infections such as wound infection, 

memingitis, osteomyelitis, septic arthritis, endocarditis, peritonitis, eye and urinary tract 

infections. A. hydrophila, A. caviae, A. veronii biovar sobria, A. veronii biovar veronii, A. jandaei, 

A. trota, and A. schubertii are known to be pathogenic to humans. While all of these species have 

been isolated from fecal specimens, A. hydrophila, A. veronii biovar sobria, and A. caviae are 

most frequently associated with gastrointestinal disease. A high number of asymptomatic persons 

carry aeromonads in their stools (Schiavano et al., 1998). Expression of virulence factors is 

multifactorial and host susceptibility dependent (Kuijper and Peeters 1991). 

Kuhn et al. (1997) suggests that only certain strains of aeromonads are associated with 

gastroenteritis in humans, though humans continually ingest aeromonads in food and water. 

Despite demonstration of virulence factors such as enterotoxins, cytotoxins, hemolysins, 

aerolysins, proteases, hemagglutinins, and invasins, it has not been possible to predict 

pathogenicity for the human gastrointestinal tract based upon presence and production of 

recognized virulence factors alone (Thornley et al., 1997). The difficulty in unequivocally 

determining the pathogenesis of aeromonads results from their extreme strain heterogeneity and 

the lack of a suitable animal model system (Albert et al., 2000). 

Aeromonas cause gastroenteritis in children, the elderly, and in immunocompromised 

individuals. Several putative virulence factors have been identified, and food isolates frequently 

produce one of more of these factors. Presence of aerolysin (cytotoxic hemolytic exotoxin), heatlabile or heat-stable cytotoxins, phospholipases and various extracellular enzymes (amylase, 

proteinase, deoxyribonuclease, elastase, chitinase, etc.) does not consistently correspond to 

virulence in humans (Dumontet et al., 2003). Strains containing the ast gene (heat-stable 

cytotonic enterotoxin) were more common in environmental isolates than in clinical isolates from 

children with diarrhea (Albert et al., 2000). Apparently, both the alt (heat-labile cytotonic 

enterotoxin) and ast genes must be present for a strain to have the ability to cause watery diarrhea, 

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while strains containing only the alt gene are associated with loose stools. 

The relationship of aeromonads to waterborne and foodborne disease is controversial 

(Joseph and Carnahan 2000). Pathogenic strains, primarily HG-1, HG-4 and HG-8Y, are not 

frequently isolated from drinking water (Havelaar et al., 1992; Hanninen and Siitonen 1995; 

Hanninen et al., 1997b), while others report the presence of pathogenic strains to be frequent in 

the aquatic environment, including drinking water (Goni-Urriza et al., 1999). 

Virulence factors associated with aeromonads include straight (Sato et al., 1989) and 

flexible (Ho et al., 1990) type IV pili. Two distinct class IV pili (Bundle forming and Tap) are 

produced (Kirov et al., 1999; Kirov et al., 2000), in addition to collagen-binding protein (Gullberg 

et al., 1989), hemagglutinin (Majeed and MacRae 1994), and S-layers (Kokka et al., 1991a). 

Several extracellular products are elaborated, including cytotoxic and cytotonic enterotoxins, 

hemolysins, and various hydrolytic enzymes. The presence of both Type II and Type III 

secretions systems has been demonstrated (Sha et al. 2005; Yu et al. 2005) and several new 

virulence factors such as enolase (Sha et al. 2003), UDP N-acetylgalactosamine 4-epimerase 

(Canals et al. 2006), and DNA adenine methyltransferase (Erova et al. 2006) are being 

investigated. Despite the plethora of putative and proven virulence factors produced by 

aeromonads, the precise combination of virulence factors that invariably confer virulence on a 

particular strain has not been determined. Presence and expression of virulence genes are not 

unequivocally associated with the ability of a strain to cause disease in a susceptible host. 

Extra-intestinal infections are usually serious and potentially life-threatening, and 

aggressive antibiotic therapy is indicated. Because multi-drug resistance is commom among 

aeromnads, antimicrobial susceptibility testing is a necessary prerequisite to successful therapy. 

Gastrointestinal infections are usually self-limiting; however, antibiotic therapy may be required 

for infants or immunocompromised patients with prolonged infection. 

5.0 Health Effect in Animals 

Aeromonas infection of aquatic animals has been recognized for over 100 years, but they 

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are less commonly recognized in other vertebrates (Gosling 1996b; Austin and Adams 1996). 

While many animals shed aeromonads from their gastrointestinal tract, there is no evidence that 

they suffer from gastrointestinal disease. Aeromonas spp. cause outbreaks of disease and 

represent an economic threat to the aquaculture industry. The role of aeromonads in aquaculture 

was reviewed by Nielsen et al. (2001). 

5.1 Clinical Symptoms 

Aeromonas hydrophila has been reported to cause septicemia in snakes (Esterabadi et al., 

1973), turtles (Pasquale et al., 1994), and rabbits (Paniagua et al., 1998). Forga-Martel et al. 

(2000) reported a case of infectious abortion caused by A. hydrophila in a mare. Contamination 

from an adjacent dairy farm was suspected as the source of infection for the mare and 

transplacental infection was thought to result in fetal sepsis and abortion. Kumar et al. (2001) 

reported a case of mastitis in a dairy cow and isolated A. sobria from milk. The organism was 

resistant to ampicillin, colistin, cotrimoxazole, gentamicin, nitrofurantion, streptomycin, 

cefotaxime, cephalexin, chloramphenicol, nalidixic acid, furazolidone, norfloxacin and 

oxytetracycline. Ciprofloxicin was administered therapeutically with success. 

Disease in aquatic animals is characterized by hemorrhagic lesions, ulcers, furunculosis, 

and septicemia in frogs and fish (Gosling 1996b; Austin and Adams 1996). 

5.1.1 Mechanism of Action/Disease 

The mechanisms of disease action in animals (Gosling 1996b) and fish (Austin and Adams 

1996) were reviewed recently. The virulence factors operative in producing animal disease are 

essentially the same ones that produce disease in humans. 

5.1.2 Chronic Conditions/Sequelae 

Mastitis (Kumar et al., 2001), septic arthritis (Traubdargatz et al., 1994), and abortion 

(Forga-Martel et al., 2000) have been reported in animals. A chronic form of furunculosis has 

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been reported in fish (McCarthy 1980). 

5.2 Dose Response 

Paniagua et al. (1990) determined that doses of 7 log10 CFU of A. hydrophila (72% of 

strains) and A. sobria (63% of strains) infected intramuscularly produced disease in trout, while 

no A. caviae strains did. The dose response for animals is unknown. 

5.3 Immunity/Vaccines 

Research is underway to characterize immunity in fish and to produce effective vaccines 

to prevent economic loss in aquaculture. Live attenuated cells (Vaughan et al., 1993), outer 

membrane proteins (Hirst and Ellis, 1994), recombinant serine protease (Bennett et al., 1992), and 

lipopolysacccharide (Rodgers 1990) have been investigated with varying degrees of success. 

5.4 Therapeutic Measures 

Apart from aquaculture and veterinary medicine, little information is available on 

treatment of Aeromonas infections. Generally, the same antibiotics and therapeutic management 

techniques are used in human and veterinary medicine. 

5.4.1 Antibiotic Treatment 

Specific antibiotic therapy is used to treat domestic animals and to reduce losses in the 

aquaculture industry. Sulfonamides, oxytetracycline, and quinolones have been used successfully 

(Martinsen et al., 1991). 

5.4.2 Other Therapeutic Measures 

Management consists of removal of diseased animals and water treatment to control the 

density of aeromonads in aquaculture. Maintaining high standards of water quality, disinfection of 

equipment, temperature control, and stress reduction reduce the prevalence of disease (Austin and 

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Adams, 1996). 

5.5 Sensitive Subpopulations 

Animals in aquaculture are crowded and vulnerable to disease caused by Aeromonas spp. 

5.6 Summary 

Aeromonas spp. cause considerable economic loss in aquaculture, and methods are being 

developed to manage disease and increase production. Aeromonads are a minor veterinary 

pathogen, and the therapeutic measures resemble those practiced in human medicine. 

6.0 Epidemiology and Disease Outbreaks 

6.1 Transmission Routes and Mechanisms 

Aeromonas spp. are ubiquitous in the environment and there are multiple opportunities for 

transmission to humans through food, water, animal contact, and direct human contact. Extraintestinal infections are typically acquired following trauma in an aquatic environment, and 

intestinal infections are acquired by ingestion of contaminated food or water. Intestinal infections 

in immunocompromised patients may disseminate resulting in septicemia with multiple organ 

involvement. Inhalation of surface water in near drowning incidents has been reported to cause 

pneumonia. 

6.1.1 Foodborne Transmission 

Aeromonas spp. have been recognized as potential foodborne pathogens since 1984 

(Buchanan 1984). Kirov (1993a) reviewed the public health significance of Aeromonas spp. in 

foods, and Merino et al. (1995) reviewed aeromonads as emerging pathogens present in foods. 

Aeromonads are common on foods, especially green vegetables, and they are found in raw milk, 

ice cream, meats, and seafood. 

