AEROMONAS HYDROPHILA AND MOTILE AEROMONAD SEPTICEMIAS OF FISH

Rocco C. Cipriano

U.S. Geological Survey, Leetown Science Center, National Fish Health Research Laboratory 

11700 Leetown Road, Kearneysville, West Virginia 25430

FISH DISEASE LEAFLET 68

UNITED STATES DEPARTMENT OF THE INTERIOR

Fish and Wildlife Service Division of Fishery Research Washington, D. C. 20240

2001

Revision of Fish Disease Leaflet 68 (1984), "Aeromonas Hydrophila And Motile Aeromonad 

Septicemias Of Fish,” by R. C. Cipriano, G. L. Bullock, and S. W. Pyle

INTRODUCTION

Aeromonas hydrophila and other motile aeromonads are among the most common bacteria in

freshwater habitats throughout the world, and these bacteria frequently cause disease among

cultured and feral fishes. From descriptions of fish diseases in the early scientific literature, Otte 

(1963) speculated that septicemic infections in fish caused by motile aeromonads were common

throughout Europe during the Middle Ages. Although the bacterial etiology of these early reports 

was inconclusive, the pathology was similar to that observed with red leg disease in frogs, in

which A. hydrophila was identified as the causal organism. Because many bacteria isolated from

fish with hemorrhagic septicemias in fish were often misidentified, it is now recognized that

certain isolations of bacteria ascribed to the genera Pseudomonas, Proteus, Bacillus, Aerobacter, 

and Achromobacter actually belonged to the genus Aeromonas. The exact etiology of disease 

involving aeromonads is complicated by the diverse genetic, biochemical, and antigenic

heterogeneity that exists among members of this group. Consequently, motile aeromonads are

often referred to as a complex of disease organisms that are associated with bacterial

hemorrhagic septicemias and other ulcerative conditions in fishes. 

Although motile aeromonads appropriately receive much notoriety as pathogens of fish, it is

important to note that these bacteria also compose part of the normal intestinal microflora of

healthy fish (Trust et al. 1974). Therefore, the presence of these bacteria, by itself, is not

indicative of disease and, consequently, stress is often considered to be a contributing factor in

outbreaks of disease caused by these bacteria. Such stressors are most commonly associated 

with environmental and physiological parameters that adversely fish under intensive culture. As
one such example, Eisa et al. (1994) have shown that the prevalence of motile aeromonad

septicemia in cultured and wild Nile tilapia (Oreochromis niloticus) was 10.0% and 2.5%

respectively; it was 18.75% and 6.25% in cultured and wild Karmout catfish, respectively. 

Ventura and Grizzle (1987) produced systemic infections more readily among channel catfish

(Ictalurus punctatus) by abrading their skin prior to exposing the fish to the bacterium. These 

researchers further showed that A. hydrophila infected internal organs through the digestive tract 

or through uninjured skin under conditions of crowding (13.1 g of fish/L) and high temperature

(24oC). Such infections did not occur when catfish were held at a lower density (5.2 g of fish/L) 

and temperature (18oC). In another interesting study, Peters et al. (1988) subjected subordinate 

rainbow trout (Oncorhynchus mykiss) to social stresses of cohabitation with dominant cohorts

and then exposed these fish to infection by A. hydrophila. By comparison to their dominant

counterparts, the subordinate trout showed physical evidence of stress based on elevated plasma 

glucose concentrations and increased leukocyte volumes. Following exposure to the pathogen,

the bacterium was also recovered from more organs and with greater prevalence among the

subordinate fish than from their dominant cohorts. In feral fish, other factors may be important

stressors that trigger motile aeromonad septicemias. Toranzo et al. (1989), for example, 

described the occurrence of such an epizootic associated with spawning stress among gizzard

shad (Dorosoma cepedianum) in the Potomac River (Maryland, U.S.A.).

In the United States, motile aeromonads primarily cause disease in cultured warm water fishes:

minnows, bait fishes, carp (Cyprinus carpio), channel catfish (Ictalurus punctatus), striped bass 

(Morone saxatilis), largemouth bass (Micropterussalmoides) and tilapia. The pathogen may also 

affect a variety of cool and cold-water species, but is not necessarily restricted to fresh water 

environments. Rahim et al. (1985) have isolated A. hydrophila from wounds of five species of 

brackish water fish including species; Platosus anguillaris, Lates calcarifer, Epinephelus

megachir, Labeo ruhita and Serotherodon nilotica. Thampuran et al. (1995) have not only

isolated motile aeromonads from raw and processed products of marine fish, but also from

marine fishing grounds, as well. 