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The presence of aeromonads in fish and seafoods lead the U.S. Food and Drug 

Administration to designate them as a “new” foodborne pathogen (Tsai and Chen 1996). Butt et 

al. (2004) reviewed foodborne diseases associated with seafood consumption. Foodborne illnesses 

cause an estimated 76 million illnesses in the U.S. each year, and seafood is implicated in 10-19% 

of these illnesses (Butt et al., 2004). The causative agent is identified in approximately 44% of 

illnesses, with viruses accounting for approximately 50% of known illnesses, and 9% were caused 

by bacteria. The prevalence of seafood-related illnesses is dependent upon cultural dietary habits, 

and 20% of foodborne illnesses in Australia and 70% of illnesses in Japan are associated with 

consumption of raw or under-cooked seafood. In the U.S., shellfish cause about 64% and finfish 

cause 31% of outbreaks. A. hydrophila, A. caviae, and A. veronii biovar sobria are commonly 

found in seafoods (Tsai and Chen 1996). Aeromonas spp. have been recovered from 93% of fish, 

100% of fish eggs, and 16% of shrimp (Hanninen et al., 1997a). Wang and Silva (1999) reported 

that 36% of channel catfish samples from 3 processing plants contained A. hydrophila and 35.7% 

contained A. veronii biovar sobria. The rates of contamination were higher in summer months 

when water temperatures are warm (Nishikawa and Kishi 1988). Cockle ingestion resulted in A. 

hydrophila gastroenteritis (Bernardeschi et al., 1988) and shellfish shucking has caused 

septicemia from superficial abrasions on the hands of a worker (Flynn and Knepp 1987). 

6.1.2 Waterborne Transmission 

Diarrheal disease was associated with drinking untreated well water (Holmberg et al., 

1986). The use of ribotyping to link environmental and clinical strains of aeromonads has been 

widely applied; however, strain diversity has confounded success in all but a handful of studies. 

Altwegg et al. (1991a) used ribotyping to demonstrate that shrimp ingestion resulted in 

gastroenteritis in the first report of foodborne illness attributed to Aeromonas spp. Subsequently, 

others have shown the same ribotype in well water and stools of patients with gastroenteritis. 

Ribotyping was used to demonstrate that a patient with chronic diarrhea carried the same strain 

for years (Moyer et al., 1992a), and ribotyping was used to demonstrate person-to-person 

transmission of Aeromonas spp. between a foster child and a foster parent (Moyer and Larew 

1988). Aquarium water has been suggested as the source of aeromonads resulting in 

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gastrointestinal infection (San Joaquin et al., 1989). 

Filler et al. (2000) reported a case of acute renal failure in a 6-month old infant caused by 

A. sobria acquired from aquarium water. The infection presented with water diarrhea that 

progressed to bloody diarrhea and hemolytic uremic syndrome. Renal function was restored by 

kidney transplant at 16 months of age. 

There is no evidence of association of strains of Aeromonas with enteric infection through 

the waterborne route among the general population (Leclerc 2003). Implementation of a general 

water safety plan by health care facilities affords sufficient protection of compromised 

populations. 

6.1.3 Person-to-person Transmission 

Ashdown and Koehler (1993) did not observe person-to-person transmission to secondary 

household contacts in 45 cases of Aeromonas gastroenteritis in Australia. Person-to-person 

transmission was shown by ribotyping in the case of a foster child with A. sobria and A. caviae 

infection, where the foster mother developed diarrhea and A. caviae matching the child’s isolate 

was isolated from her stool (Moyer and Larew 1988). Transmission among children in daycare 

centers (Sempertegui et al., 1995), nursing homes (Bloom and Bottone 1990), and patients in 

intensive care (Torre et al., 1996) have been reported. 

6.1.4 Animal-to-person Transmission 

Animal-to-person transmission may occur through direct contact, or by ingestion of 

contaminated food products of animal origin. 

6.1.5 Environmental Transmission 

Extra-intestinal infections originate from environmental sources directly from soil or 

water contact, or indirectly by ingestion and bacteremic dissemination of aeromonads from the 

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gastrointestinal tract. 

6.2 Disease Outbreaks 

Aeromonas spp. have been reported to be the cause of the disease outbreaks shown in 

Exhibit 6.1. Foodborne outbreaks are discussed below in section 6.2.4. The source of infection is 

difficult to determine and is frequently unknown. 

Exhibit 6.1. Cases or Outbreaks Caused by Aeromonas spp. 

Location Number Ill Symptoms Source Organism Reference 

Florida, 1 septicemia seafood A. hydrophila Flynn and Knipp 

community shucking 1987 

acquired (crabs) 

Italy, 1 gastroenteritis, Cockles A. hydrophila Bernardeschi et al., 

community septicemia 1988 

acquired 

U.S., 

nursing home 

4 or 11 gastroenteritis unknown A. hydrophila Bloom and Bottone 

1990 

Canada, Glover et al., 1992 

industrial camp 

Mexico, 6 or 25; gastroenteritis unknown A. caviae de la Morena et al., 

day care 5 of 24 1993 

center 

U.S., 

nursing home 

1 gastroenteritis unknown Aeromonas 

spp. 

Sims et al., 1995 

Libya, 28 gastroenteritis unknown A. sobria Taher et al., 2000 

community 

acquired 

U. S., 1 gastroenteritis. aquarium A. sobria Filler et al., 2000 

community acute renal water 

acquired failure 

Australia, 26 cellulitis mud A. hydrophila Vally et al., 2004 

community football 

acquired 

Modified from Joseph, S. W. 1996 

6.2.1 Outbreaks Associated with Drinking Water 

Aeromonads are frequently isolated from drinking water (Havelaar et al., 1990; Holmes 

and Nicholls 1995; Holmes et al., 1996), and temporal and seasonal associations between 

presence of aeromonads in drinking water and their presence in the stools of patients with 

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gastroenteritis have been reported (Burke et al., 1984b). While some investigators claim that 

drinking water is responsible for outbreaks of Aeromonas gastroenteritis (Ghanem et al., 1993), 

epidemiological evidence linking water ingestion to gastrointestinal illness has been limited to 

untreated drinking water supplies, and no waterborne disease outbreaks associated with treated 

drinking water supplies have been reported. Molecular typing studies have shown that the strains 

most frequently found in feces belong to HG-1 and HG-4, while HG-2, HG-3 and HG-5A are 

more commonly found in drinking water and the environment (Moyer et al., 1992b; Hanninen and 

Siitonen 1995), suggesting that environmental strains are fundamentally different from clinical 

strains. The high infectious dose, the differences in temperature optima, and the variation in 

expression of putative virulence factors between clinical and environmental strains suggests that 

outbreaks of gastrointestinal illness resulting from water ingestion are unlikely to occur. 

6.2.2 Outbreaks Associated with Recreational Water 

While superficial infections resulting from environmental exposure are relatively 

common, no outbreaks attributable to recreational water exposure have been reported. 

6.2.3 Outbreaks Associated with Other Water Sources 

One report linked exposure to aquarium water to a case of gastroenteritis caused by A. 

sobria with fatal disseminated disease in a 6-month old child (Filler et al., 2000). 

6.2.4 Food

 Aeromonas spp. have been reported as the cause of individual cases and point source 

outbreaks of foodborne disease as shown in Exhibit 6.2. Raw or under cooked seafood and meat 

are most frequently implicated, but snails, soups and egg salad have resulted in illness or 

outbreaks. 

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Exhibit 6.2. Foodborne Gastroenteritis Associated with Aeromonas spp. 

Incubation Aeromonas spp. Isolated 

Location Attack Rate Period (h) Reference 

Food Feces 

School, Russia “mass” 

poisoning 

ND Fish ND Kalina 1977 

Restaurant, 

England 

3 of 3 24-36 oysters ND McSwiggan 

and Jones 

1978* 

Hungary “several” 20 Soups, starchy ND Janossy and 

cases broths Tarjan 1980 

Pub, Scotland > 20 < 24 prawns no Small 1980* 

Dinner party, 

Nigeria 

1 of 2 20 snails yes Agbonlahor et 

al., 1982 

Florida 7 of 7 22-34 oysters yes Abeyta et al., 

1986 

China 115 ND pork yes Zeng Shan et 

al., 1988 

School, Japan 29 of 37 ND ND yes Kobayashi and 

Ohnaka 1989 

Household, 4 of 5 ND sashimi (fish yes Kobayashi and 

Japan balls) Ohnaka 1989 

Restaurant, 

England 

14 < 24 prawns ND Hospital 

report* 

Restaurant, 

England 

2 6-8 prawns no Hospital 

report* 

Specialty 1 48 shrimp cocktail yes Altwegg et al., 

store, 1991a 

Switzerland 

Japan 2 of 8 ND seafoods yes Tanaka et al., 

1992 

Hospital 

cafeteria, U.S. 

4 < 24 egg salad 

sandwich 

yes Bottone 1993 

Smorgasbord, 

Sweden 

22 20-34 meats yes Krovacek et 

al., 1995 

Norway 3 of 4 ND raw fish yes Granum et al., 

1998 

Modified from Kirov 1993a 

*Cited by Todd et al., 1989 

ND - no data 

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6.2.5 Travelers 

Echeverria et al. (1981) noted that travelers were at risk of acquiring diarrheal disease 

caused by Aeromonas spp. Hanninen et al. (1995) reported travelers’ diarrhea in Finnish tourists 

who visited Morocco, and Yamade et al. (1997) reported Aeromonas infections in Japanese 

travelers to developing countries. Both Sierra et al. (2001) and Vila et al. (2003) reported 

Aeromonas spp. as the cause of diarrhea in 2% of travelers to Africa, Latin America, and Asia. 

Rautelin et al. (1995) reported a case of A. caviae diarrhea in a 45-year old male who 

traveled to Turkey. A. caviae was isolated from his stool on 14 occasions over 17 months. 

Treatment with three courses of ciprofloxacin were required to eradicate the infection. Ribotyping 

revealed that all isolates were the same strain. 

6.2.6 Other Sources 

Lehane and Rawlin (2000) reviewed zoonoses acquired from fish and reported that 

aeromonads caused cellulitis, myositosis, and septicemia following injuries from handling fish, 

working in aquaculture, or keeping fish as pets. 

Several investigators have reported Aeromonas infections concurrently with use of therapeutic 

leeches. Indergand and Graff (2000) and Braschler et al. (2003) both reported symbiotic 

relationships between Aeromonas veronii biovar sobria and medicinal leeches. Cellulitis is the 

most common infection following application of medicinal leeches (Fenollar et al., 1999), and up 

to 20% of patients undergoing leech therapy experience infections (Mercer et al., 1987). 