TAXONOMY AND CLASSIFICATION

Because the biochemistry, genetics, and serology of the motile Aeromonas taxon are 

heterogeneous, the taxonomic position of this genus has been unstable. Kluyver and van Niel 

(1936) transferred many organisms that were associated with hemorrhagic septicemias in fish 

within the genera Bacillus, Pseudomonas, Proteus, and Aerobacter into the new genus 

Aeromonas. These aeromonads were short, gram-negative, motile bacilli with a single flagellum 

that fermented glucose with or without the production of gas. Snieszko (1957) later divided the 

genus into three species: A. hydrophila, A. punctata, and A. liquefaciens. In Snieszko’s scheme, 

Aeromonas liquefaciens contained most of the fish pathogens. Schubert (1967) confirmed that 

there was enough biochemical similarity to establish the genus Aeromonas, but invalidated 

species-specific distinctions. Later, Popoff and Vernon (1976) demonstrated that the motile 

aeromonads could be classified into two distinct species: A. hydrophila (composed of the 

organisms previously described as A. punctata and A. liquefaciens), and a new species that they 

named A. sobria. Biochemically, A. hydrophila hydrolyzes esculin and ferments both salicin and 

arabinose, whereas A. sobria does not utilize these compounds (Lallier et al. 1981). Motile 
aeromonads may be pleomorphic but generally produce circular, smooth, raised colonies on agar. 

Upon microscopic examination, the bacteria appear as short (0.5 X 1.0 ~m), gram-negative 

bacilli. Phenotypically, motile aeromonads are cytochrome oxidase positive, ferment glucose 

with or without the production of gas, and are insensitive to the vibriostatic agent 0/129 (2,4-

diamino,6,7-di-isopropyl pteridine). In addition, the bacteria produce 2,3-butanediol and reduce 

nitrate to nitrite. Hsu et al. (1984) noted that all (n = 164) of the isolates of motile aeromonads 

that they studied produced acid from fructose, galactose, maltose, mannitol, trehalose, dextrin, 

and glycogen; 99.4% of the strains produced acid from glucose, 98.8% from mannose, and 

98.2% from glycerol. Acid production from other carbohydrates (arabinose, salicin, cellobiose, 

sucrose, and lactose) varied. Shotts et al. (1985) also found that all A. hydrophila complex strains 

hydrolysed albumin, casein, and fibrinogen; most strains also digested gelatin (99.9%), 

hemoglobin (94.3%), and elastin (73.2%), but none of the strains hydrolyzed collagen. Further 

phenotypic differentiation of the seven most commonly isolated motile aeromonads obtained 

from clinical isolates may be accomplished using the criteria of Carnahan et al. (1991) and 

modified by Joseph and Carnahan (1994) as described in Table 1. However, Austin and Austin 

(1989) have shown that A. hydrophila, A. sobria, and A caviae comprise the most predominant 

clinical isolates that are typically associated with fish. 

Table 1: Differentiation of common motile aeromonads isolated from clinical specimens as

described by Carnahan et al. (1991) as modified by Joseph and Carnahan (1994). All

bacterial isolates to this point would be short gran negative, oxidase-positive bacilli that ferment 

glucose and are resistant to the Vibrostatic agent 0129.

Characteristica 

hydrophiA. 

la

bv. sobriaA. veronii bv. veroniiA. veronii A. caviaeA. scubertiiA. janddaei

A. trota

Esculin hydrolysis + - + + - - -

Voges-Proskauer reaction + + + - V + -

Pyrazinamidase activity + - - + - - -

CAMP-like factor (aerobic only) + + + - - V -

Arabinose fermentation V - - + - - -

Mannitol fermentation + + + + - + +

Sucrose fermentation + + + + - - -

Ampicillin susceptibility R R R R R R S

Carbenicillin susceptibility R R R R R R S

Cephalothin susceptibility R S S R S R R
Colistin susceptibilityb V S S S S R S

Lysine decarboxylase + + + - + + +

Ornithine decarboxylase - - + - - - -

Arbutin hydrolysis + - + + - - V

Indole production + + + - + +

H2S productionc+ + + - - + +

Gas from glucose + + + - - + +

Hemolysis 

(TSA with 5% sheep erythrocytes) + + + V + + V

a +, positive for >70% of isolates; -, negative, i.e. positive for <30% of isolates; V, variable; R, 

resistant; S, susceptible.