Ouderkirk et al. (2004) reported a case of meningitis following leech therapy. Use of tap water to 

fill aquariums used for maintaining leeches is discouraged (Sartor et al., 2002). 

6.3 Summary

Aeromonas spp. are ubiquitous in the environment and there are multiple opportunities for 

transmission to humans through food, water, animal contact, and direct human contact. 

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Aeromonads are common on foods, especially green vegetables, and they are found in raw milk, 

ice cream, meats, and seafood. The presence of aeromonads in fish and seafoods lead the U.S. 

Food and Drug Administration to designate them as a “new” foodborne pathogen (Tsai and Chen 

1996). While aeromonads are frequently isolated from drinking water (Havelaar et al., 1990; 

Holmes and Nicholls 1995; Holmes et al., 1996), and claims that drinking water is responsible for 

Aeromonas gastroenteritis (Ghanem et al., 1993), epidemiological evidence supporting water 

ingestion to illness has been limited to untreated drinking water supplies, and no waterborne 

disease outbreaks associated with treated drinking water supplies have been reported. There is no 

evidence of association of strains of Aeromonas with enteric infection through the waterborne 

route among the general population (Leclerc 2003). Implementation of a general water safety plan 

by health care facilities affords sufficient protection of compromised populations (CDC 2001). 

Lehane and Rawlin (2000) reviewed zoonoses acquired from fish and reported that aeromonads 

caused cellulitis, myositosis and septicemia following injuries from handling fish, working in 

aquaculture, or keeping fish as pets. 

7.0 Risk Assessment 

The health implication of heterotrophic bacteria, including Aeromonas spp. is 

controversial (Edberg et al., 1996; Grabow 1996; Rusin et al., 1997). Payment et al. (1991) found 

that 35% of gastrointestinal disease was attributed to consumption of water meeting current 

treatment standards. This finding has not been confirmed in recent epidemiological studies 

(Colford et al., 2005). Pavlov et al. (2004) studied virulence factors in heterotrophic bacteria 

isolated from treated and untreated drinking water and concluded that while the presence of 

potential pathogens in drinking water is of epidemiological concern, the extent of health risk 

cannot be defined and further studies are needed. Aeromonas was isolated from 18.1% of 

samples; however, Rusin et al. (1997) report the risk of infection to be less than 6.8 cases per 

10,000 population exposed and 95% of the time would be less than 1/1000 for a single day 

exposure. 

One means of assessing the risk of infection associated with Aeromonas spp. is to 

characterize isolates from various sources for presence of putative virulence factors. Several 

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investigators have demonstrated production of multiple virulence factors and multidrug resistance 

in Aeromonas spp. isolated from surface waters (Gibotti et al., 2000; ). While putative virulence 

factors have been demonstrated in water isolates, Havelaar et al. (1992) showed that aeromonads 

from human fecal specimens were not very similar to strains isolated from drinking water. The ast 

gene was significantly more common in environmental isolates than in isolates from children with 

diarrhea, while isolates possessing both the alt and ast genes were significantly more numerous 

for children with diarrhea than from other sources (Isonhood and Drake 2002). 

Biscardi et al. (2002) isolated cytotoxic A. hydrophila from Italian bottled mineral and 

thermal waters. Overall, 61 bottled waters yielded 4 positives (6 strains) and 23 thermal waters 

yielded 6 positives (12 strains). All 6 isolates from mineral water were cytotoxic and contained 

the aerolysin gene, while 7 of 12 environmental strains were cytotoxic and 11 of 12 strains 

contained the aerolysin gene. 

Murphy et al. (1995) reviewed hospitalized cases of Aeromonas infection over a 15-year 

period and reported that 52.5% of extra-intestinal Aeromonas infections were hospital-acquired 

and 55.9% of infections occurred in patients with serious underlying disease. Only 13.6% of 

isolates were from community-acquired cases in previously health individuals. Polymicrobial 

infections occurred in 40.7% of patients. Post-operative infection accounted for 27.1% of cases. 

Clinically significant infections occurred in 79.7% of cases. The mortality rate was 5.1%. 

Aeromonas spp. were isolated from patients with bacteremia, skin and soft tissue infections, 

respiratory tract infections, genitourinary tract infections, biliary tract infections, and peritonitis. 

Malignant disease was a common underlying illness. 

Immunocompromised, infants less than 2 years of age, and individuals with severe 

wounds associated with trauma are the primary risk groups for Aeromonas infection. The 

mortality of disseminated cases is 25-75% (Janda and Abbott 1998). 

Drinking water standards have not provided absolute public health protection, though they 

have been responsible for reducing the number of waterborne disease outbreaks associated with 

treated public drinking water supplies (Padiglione et al., 1997). Any deviation from usual plant 

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performance may signal a problem that potentially increases the risk of contaminants entering the 

distribution system, and such events require immediate intervention (Padiglione et al., 1997). 

A. hydrophila was placed on the U.S. EPA Candidate Contaminant List in 1998 (USEPA 

1998). This list is published every five years. It identifies chemicals and potential waterborne 

pathogens for further evaluation, research and regulatory consideration, which includes risk 

assessment. 

7.1 Risk Assessment Paradigms/Models 

The current framework for microbial risk assessment is based upon the report, A 

Conceptual Framework for Assessing the Risks of Human Disease Following Exposure to 

Waterborne Pathogens, published in condensed form in 1996 (ILSI 1996). This framework was 

further revised during the Workshop on Water and Foodborne Pathogen Risk Assessment (May 

1999), cosponsored by the ILSI Risk Science Institute and EPA Office of Ground Water and 

Drinking Water (ILSI 2000). Microbial risk assessments involve analyses of data related to 

characterization of potential exposure and characterization of associated human health effects. 

The characterization of pathogen virulence factors is thought to be of paramount importance in 

determining the potential for a particular strain to cause disease in an exposed host. This concept 

has lead to exhaustive characterization of virulence factors in collections of isolates from clinical 

and environmental sources. 

7.1.1 Assessment of Virulence Factors as an Indication of Risk 

Figueroa et al. (1988) proposed that all strains containing virulence factors be considered 

pathogenic, but Schiavano et al. (1998) isolated strains of A. veronii biovar sobria capable of 

producing cytotoxin and showing invasive properties from asymptomatic hosts. While our 

knowledge of virulence factors of aeromonads is incomplete, carrying the genes responsible for 

production of known virulence factors, and demonstration of structural properties suggestive of 

virulence is not sufficient evidence to classify aeromonads as human pathogens. Unrecognized 

host factors undoubtedly play the pivotal role in determining the condition under which virulence 

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factors are expressed and disease occurs. 

7.1.1.1 Studies Characterizing Virulence Markers in Clinical and Environmental Isolates 

Gonzalez-Serrano et al. (2002) used the presence of the hemolysin genes aerA and hlyA 

genes to predict virulence in isolates from freshwater fish and a case of human diarrhea. The 

strain from a case of human diarrhea was positive for both hemolysin genes. Nine of 11 strains of 

A. hydrophila contained both hemolysin genes, while 2 of 11 A. hydophila strains lacked the hlyA 

gene. All three strains of A. veronii biovar sobria lacked both hemolysin genes. 

Growth temperature may affect the expression of several virulence-associated factors 

(Gonzalez-Serrano et al., 2002). Clinical isolates (invariably isolated at 35o C) express virulence 

traits more frequently at 37o C; whereas, isolates from foods or the environment express virulence 

traits more efficiently at refrigeration temperatures (Knochel 1989; Kirov 1997). Curiously, the A. 

veronii biovar sobria strain that was aerA and hlyA negative showed enterotoxic activity in the 

suckling mouse assay. Wong et al. (1998) has suggested that aerA (aerolysin) and hlyA (CT-like 

hemolysin) must be produced together to contribute to virulence of A. hydrophila, PCR detection 

of aerA and hlyA genes (Pollard et al., 1990; Heuzenroeder et al., 1999) may not be sufficient to 

characterize a strain as potentially virulent, and other factors determine whether or not a strain is 

pathogenic (Gonzalez-Serrano et al., 2002). No criteria presently exist for classification of 

environmental strains as enteropathogenic. 

Chacon et al. (2003) studied the distribution of virulence genes in clinical isolates from 

wounds (7), stools (92), abscesses (2), urine (3), sputum (1) and blood (3), and food, and 

environmental isolates from cake (2), fish (23), seawater (15), freshwater (83), and other sources 

(3). Markers included aerA gene, hemolysis on sheep and human blood, serine protease gene, 

GCAT gene, lipase gene, and DNAse gene. 

Several genes have been identified that play a role in pathogenesis of aeromonads (Janda 

2002). These genes encode for secreted enzymes such as lipase, hemolysin, enterotoxins, 

proteases, and nucleases (Pemberton et al., 1997; Heuzenroeder et al., 1999; Merino et al., 1999). 

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Detection of virulence genes is a crucial step in determining the potential pathogenicity of 

Aeromonas isolates (Wong et al., 1998; Heuzenroeder et al., 1999. 

The aerolysin/hemolysin genes of Aeromonas spp. have frequently been targeted in 

molecular probe and PCR methods (Wang et al., 1996; Khan et al., 1998). Differences in PCR 

results for characterizing virulence genes results from choice of primers, though several 

investigators have used the primers developed by Kingombe et al. (1999) in an effort to 

standardize and compare results. The degree of similarity of sequences of Aeromonas spp. 

deposited in GenBank and the European Molecular Bacteriology Laboratory (EMBL) show 

marked variability with sequence similarity for aerolysin/hemolysin (50.2%), lipase (74.3%), 

DNAse (50.8%), serine protease (92.3%), and GCAT (91.9%) (Chacon et al., 2003). 

Aerolysin/hemolysin genes were not detected in A. media, A. allosaccharophila, and A. 

schubertii (Chacon et al., 2003). Considerable discrepancies were found between presence of 

virulence genes and the phenotypic evidence of gene product expression (Chacon et al., 2003). 

Such discrepancies may result from differences in growth conditions that mediate expression of 

virulence genes under one set of conditions but not under other conditions. The absence of 

expression of one or more virulence factors may not indicate absence of virulence. No significant 

differences were found between presence of virulence factors among clinical and environmental 

strains for the two most clinically relevant species, A. caviae, and A. veronii. A statistically 

significant relationship between the presence of aerolysin/hemolysin and serine protease supports 

the role of protease in activation of aerolysin (Chacon et al., 2003). 

Aerolysin/hemolysin, DNAse, and β-hemolysis were prevalent in clinical isolates (Chacon 

et al., 2003). DNAse was present more frequently in clinical than environmental strains. Lipases 

were present in both clinical and environmental strains. Aerolysin/hemolysin genes were more 

frequent in clinical isolates that in environmental strains. β-hemolysis was more common in 

clinical strains using sheep blood, but not human blood. 

Sechi et al. (2002) characterized virulence factors from clinical and environmental strains 

of Aeromonas using slime, hemolysin, gelatinase, adhesin for epithelial cells, and the virulence 

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genes coding for the cytolytic enterotoxin, type IV pili (tap), bundle forming pili (bfpA and 

bfpGH). Patient strains had greater prevalence of hemolysin, protease, and gelatinase production, 

as well as greater adhesion. 

7.1.1.2 Studies Characterizing Virulence Markers in Clinical Isolates 

Schiavano et al. (1998) reported cytotoxic activity together with the presence of adhesins 

and invasins in 4 of 8 stool isolates for patients with diarrhea; however, one or more of these 

putative virulence factors were also found in isolates from normal patients. Hayashi et al. (2002) 

compared endoscopic findings in patients with enterocolitis caused by Aeromonas with V. 

parahaemolyticus and Staphylococcus aureus. Aeromonas infections were high in the sigmoid 

colon, but characteristic pathology was not observed. 

Ghenghesh et al. (1999b) compared virulence factors and antibiotic susceptibility in 

aeromonads from children with diarrhea compared to matched normal controls in Lybia. They 

used hemolysis and hemagglutination as virulence traits, and isolated aeromonads from14.6% 

from diarrhea and 17.8% from controls. Only hemagglutinin showed a significant difference with 

greater prevalence in diarrheal patients. 

Mokracka et al. (2001) characterized virulence factors of CT-like toxin, adhesive ability, 

and siderophores in clinical isolates of A. caviae. All isolates were adherent and produced 

siderophores, while 6 of 13 produced CT-like toxin. None of the isolates produced hemolysins. 

All isolates had high LD50 values in mice, which suggested that there is no relationship between 

pathogenicity in mice and the ability to produce gastroenteritis in humans. 

Sinha et al. (2004) enriched fecal samples in APW, pH 9, incubated overnight, then 

inoculated onto 5% sheep blood agar with 30 mg/L ampicillin and xylose/deoxycholate citrate 

agar. Isolates were identified using Aerokey II (Carnahan et al., 1991b). PCR primers included alt 

(Granam et al., 1998), ast and act (Galindo et al 2005), hlyA (Heuzenroeder et al., 1999) and aer 

(Pollard et al., 1990). Ribotyping was performed using BglI restriction enzyme and a 7.5 kb 

BamHI fragment of plasmid pKK3535 as the probe. RFLP (RAPD-PCR) was used for DNA 

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fingerprinting. Among 78 patients with watery diarrhea, serotypes O:85, O:16: and O:83 were 

found most frequently, but 27 other serotypes were identified (Sinha et al., 2004). Four strains 

were rough, 5 were nontypable, and 1 was not available for serotyping. The presence of toxin 

genes in isolates from patients with diarrhea is shown in Exhibit 7.1. RFLP failed to show 

clonality among strains with one or more virulence factors. 

Exhibit 7.1. Distribution of Toxin Genes in Clinical Isolates of Aeromonas spp. 

Species No. Strains 

Number of Isolates with Toxin Genes (%) 

alt act ast hlyA aer 

A. caviae 74 72 (97.3) 1 (1.4) 1 (1.4) 3 (4.1) 3 (4.1) 

A. veronii 

biovar 

sobria 

25 12 (48.0) 21 (84.0) 2 (8.0) 6 (24.0) 1 (4.0) 

A. 

hydrophila 

35 31 (88.6) 14 (40.0) 0 31 (88.6) 10 (28.6) 

A. veronii 

biovar 

veronii 

9 7 (77.7) 7 (77.7) 0 0 0 

A. trota 10 5 (50.0) 0 1 (10.0) 1 (10.0) 0 

A. schubertii 4 3 (75.0) 2 (50.0) 0 1 (25.0) 0 

A. jandaei 1 1 (100) 0 0 0 0 

Unidentified 6 5 (83.3) 3 (50.0) 0 4 (66.7) 0 

From Sinha et al., 2004. 

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7.1.1.3 Studies Characterizing Virulence Markers in Environmental Isolates 

Gene disruption experiments have shown that enterotoxin Act and Alt (Chopra et al., 

1994; Chopra et al. 1996; Xu et al., 1998; Sha et al., 2002), enterotoxin Ast (Chakraborty et al., 

1984; Sha et al., 2002), elastase (Cascon et al., 2000a) and flagellin (Rabaan et al., 2001) are 

directly involved in pathogenesis in animal models and cell lines. 

Ormen and Ostensvik (2001) demonstrated widespread occurrence of aerolysin-producing 

cytotoxic Aeromonas spp. in ambient water and drinking water in Norway. 79% of isolates 

carried the aerolysin gene and 83% were found to be cytotoxic to Vero cells. 

Messi et al. (2003) screened 30 water isolates for bacteriocin-like substances. A. 

hydrophila showed antibacterial activity to one or more indicator bacteria or pathogens. These 

data support the empirical finding of inhibition of indicator bacteria in coliform tests. 

Schubert (2000) examined 650 isolates from surface water and water treatment plants for 

adhesion and maximum growth temperature. INT-407 cells and 39o C were used to determine 

potential pathogenicity of environmental strains. Clinical strains grew at 39o C and adhered; while 

most environmental strains lacked the ability to grow at 39o C or adhere to cells. Strains 

encountered in drinking water are very unlikely to cause gastrointestinal disease. 

Balaji et al. (2004) determined the cytotoxicity of 39 environmental isolates. All isolates 

produced cytotoxin in Vero cells. 

Bondi et al. (2000) constructed virulence and antibiotic resistance profiles of 30 A. 

hydrophila environmental isolates. Hemolysin, cytotoxin, and adhesive capacity were use as 

virulence factors. Isolates were positive for adherence (70%), hemolysin (87%), and cytotoxin 

(100%). 

Sen and Rodgers (2004) characterized the presence of six virulence factors in Aeromonas 

spp. isolated from U.S. drinking water supplies using PCR for detection of elastase (ahyB), lipase 

(pla/lip/lipH3/alp-1), flagella A and B (flaA, flaB), and enterotoxin (act, alt, ast) genes. Genes 

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were present in 88% (ahyB), 88% (lip), 59% (fla), 43% (alt), 70% (act), and 30% (act) of strains. 

Only 1 of 205 isolates tested had all virulence genes. In the work of Sen and Rodgers (2004), 67% 

of isolates had at least four virulence genes, and 100% of isolates had at least one gene. Only 1 

strain of A. hydrophila HG-1 had all three toxin genes act, alt and ast, and this isolate had all 

virulence genes. The combination of virulence factors that would be required to predict infectivity 

remains to be determined because strains expressing multiple putative virulence factors have been 

isolated from asymptomatic individuals. 

Hanninen and Siitonen (1995) used a combination of phenotypic methods and ribotyping 

to classify 79% of water isolates. Many environmental strains have not been assigned to a 

hybridization group and new species continue to be recognized. 

Martinez-Murcia et al. (2000) used RFLP of PCR amplified intergenic spacer region 

between the 16S and 23S rRNA genes to demonstrate colonization of a public water supply with a 

particular strain of A. veronii. The method could be used to trace strains during epidemiological 

investigations of foodborne and waterborne outbreaks. 

Rahim et al. (2004) characterized environmental isolates using an act gene probe to 

evaluate cytotoxic enterotoxin potential. 32 of 69 environmental isolates had the act gene. Isolates 

were examined for enterotoxigenicity by suckling mouse assay, hemolytic activity on blood 

plates, CAMP-like factor and cytotoxicity in Vero cells. The CAMP-hemolysis phenotype was 

significantly associated with rabbit ileal loop assay, vero cell assay, and hemolysin production 

suggesting that the CAMP-hemolysin assay can be used to evaluate enterotoxin production 

potential of aeromonads. 

Mateos et al. (1993) reported that environmental strains of Aeromonas were inhibitied at 

37o C and production of hemolytic and cytotoxic activity decreased at this temperature. All 

environmental isolates of A. hydrophila were pathogenic for trout while only 44% of human 

strains caused disease in fish. 

Demarta et al. (2000) used ribotyping to show the same strain in ill and well family 

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members, suggesting host factors determined susceptibility and expression of disease. The same 

ribotype was isolated in the drinking water supply in the same region but not in samples from the 

household distribution system. Others (Moyer et al., 1992b; Hanninen and Siitonen 1995; 

Borchardt et al., 2003) have been unable to demonstrate strain similarity between patient and 

environmental isolates. Moyer reported the first known case where the same ribotype was 

isolated from a patient and the household water supply at the 7th International Symposium on 

Aeromonas and Plesiomonas, Orihuela, Spain in September 2002. 