bMIC (single dilution), 4 μg/mL.
PATHOLOGY

Motile aeromonads cause diverse pathologic conditions that include acute, chronic, and covert

infections. Severity of disease is influenced by a number of interrelated factors, including

bacterial virulence, the kind and degree of stress exerted on a population of fish, the physiologic 

condition of the host, and the degree of genetic resistance inherent within specific populations of

fishes. Motile aeromonads differ interspecifically and intraspecifically in their relative

pathogenicity or their ability to cause disease. Under controlled laboratory conditions, De

Figueredo and Plumb (1977) found that strains of motile aeromonads isolated from diseased fish 

were more virulent to channel catfish than were those isolated from pond water. Lallier et al.

(1981) performed studies on rainbow trout (Oncorhyncus mykiss, formerly Salmo gairdneri) to 

compare the relative virulence of A. hydrophila and A. sobria, as taxonomically described by

Popoff and Vernon (1976). Their results indicated that strains of A. hydrophila isolated from 

either healthy or diseased fish were more virulent than strains of A. sobria. Additionally, A. 

sobria was not isolated from fish with clinical signs of motile aeromonad septicemia (Boulanger 

et al. 1977). Paniagua et al. (1990), for example, collected aeromonad isolates along the River 

Porma, Leon Province (Spain) and found that their isolates grouped within three species; A. 

hydrophila (n=74 strains), A. sobria (n =11 strains), and A. caviae (n = 12 strains). The authors 

additionally observed that 72.02% of A. hydrophila isolates and 63% of A. sobria isolates were 

virulent for fish by intramuscular challenge, but all of the strains of A. caviae were avirulent. 

Pathologic conditions attributed to members of the motile aeromonad complex may include

dermal ulceration, tail or fin rot, ocular ulcerations, erythrodermatitis, hemorrhagic septicemia,

red sore disease, red rot disease, and scale protrusion disease. 

Figure 1: Scale protrusion on 

a carp (Cyprinus carpio) 

caused by Aeromonas 

hydrophila

In the acute form of disease, a fatal septicemia may occur so rapidly that fish die before they

have time to develop anything but a few gross signs of disease. When clinical signs of infection

are present, affected fish may show exophthalmia, reddening of the skin, and an accumulation of
fluid in the scale pockets (Faktorovich 1969). The abdomen may become distended as a result of 

an edema and the scales may bristle out from the skin to give a “washboard” appearance (Figures 

1 and 2). The gills may hemorrhage and ulcers may develop on the dermis. Ogara et al. (1998) 

noted severe eye pathology and heavy mortality among yearling and older rainbow trout

accompanying a severe outbreak of motile aeromonad septecemia. The condition at first affected 

one eye, progressed into the other eye, after which the orbits ruptured causing blindness and 

death. Similarly, Yambot and Inglis (1994) described an acute mortality among Nile tilapia in

which the most apparent clinical signs included an opaqueness in one or both eyes, accompanied 

by exophthalmia and eventual bursting of the orbit. Motile aeromonads were isolated from the 

eyes, liver and kidneys of affected fish. Histopathologically, fish may exhibit epithelial

hyperplasia in the foregut; leptomeningeal congestion in the brain, as well as a thrombosis and

inflammation in the perisclerotic region and corneal epithelium of the eye (Fuentes and Perez

1998). There my also be a severe branchitis, as indicated by leukocytic infiltration and dilation

of the central venous sinus Grizzle and Kiryu 1993). These authors also noted that catfish with

septecemic or latent infections had enlarged nuclei in the branchial epithelium and that there was 

a significant correlation between the presence of these gill lesions and the severity of hepatic

and pancreatic lesions. Rodriguez et al. (1993) further noted that there was an increase in 

bacterial acetylcholinesterase activity in the brain tissue of moribund fish.

Figure 2: Severe distention and

accumulation of ascites in the

abdomen of a goldfish

(Carassius auratus) caused by

Aeromonas hydrophila. Also

note the “washboard” effect of

the dermis caused by the

protrusion of scales from the

body surface. 