7.1.1.4 Studies Characterizing Virulence Markers in Food Isolates 

Many investigators have characterized food isolates for their ability to produce putative 

virulence factors (Kumar et al., 2000b; Kingombe et al., 2004). Aermonas spp. exhibit a high 

degree of genetic heterogeneity. When PFGE was used to characterize a group of cultures isolated 

from ready-to-eat foods, 24 unique patterns were detected in 27 strains of A. hydrophila and 20 

unique patterns were detected in 23 strains of A. caviae. 

Tsai et al. (1997) evaluated hemolysin and cytotoxin production at various temperatures 

and determined that A. hydrophila produced both virulence factors at 5o, 28o and 37o C; however, 

the activity was greater when cultures were incubated at 28o C. Toxin production may occur in 

meats at refrigerator temperature (Majeed and MacRae 1991). Kirov et al. (1993a) studied 

virulence factors in strains isolated from milk, and concluded that aeromonads could grow to high 

numbers in refrigerated milk without spoilage. Most milk isolates were not enterotoxigenic; 

however, a strain of A. sobria from milk produced hemolysins, cytotoxins, and enterotoxin. 

Strains grown at low temperature have enhanced adhesin production as the result of 

increased production of pili (Kirov et al., 1993b). Studies suggest that the presence of pre-formed 

enterotoxins in foods are of little concern, since toxin production is considerably reduced in food 

products compared to the amounts of toxins produced in laboratory media. This observation 

explains why so few foodborne outbreaks are attributed to Aeromonas spp. despite their presence 

at high numbers in foods. 

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Alcides et al. (2003) isolated Aeromonas spp. were isolated from parsley and watercress 

from street markets in Brazil. Strains expressed virulence factors and antibiotic resistance to 

trimethoprim (39%) and gentamicin (21%). Cell populations remained constant for 7 days at 5o C. 

A. caviae, A. hydrophila, and unidentified aeromonads were isolated. Vegetables may pose a risk 

factor to immunocompromised populations when they are eaten raw. 

Of 141 food isolates examined for virulence factors, 66% produced cytolytic enterotoxin, 

as did 67% of clinical isolates; however, 58% of environmental isolates also produced enterotoxin 

(Kingombe et al., 1999). Wang and Silva (1999) reported that 86% of catfish isolates were 

hemolytic. 

Okrend et al. (1987) demonstrated the capability of isolates from meats to produce toxins, 

and Tsai and Chen (1996) reported toxigenic strains isolated from seafood. 

Phospholipase and proteases are thought to be responsible for invasive properties 

(Ishiguro et al., 1985). Hemolysins (Santos et al., 1999), enterotoxin and cytotoxin (Wong et al., 

1998), and adherence factors (Neves et al., 1994) all have virulence-enhancing potential. The 

complex multifactorial pathogenic mechanisms in aeromonads are not completely understood. 

Yadav and Kumar (2000) isolated 5 (5%) Aeromonas strains from 100 clinical specimens 

and 40 (14%) Aeromonas strains from 285 food samples (fish, milk and ice cream). A. hydrophila 

(9), A. sobria (6) and A. caviae (1) were identified from foods. A. hydrophila (2) and A. sobria (3) 

were isolated from stools. Enterotoxigenicity was shown in 48.8% to 57.5% of food isolates and 

60% of stool isolates. 

Kingombe et al. (2004) surveyed 78 meats and 123 processed, ready-to-eat foods for 

Aeromonas spp. containing virulence genes for cytotoxic enterotoxin and hemolysin by using 

conventional culture and a newly developed PCR method for direct detection of virulence genes 

in food samples. Using culture methods, 32.3% of samples contained aeromonads, while PCR 

detected target genes in 25.4% of samples. Fatty acid methyl ester (FAME) analysis and 

amplified fragment length polymorphism (AFLP) were used to determine the hybridization group 

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for 97% of isolates. HG-3 (50%) and HG-8/10 (38%) were most frequently isolated. The 

combination of PCR-RFLP and PCR-amplicon sequence analysis (PCR-ASA) allowed 

classification of strains to hybridization group, thereby providing a sensitive, specific system for 

assessment of aeromonads harboring virulence factors in food samples. 

Nishikawa and Kishi (1988) isolated Aeromonas spp. from 11.1% of diarrheal stools 

compared to 2.2% of normal stools. Among food isolates, A. hydrophila and A. sobria 

predominated, while A. caviae was most common in seafood and vegetables. The common 

occurrence of aeromonads in foods suggested that the primary source of exposure may be 

foodborne rather than waterborne. Stool, food and water isolates all produced putative virulence 

factors, and differentiation of pathogenic from non-pathogenic strains was not possible from in 

vitro tests. 

Hanninen et al. (1997a) isolated aeromonads from fish, fish eggs, shrimp, and water to 

determine the distribution of genomospecies in these samples. Hybridization groups, HG-2 and 

HG-3, were found most frequently in all samples; however, no strains of HG-2 were found in fish 

eggs. Twenty-six Aeromonas strains are suspected of being associated with foodborne illness. 

They include HG-1 (2), HG-2 (7), HG-3 (3), HG-7 (7), HG-8/10 (2), and 5 untyped strains. 

Strains of HG-1 were not found in market fish, fish eggs, shrimp, or fresh water samples. Strains 

isolated from suspected cases of diarrheal illness typically belong to HG-1 (A. hydrophila), HG-4 

(A. caviae), or HG-8/10 (A. veronii biovar sobria). Because HG-2 and HG-3 are uncommon in 

diarrheal stools, their role in causing diarrhea is unclear. 

Karabasil et al. (2002) examined Aeromonas isolated from fish and other seafood from 

retail markets in Hungary for their ability to produce toxins associated with virulence. Nine 

isolates were recovered from 78 samples, 6 A. hydrophila and 3 A. sobria. All 9 strains produced 

varying amounts of cytotoxin in Vero cell cultures. Because enrichment broth was used for 

primary isolation, it was not possible to quantitatively determine the number of organisms per 

gram of sample. 

Quantitative culture revealed no significant differences between the number of 

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aeromonads in ambient water and on the skin, gills, or visera for fish at the time they are caught 

(Gonzalez et al., 2001). Aeromonads grow on fish skin and visera during storage on ice. Counts 

ranged from 2-5 log10 CFU/g with intestinal contents demonstrating the higher counts. Bacterial 

counts increased 2-3 log10 CFU/g during iced storage up to 14 days. A. hydrophila, A. veronii 

biovar sobria, A. caviae, A. jandaei, A. schubertii, A. bestiarum, and the unnamed hybridization 

group HG-11 were isolated. The diversity of strains was representative of the degree of water 

pollution. Aeromonas spp. contribute to spoilage of iced freshwater fish, and may pose a public 

health concern for fish handlers. Cooking is sufficient to protect persons ingesting fresh fish. 

Martins et al. (2002) examined a series of clinical and food isolates for production of 

enterotoxin, hemolysin and cytotoxin, and the results are summarized in Exhibit 7.2. These results 

are consistent with those of other current investigators; however, others have reported higher 

correlation between hemolysin and cytotoxin production when hemolysin was detected using 

rabbit erythrocytes. Methods for determining hemolytic and cytotoxic activity of aeromonads has 

not been standardized, and cautious comparisons of between published reports are advised. 

Exhibit 7.2. Putative Virulence Factors Produced by Aeromonas spp. Isolated from 

Clinical and Food Sources 

Source 

Presence of Virulence Factor 

Hemolysin Cytotoxin Enterotoxin 

Clinical Specimens 43.1% 89% 29.4% 

Foods 17.1% 72.7% 18.2% 

From Martins et al., 2002 

A collection of 107 isolates from ready to eat meats and cheese in Italy were examined for 

cytotoxin (Montagna et al., 1998). Cytotoxin activity (Vero cells) was demonstrated in all 

isolates, but cytotoxin activity of A. hydrophila/caviae strains was temperature dependent, with 

100% cytotoxity at 5o C but 98.2% cytotoxicity at 31o C. 

Tsai and Chen (1996) cultured 66 seafood samples and reported that 17 (25.8%) were 

contaminated with Aeromonas spp. Aeromonas hydrophila was cultured from 50% of oysters 

(3.2-5.8 log10 CFU/g), 22.2% of fish fillets (1.9-3.9 log10 CFU/g), and 14.3% of shrimp (2.1-2.7 

log10 CFU/g). No Aeromonas spp. were isolated from 12 fish ball samples. From this survey, 48 

isolates were examined for hemolysin and cytotoxin. Hemolysin was found in 79.2% of isolates 

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(n = 38) and 91.7% of isolates (n = 44) produced cytotoxin. These isolates grew best and 

virulence factors were produced at higher titers at 28o C rather than at 37o C. 

Scoglio et al. (2001) examined 18 strains of Aeromonas isolated from foods for presence 

of virulence factors including caseinase, lipase and lecithinase, DNAse mucinase, elastase, 

hemolysin, cytotoxin, and adherence. Most A. hydrophila (7 strains) and A. caviae (11 strains) 

produced caseinase, lecithinase, lipase, and DNAse, while all strains were negative for elastase 

and mucinase. All but 3 strains of A caviae were cytotoxic in either Vero or HEp-2 cells. 

Adherence was demonstrated in 11 strains. Strains with multiple virulence factors are found in 

seafoods that may have the potential to cause gastrointestinal disease. 

7.1.1.5 Studies Characterizing Virulence Markers in Animal Isolates 

Boynukara et al. (2004) characterized A. sobria strains from animal feces using SDSPAGE. Turkish van cat, chicken, gull, and trout strains were included in the study. 

7.2 Risk Assessment Case-Control Studies 

Hellard et al. (2001) conducted a double-blinded epidemiological study to determine the 

contribution of drinking water to the incidence of gastrointestinal disease in Melbourne, Australia. 

They found no evidence of waterborne disease, even though the water supply was drawn from a 

forest catchment and the chlorination was the only treatment. A similar study conducted in the 

U.S. by Colford et al., (2005). The U.S. study showed no increased risk of acquiring 

gastrointestinal infections from exposures to treated drinking water. 

7.2.1 Laboratory Studies 

Nzeako and Okafor (2002) studied patients with diarrhea in Nigeria during the rainy 

season between April and October. Found A. hydrophila in 13% of patients with the highest 

recovery in children age 6-10 years (6%), followed by children age 11-15 years (3.6%). The 

asymptomatic carriage rate was 1%. Food and drinking untreated water were thought to be the 

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sources of Aeromonas spp. 

The distribution of Aeromonas spp. isolated in Bangladesh are shown in Exhibit 7.2, and 

the presence of toxin genes in clinical and environmental isolates is shown in Exhibit 7.3. 

Exhibit 7.2. Distribution of Aeromonas Strains from Various Sources in Bangladesh 

Source No. 

Isolates 

Number of Isolates (% Positive) 

A. 

hydrophila 

A. bestiarum A. veronii 

biovar 

sobria 

A. caviae A. media A. 

eucrenophila 

A. trota A. jandaei A. species 

(unknown) 

Diarrheal 

children 

115 11 (9.6) 3 (2.6) 21 (18.3) 39 (33.9) 14 (12.2) 1 (0.9) 25 (21.8) 0 (0.0) 1 (0.9) 

Control children 27 1 (3.7) 0 (0.0) 4 (14.8) 11 (40.7) 1 (3.7) 1 (3.7) 6 (22.2) 1 (3.7) 2 (7.4) 

Environment 120 18 (15.0) 0 (0.0) 16 (13.3) 7 (5.8) 2 (1.7) 0 (0.0) 72 (60.0) 4 (3.3) 1 (0.8) 

Total 262 30 (11.5) 3 (1.2) 41 (15.7) 57 (21.8) 17 (6.5) 2 (0.8) 103 (39.3) 5 (1.9) 4 (1.5) 

From Albert et al., 2000 

Exhibit 7.3. Presence of Toxin Genes in Diarrheal Children, Control Children and the 

Environment 

Source No. Isolates alt ast alt + ast 

Diarrheal children 115 19 (16.5) 18 (15.7) 65 (55.7) 

Normal children 27 9 (33.3) 7 (25.9) 6 (22.2) 

Environment 120 20 (16.7) 36 (30.0) 40 (33.3) 

From Albert et al., 2000 

Isolates with both the alt and ast genes were from patients with watery diarrhea; while 

isolates with only the alt gene came from patients with loose stools (Albert et al., 2000). A. 

caviae was found most frequently; however, loose or watery stools also occurred in patients 

whose isolates were A. veronii biovar sobria, A. media, or A. hydrophila. 

7.2.2 Studies in Younger Children 

In a case-control study in Bangladesh in children under the age of five years, potential 

bacterial pathogens were isolated from 74.8% of symptomatic children and 43.9% of controls 

Albert et al., 1999). Aermonas infection did not show seasonality as in temperate climates. 

Aeromonas spp. were isolated from 99 (12.2%) sick individuals and 44 (5.4%) healthy individuals 

(P value 0.0001) (Albert et al., 1999). Aeromonas infections were more common during the first 

two years of life. These results differ from the findings in Thailand where no association between 

aeromonds and diarrhea was found (Echeverria et al., 1989). 

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Dallal and Moezardalan (2004) conducted a case-control study of diarrhea in children in 

Iran. Isolation rate was significantly higher from cases than controls. Aeromonads were isolated 

from 14 of 310 ( 4.5%) of cases of diarrhea in children. 

7.2.3 Field Studies 

Studies of military troops deployed in various regions of the world have not shown 

significance incidence of gastrointestinal disease caused by aeromonads related to their 

deployment (Haberberger et al., 1991; Oyofa et al., 1995). Joseph (1996) compiled the available 

data on gastrointestinal disease causation for Aeromonas spp. and found widely differing rates of 

asymptomatic carriage. He concluded that while some strains of aeromonads are undoubtedly 

capable of causing diarrhea in susceptible hosts, the factors contributing to their pathogenesis are 

not sufficiently characterized to permit reliable predication based upon presence and expression 

of known virulence factors. 

7.2.4 Animal Studies 

Pretreatment of experimental animals with antibiotics has been suggested to render them 

susceptible to gastrointestinal infection following intragastric inoculation with Aeromonas spp. 

(Haberberger et al., 1991). Kelleher and Kirov (2000) found that while intestinal colonization 

occurred, pathology characteristic of Aeromonas infection was absent in Norwegian rats. The 

clindamycin-treated rat model is not suitable for studies on the pathogenesis of aeromonads, and 

no other animal model has been identified. 

Krzyminska et al. (2001) reported serial passage in mice-enhanced virulence of A. caviae 

isolated from diarrheal patients. Adhesion and siderophore properties increased, and the LD50 

decreased with some strains. An increase of cross-reactive cholera-like toxin occurred with serial 

passage in mice. These studies confirm that A. caviae strains may be virulent, and demonstrate the 

adaptive nature of pathogenic aeromonads. The host factors responsible for pathogenic 

enhancement were not identified. These data suggest that human-to-human transmission of A. 

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caviae would likely result in more severe gastrointestinal infections that would occur when 

environmental starins are ingested. 

7.2.5 Typing Studies 

Demarta et al. (2000) showed that strains isolated from the household environment 

matched strains isolated from stools of children age 4 months to 6 years with gastroenteritis by 

using ribotyping. It was not determined whether the patients contaminated the environment or 

vice versa. The high rate of asymptomatic carriers and the wide distribution of potentially 

pathogenic strains in the environment suggest that host susceptibility factors determine the course 

of infection. 

7.3 Risk Factors 

Aeromonas spp. are infrequently cultured from treated drinking water. When detected, 

they are usually present at concentrations < 10 CFU/100 mL. Concentrations in groundwater are 

also low, typically < 1 CFU/100 mL; however, wells and associated distribution systems may 

become colonized, and well water may contain > 200 CFU/100L (Holmes et al., 1996). 

Kilb et al. (2003) found that aeromonads colonizing rubber-coated valves in drinking 

water distribution systems were released into drinking water at a cell density of 17 CFU/cm2

. 

However, sloughing of biofilms of this cell density are unlikely to constitute a public health risk 

of infection, considering humans regularly ingest foods containing aeromonads at 2-5 log10 

CFU/g without ill effects. 

Ambient water has been shown pose a risk of food contamination. Pianetti et al. (2004) 

found Aeromonas spp. in 14 of 52 samples of irrigation water in Italy at concentrations from 2 to 

4 log10 CFU/mL. This suggests that irrigation water may be a source of contamination for 

vegetables, and that because some strains produced virulence factors, they could serve as a source 

of infection to humans. 

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7.4 Potential for Human Exposure 

Point-of-use (POU) devices on household taps increased the number of bacteria ingested 

in drinking water due to amplification in activated carbon (Chaidez and Gerba 2004). Aeromonas 

was rarely isolated and when found, it was present in low numbers (10 CFU/mL in tapwater; 29.5 

CFU/mL in POU effluent). There is no evidence to implicate Aeromonas as a cause of illness 

from consumption of treated water (van der Kooij 2003). 

Because aeromonads are capable of growing to large numbers in foods, it is theoretically 

possible that food-poisoning could result from colonization of the intestinal tract or from 

ingestion of pre-formed toxins. However, experience provides little evidence for the role of preformed toxins in causing gastroenteritis following ingestion of foods contaminated by 

aeromonads (Kirov 1993a). Many strains that grow at low temperatures and produce enterotoxin 

in culture media do not grow at 37o C, so they pose little threat to humans (Todd et al., 1989). 

Since aeromonads are sensitive to heat and most food preservation methods, 

recontamination after cooking is suspected as the cause for outbreaks resulting from ingestion of 

cooked products (Marino et al., 1995). Ingestion of raw foods and untreated water remain the 

primary modes of transmission for aeromonads causing foodborne and waterborne illness. 

Host factors seem to be related strongly to diarrheal disease caused by aeromonads 

(Schiavano et al., 1998). While humans are constantly exposed to aeromonads from various 

environmental sources, manifestations of disease occur infrequently and only in persons with 

predisposing conditions. 

7.5 Risk Management 

Occurrence and growth of aeromonads in drinking water distribution systems is dependent 

upon access from ambient waters, temperature, and nutritional parameters conducive to growth, 

and presence of a disinfectant residual to suppress microbial populations. Aeromonas spp. are 

rarely detected in distribution system water below 14o C and with free chlorine residuals above 

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0.2 mg/L. Limitation of available organic carbon restricts nutrient thereby limiting growth 

potential. 

Factors for control of access and growth of aeromonads include: 

$ operating filter beds at optimal efficiency; 

$ maintaining a free chlorine residual of at least 0.2 mg/L in all parts of the distribution 

system; 

$ monitoring and maintaining the microbiological quality of GAC used in treatment 

processes; 

$ control of biofilms by preventive maintenance, e.g. hydrant flushing; and 

$ ensuring disinfection during maintenance and new construction. 

These actions will ensure adequate treatment to minimize the opportunity for release of 

aeromonads into distribution water and to prevent colonization of biofilms. 

7.6 Summary 

Differentiation of pathogenic from non-pathogenic strains of Aeromonas spp. is 

difficult to impossible because the presence of traits typically associated with virulence are 

widespead among environmental strains (Thornley et al., 1997). Several authors have suggested 

that the presence of aeromonads capable of producing virulence factors in drinking water is a 

threat to public health (Bondi et al., 2000; Handfield 1996; Kuhn et al., 1997). However, the lack 

of correlation between presence of virulence factors and the ability of strains to cause 

gastroenteritis in humans suggests that the potential is not realized in any epidemiologically 

meaningful way. Demonstration of virulence genes is not sufficient evidence to ascribe 

pathogenicity since Pin et al. (1995) and Schiavano et al. (1998) have found virulence genes in 

isolates from healthy humans. 

The presence of genes does not correlate with expression of gene products (Bondi et al., 

2000). The presence of virulence factors does not correlate with manifestations of disease in 

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animals or humans (Bondi et al., 2000). There is no predictive value in genetic, molecular, or 

phenotypic characteristics for Aeromonas (Bondi et al., 2000). There is no significant correlation 

between hybridization group and virulence properties of individual strains (Marino et al., 1995). 

The public health significance of finding virulence genes in isolates from drinking water 

must been interpreted carefully since the relationship between the presence of virulence factor 

genes and the ability of these strains to cause disease has not been firmly established (Sen and 

Rodgers 2004). Host factors may play an important role in determining susceptibility to infection 

(Sen and Rodgers 2004). 

Various types of foods are considered possible sources of potentially pathogenic 

aeromonads (Abeyta et al., 1986; Altwegg et al., 1991a). Unfortunately, no reliable methods 

presently exists for differentiation of pathogenic and non-pathogenic strains that are predictive for 

the ability of a strain to produce gastrointestinal disease in humans (Kingombe et al., 2004). 

Because differences between clinical strains have not been demonstrated (Kingombe et al., 1999), 

the presence of aeromonads in foods carries potential public health risk to susceptible hosts. 

The vast diversity among aeromonads suggests that predictive models based upon a few 

strains are not necessarily reliable (Merino et al., 1995). This was certainly the case in the human 

feeding study of Morgan et al. (1985), where high number of aeromonads failed to produce illness 

by ingestion. 

Application of the ILSI model to risk assessment of Aeromonas spp. results in estimates of 

potential risk if infection from various sources that are unsupported by clinical and 

epidemiological data. 

8.0 Water Treatment 

Aeromonads have been reported in water distribution systems around the world (Havelaar 

et al., 1987; van der Kooij 1988; Hanninen and Siitonen 1995; Huys et al., 1997). Regrowth in 

distribution system water has been demonstrated to be a function of water temperature and 

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availability of assimilable nutrient. Aeromonads are known to colonize the surfaces in distribution 

system producing biofilms (Burke et al., 1984a). Most drinking water treatment process are 

capable of reducing the density of aeromonads from 3-4 log10 CFU to less than 1 CFU/100 mL 

(WHO 1996). 

8.1 Conventional Water and Wastewater Treatment 

In a survey of water treatment plants in Belgium, Meheus and Peeters (1989) found that 

the mean reductions for stages of treatment was 30-60% for flocculation/sedimentation, 70-90% 

for rapid sand filtration, 80-90% for GAC, and 98-100% for slow sand filtration. 

Hyperchlorination in conjunction with direct filtration removed 99-100% of aeromonads.

 Aeromonas cell densities in surface water reach 3-4 log10 CFU/100mL in summer months 

in Indiana (Chauret et al., 2001). Counts from water filtered through GAC ranged from < 1 to 490 

CFU/100 mL. Aeromonads were not detected in treated water plant effluent, however A. 

hydrophila was detected in 7.7% of biofilm samples from the distribution system. The issue of 

regrowth vs. distal contamination as the source of aeromonads in biofilms is difficult to assess. 

El Taweel and Shaban (2001) sampled eight water treatment plants in Egypt for removal 

of indicator bacteria and Aeromonas. The number of aeromonads at plant intakes (raw water) 

ranged from 2.26-5.5 log10 CFU/100 mL. The number of aeromonads in finished water ranged 

from below detection limit to 47 CFU/100 mL in two or eight plants (mean 7-11 CFU/100 mL) 

and no aeromonads were detected in finished water produced by the other six plants. Because 

aeromonads exeeded coliforms in raw water contaminated with sewage, they are considered 

pollution indicators that are complementary to coliform monitoring (El Taweel and Shaban 2001). 

8.1.1 Slow Sand Filtration 

Slow sand filtration effectively removes 98-100% of aeromonads from raw water (Meheus 

and Peeters 1989). 

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8.1.2 Activated Sludge Plant/Trickling Filter Plant 

Aeromonas spp. are present in raw sewage and the effluent of wastewater treatment plants 

(Stecchini et al., 1994). Overall reduction of aeromonads varies by treatment process, with 93% 

reduction in stabilization ponds (Monfort and Baleux 1990; Boussaid et al., 1991), and 99.9% 

reduction in activated sludge plants (Poffe and Op de Beeck 1991), and 98% in trickling filter 

plants (Poffe and Op de Beeck 1991). A. caviae is the dominant aeromonad in raw sewage and 

sewage effluents (Stecchini et al., 1994), but A. hydrophila has been found at 6 log10 CFU/g in 

partially dried sludge (Poffe and Op de Beek 1991). Ramteke et al. (1993) observed high levels of 

A. caviae in sewage pollution river water, and they proposed A. caviae as an indicator of fecal 

pollution. 

8.1.3 Treatment Plants and Distribution Systems 

Holmes and Nicolls (1995) and Chauret et al. (2001) reported the presence of Aeromonas 

in pipe biofilms in drinking water supplies. Biofilms are defined as a consortium of 

microorganisms within extensive polymer matrices (Costerton et al., 1987). Organisms forming 

biofilms typically produce extracellular polysaccharides and accumulate in hydrated structures on 

surfaces. It is estimated that 99% of all bacteria in natural environments exist in biofilms. The 

dynamic nature of biofilms suggested that a mechanism for cell-to-cell communication termed 

“quorum sensing” may exist to regulate attachment and maturation of biofilms (Williams and 

Stewart 1994). In many bacteria, N-acylhomoserine lactone (AHL) interacts with a sensor 

regulator of transcription. A. hydrophila possesses the AHL-dependent quorum-sensing system as 

an expression of the ahyRI locus (Lynch et al., 2002). Sha et al. (2005) showed that aeromonads 

used quorum sensing and that over-production of AHL was associated with reduced virulence. 

Huys et al. (1995) showed that water plant isolates were dominated by A. hydrophila 

(38.8%), A. caviae complex (22.7%), and A. sobria complex (16.7%), with HG-3, HG-5A/B, and 

HG-8 as the representative genomospecies. 

MacDonald and Brozel (2000) evaluated biofilm production in a simulated recirculating 

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cooling water system using fluorescent in situ hybridization with rRNA-targeted oligonucleotide 

probes for Pseudomonas, Shewanella, Aeromonas, Proteobacteria, and Eubacteria. Aeromonads 

were not detected in resulting biofilms. While presence of aeromonads in distribution system 

biofilms has been demonstrated, the conditions leading to their colonization are unknown. 

Kilb et al. (2003) examined biofilm formation on rubber-coated valves in drinking water 

distribution systems. Citrobacter spp. were determined to be the predominant bacteria colonizing 

valves. However, in the 21 biofilms samples examined, 2 contained Aeromonas spp., 1 contained 

A. hydrophila at a cell density of 17 CFU/cm2

, and 1 had an unidentified Aeromonas spp. 

Aeromonas spp. adhere to distribution system material such as stainless steel, copper, 

polybutylene after exposures as short as 1-4 hours (Assanta et al., 1998). Development of a 

mature biofilm is controlled through quorum sensing, a chemical signaling system by which 

bacteria poll cell density and regulate the expression of genes at high cell densities (Kirov 2003; 

Sha et al., 2005). 

8.1.4 Rapid Gravity Filters 

Rapid gravity filters remove 70-90% of aeromonads from raw water, while 

hyperchlorination in conjunction with filtration results in 99-100% reduction (Meheus and Peeters 

1989). 

8.2 Disinfection/Inactivation 

Aeromonas spp. are susceptible to free chlorine at concentrations used in water treatment, 

though failure to maintain a chlorine residual in the distribution system may result in growth of 

aeromonads (Burke et al., 1984a; van der Kooij and Hijnen 1988). 

Several studies on response to disinfection have been published (Cattabani 1986; Knochel 

1991; Ozbas and Aytec 1994). Environmental strains resemble E. coli in their response to 

chlorine exposure at 0.5 mg/L; however, strain differences in chlorine response have been 

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demonstrated. Chlorine activity was higher at pH 6 than at pH 8. Some strains survive treatment 

at 0.3 mg/L (Cattabani 1986) and at 0.5 mg/L (Ozbas and Aytac 1994), while Ozbas and Aytac 

(1994) reported survivers at chlorine concentrations up to 2.5 mg/L after 30 min. exposure. 

Chlorine concentrations of 0.1, 0.2 and 0.5 mg/L at pH 6,7, and 8 and temperature of 4, 

21, and 32o C were used to determine disinfection response of Aeromonas spp. Strains isolated 

from the environment were inactivated within 1 min., but strains isolated from treated water 

distribution systems demonstrated less sensitivity to chlorine than the other strains. 

Aeromonads are isolated from chlorinated waters, including distribution system water 

where colifoms counts are below the detection limit, leading to the suggestion that they are 

moderately resistant to chlorine (LeChevallier et al., 1983). Later studies do not support that 

aeromonads are any more resistant to chlorine than other waterborne bacteria, e.g. E. coli, 

Pseudomonas spp. Acinetobacter spp., etc. Knochel (1991) reported that aeromonads had a 

slightly higher susceptibility to combined chlorine compared to other water bacteria. Cattabiani 

(1986) reported that aeromonads were more resistant to chlorine that Vibrio spp. Three of four 

strain of aeromonas were sensitive to chlorine at 0.625 mg/L at pH 6.5 and temperature of 25o C. 

All aeromonads experienced >4 log10 reduction in 1 min. at a chlorine concentration of 5 mg/L 

(Palumbo and Buchanan 1988). 

Because Aeromonas spp. are pathogenic for fish, their presence in aquaculture waters is of 

great economic concern (Bomo et al., 2003). These investigators used slow sand filtration to 

remove and inactivate A. hydrophila and other bacteria in fish pond effluents. A bacterial load of 

8 log10 CFU/mL was applied to course and fine sand filters, and filter effluents were cultured to 

determine recovery. For course sand filters, a 4 log10 removal occurred on day 1, gradually 

decreasing to 3 log10 removal on day 3, followed by a progressive increase in removal to 5 log10 

by day 7, and 6 log10 by day 15. For fine sand filters, a 4 log removal occurred between day 1-4, 

followed by a increase in removal to 6 log10 by day 7, and 7 log10 by day 15. Sand filtration 

appears to be an effective means of removing aeromonads from surface waters during water 

treatment. 

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Lee and Deininger (2000) used ozonation to reduce the heterotrophic plate count (HPC) of 

raw water from 1.33x105 to 13 CFU/mL after treatment. The average reduction was 2.6 log10 

following ozonation. When GAC was use after ozone treatment, the HPC increased to the levels 

of raw water. Gram-positive bacteria and mycobacteria survived ozone treatment better than 

gram-negative bacteria. Since aeromonads are gram-negative bacteria that comprise a significant 

proportion of the HPC of raw water, aeromonads would be susceptible to ozonation. 

8.2.1 Disinfection 

Gavriel et al. (1998) studied the response of Aeromonas to chlorine and chloramine 

disinfection and showed that Aeromonas spp. are more susceptible to chlorination than E. coli. 

The reported inactivation studies by various treatments are shown in Exhibits 8.1 to 8.3. 

Exhibit 8.1. Inactivation of Aeromonas hydrophila by UV Irradiation 

Strain Water UV dose (mWs/cm2) Reduction (%) 

A. hydrophila PBW - 2.5 99 

A. hydrophila ND 3 90 

A. hydrophila ND 8 99 

From Gerba et al., 2003 

PBW, phosphate buffered water, ND, no data 

Exhibit 8.2. Inactivation of Aeromonas hydrophila by Chlorine Dioxide 

Strain Water ClO2 

(mg/L) 

Temp (o C) pH Time 

(min) 

Reduction (%) CT99 (mg-min/L) 

Ah M800 BDF 0.2 2 " 2 8 ND ND 0.11-0.14 

Ah M800 BDF 0.2 2 " 2 8 ND ND 0.04-0.14 

From Gerba et al., 2003 

Ah, Aeromonas hydrophila; BDF, buffer demand free; ND, no data 

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Exhibit 8.3. Inactivation of Aeromonas hydrophila by Chlorine 

Strain Water Free Chlorine 

(mg/L) 

Temp (o 

C) 

pH Time 

(min) 

Reduction (%) CT99 

(mg-min/L) 

Ah TW11 CDF 0.2 21 6 7 99 1.4 

Ah TW27 CDF 0.2 21 6 1 99 0.2 

Ah 10693H SDW 0.14 10 ND 5 99.999 ND 

Ah 10693H SDW 0.55 20 ND 5 99.999 ND 

Ah 10693H SDW 0.60 30 ND 5 99.999 ND 

Ah 10693H SDW 0.52 37 ND 5 99.999 ND 

Ah clinical Tap 0.05 5 7.8 20 90 1.0 

Ah clinical Tap 0.3 5 7.8 20 90 6.0 

From Gerba et al., 2003 

Ah, Aeromonas hydrophila; CDF, chlorine demand free; SDW, sterile distilled water; ND, no data 

8.2.2 Inactivation 

All treatments currently used by public drinking water supplies effectively control 

aeromonads in distribution water to a level that does not represent a threat to public health. 

8.2.3 Distribution Systems 

Municipal drinking water supplies may contain aeromonads in relative high levels, 

particularly in the distal ends of the system (Havelaar et al., 1990). Concentrations from <1 to 860 

CFU/100 mL have been reported (Knochel and Jeppesen 1990; Havelaar et al., 1990). Aeromonas 

counts are higher in summer months and virtually undetectable with water temperatures drop 

below 14o C (Holmes et al., 1996). Control of aeromonads in distribution water may be achieved 

by controling biofilm production and maintaining a disinfectant residual. 

 Aeromonas hydrophila was placed on the EPA Contaminant Candidate List in 1998, 

establishing it as a priority for Agency drinking water research, including assessment of 

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occurrence in public drinking water supplies under the Unregulated Contaminant Monitoring 

Regulation (UCMR) (USEPA, 1999; USEPA, 2002). Under the UCMR Screening Survey, 293 

distribution systems across the United States were monitored for aeromonads over a twelve month 

period using EPA Method 1605 for sample analysis (Best and Frebis, 2005). Other analyses 

conducted concurrently with detection and enumeration of aeromonads included pH, water 

temperature, turbidity, and free and total disinfectant residual. Aeromonads were detected in 14% 

of samples, with water systems serving more than 10,000 customers having a detection rate of 

11% and water systems serving less than 10,000 customers having a detection rate of 17%, 

suggesting that aeromonads may be present more frequently in smaller water systems. Altogether, 

32 of 42 (76%) water supplies observed 3 or fewer detections out of 18 total samples collected. 

In this study, 78% of positive samples contained 10 CFU/100 mL or less. No marked seasonal 

variation was observed. Aeromonads were more likely to occur in systems using ground water, 

rather than those using surface waters. No apparent differences were found in detection rates in 

high flow vs. low flow parts of the distribution systems, and there was no correlation between 

detection of aeromonads in areas of the distribution systems and low disinfectant residual. While 

aeromonads occur in drinking water distribution systems, there have been no reported outbreaks 

of waterborne disease associated with aeromonads. 

8.2.4 Survival 

Water temperature, available nutrient and disinfection residual affect the regrowth 

potential of Aeromonas in drinking water distribution systems (Burke, et al., 1984a; Holmes et al., 

1996). A. hydrophila can survive and establish biofilm up to 0.6 mg/L of monochloramine 

(Mackerness et al., 1991). Holmes et al. (1996) reported that aeromonads persisted in 10% of pipe 

lengths after disinfection with 1 mg/chlorine. Freely suspended cells are susceptible to this level, 

but elevated levels are required to destroy Aeromonas in biofilm. 

Bomo et al. (2004) investigated biofilm formation on stainless steel, polyvinyl chloride 

and glass surfaces using recycled water systems, and PVC an stainless steel using potable water 

systems. Biofilm formed on all materials at 1-2 log10 CFU/cm2

. Culture methods proved 

unreliable for detecting aeromonads in water samples, suggesting that molecular methods may be 

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required in addition to culture methods for characterization of Aeromonas populations in 

distribution systems. None of the aeromonads were identified as potential human pathogens. The 

public health implications of aeromonads in drinking water distribution systems are unknown. 

8.3 Summary 

Aeromonads have been reported in water distribution systems around the world (Havelaar 

et al., 1987; van der Kooij 1988; Hanninen and Siitonen 1995; Huys et al., 1997). Most drinking 

water treatment processes are capable of reducing the density of aeromonads from 3-4 log10 to 

less than 1 CFU/100 mL (WHO 1996). In a survey of water treatment plants in Belgium, Meheus 

and Peeters (1989) found that the mean reductions for stages of treatment was 30-60% for 

flocculation/sedimentation, 70-90% for rapid sand filtration, 80-90% for GAC, and 98-100% for 

slow sand filtration. Hyperchlorination in conjunction with direct filtration removed 99-100% of 

aeromonads. Aeromonads are isolated from chlorinated waters, including distribution system 

water where colifoms counts are below the detection limit, leading to the suggestion that they are 

moderately resistant to chlorine (LeChevallier et al., 1983). Later studies do not support that 

aeromonads are any more resistant to chlorine than other waterborne bacteria, e.g. E. coli, 

Pseudomonas spp., Acinetobacter spp., etc. Knochel (1991) reported that aeromonads had a 

slightly higher susceptibility to combined chlorine compared to other water bacteria. Water 

temperature, available nutrient and disinfection residual affect the regrowth potential of 

Aeromonas in drinking water distribution systems (Burke, et al., 1984a; Holmes et al., 1996). 

9.0 Indicators for Occurrence and Treatability 

A. hydrophila has been proposed as an indicator of tropical water quality (Miranda and 

Castillo 1996); however, some investigators contend that Aeromonas spp. are indigenous to the 

aquatic environment and do not correlate with fecal indicators.. Chao et al. (2003) conclude that 

the use of the commonly employed indicators for assessing subtropical water quality is 

questionable, especially for pristine waters such as springs and groundwater. The presence of 

Aeromonas spp. does not correlate with the presence of coliform bacteria in drinking water 

supplies (Fernandez et al., 2000). Aeromonas spp. did not correlate to chlorination levels in the 

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distribution system, but they proposed using Aeromonas spp. as an additional indicator of 

drinking water quality related to presence of biofilm 

9.1 Relationship of Aeromonads to Other Microbial Indicators 

Aeromonads comprise 36% of “atypical” colonies in coliform tests based upon membrane 

filtration and M-Endo broth (Brion and Mao 2000). The ratio of “atypical” colonies to coliphage 

was found to correlate with the level and type of fecal pollution in water; thus, this ratio may 

provide a supplemental indication of watershed quality (Brion et al., 2000). 

Marcel et al. (2002) characterized Aeromonas populations in a tropical estuary in Africa. 

Overall, 63% of water samples contained aeromonads. Aeromonads were most abundant during 

the rainy season when salinity levels were lowest. The highest Aeromonas counts occurred with 

the E. coli counts were high. Counts ranged from < 1 to > 104

 CFU/100 mL. Aeromonads are a 

good supplemental fecal pollution indicator. 

9.2 Summary 

A. hydrophila has been proposed as an indicator of tropical water quality (Miranda and 

Castillo 1996); however, some investigators contend that Aeromonas spp. are indigenous to the 

aquatic environment and do not correlate with fecal indicators. The ratio of “atypical” colonies to 

coliphage was found to correlate with the level and type of fecal pollution in water, thus this ratio 

may provide a supplemental indication of watershed quality (Brion et al., 2000). 

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Aeromonas: Human Health Criteria Document 

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