Systemic infections were characterized by diffuse necrosis in several internal organs and the

presence of melanin-containing macrophages in the blood Ventura and Grizzle 1988). Internally, 

the liver and kidneys are target organs of an acute septicemia. The liver may become pale or

have a greenish coloration while the kidney may become swollen and friable. These organs are

apparently attacked by bacterial toxins and lose their structural integrity (Huizinga et al. 1979). 

Even when tissue damage in the liver and kidneys is extensive, the heart and spleen are not

necessarily damaged. However, splenic ellipsoids are often centers of intense phagocytic activity
by macrophages. Bach et al. 1978 observed pathological changes in the spleens of fish injected 

with virulent A. hydrophila, whereas fish infected orally showed little or no splenic involvement. 

Bacteria were present within the reticular sheaths of the ellipsoids, where intense phagocytic 

activity by macrophages occurred. Phagocytized bacteria divided intracellularly and

extracellularly and destroyed the endothelial and reticular cells of the ellipsoids. . The lower

intestine and vent, which sometimes protrude from the body, are often swollen, inflamed, and

hemorrhagic. Additionally, the intestine is devoid of food and may be filled with a yellow

mucus-like material.

Chronic motile aeromonad infections manifest themselves primarily as ulcerous forms of

disease, in which dermal lesions with focal hemorrhage and inflammation are apparent (Figures

3 and 4). Both the dermis and epidermis are eroded and the underlying musculature becomes

severely necrotic (Huizinga et al. 1979). Inflammatory cells are usually lacking in the necrotic

musculature, whereas the adjacent epidermis undergoes a hyperplasia that results in a raised

margin. At this stage, the infection has usually become systemic and pinpoint hemorrhages

(petechiae) may occur throughout the peritoneum and musculature. Fish with only cutaneous

infections may have several types of concealed lesions including increased amounts of lipofuscin

and haemosiderin in the liver and spleen; however, most visceral organs were not necrotic

(Ventura and Grizzle 1988). 

Figure 3: Lesion produced by

Aeromonas hydrophila in the

dermis of a channel catfish

(Ictalurus punctatus).

Figure 4: Hemorrhage and

ulcerations caused on the

dermis of American shad
(Alosa sapidissima) by Aeromonas hydrophila.

Aeromonas hydrophila was generally considered to be a secondary invader in red sore disease, in 

which the primary etiological agent was believed to be the protozoan ciliate Epistylis (Rogers 

1971). Recently Hazen et al. (1978b) reexamined the etiology of red sore disease and found that 

A. hydrophila was present in 96% of the initial lesions on fish, whereas Epistylis was present in 

only 35% of such lesions. Furthermore, electron microscopy showed that Epistylis lacked 

structures that produced lytic enzymes and, therefore, could not initiate the development of

lesions. This study strongly suggested that A. hydrophila is indeed the primary etiological agent 

of red sore disease and that Epistylis is a secondary pathogen that rapidly colonizes the dermal

lesions initiated by bacterial proteolytic enzymes. 

In frogs and other amphibians, A. hydrophila infections cause distention of capillaries on the

ventral surface of the legs and abdomen, giving them the red coloration that is the source of the 

name of the disease; - red leg. Outbreaks of aeromonad septicemias in frogs and warm water 

fishes usually occur in the spring and coincide with an increase in water temperature. Resistance 

to disease is lowered at this time because aquatic organisms are often anemic and have a

substantial decrease in serum proteins resulting from periods of dormancy and starvation that

occurred during the winter. Huizinga et al. (1979) also indicated that rising water temperatures

increased metabolism, decreased overall condition, and stressed the fish. Stressed fish increased 

production of corticosteroids, which in turn increased their susceptibility to infection. 

Motile aeromonads can also cause disease in warm-blooded vertebrates. In immunocompromised human hosts, for example, A. hydrophila may cause septic arthritis, diarrhea,

corneal ulcers, skin and wound infections, meningitis, and fulminating septi cemias (von

Gravenitz and Mensch 1968; Davis 1978). Clinical isolates of A. hydrophila have been obtained 

from retail foods (fish, seafood, raw milk, poultry and red meats) and all isolates had biotypes 

identical to those of enterotoxin-positive strains (Palumbo et al. 1989). The abilitry of these

bacteria to grow competitively at 5

oC may be indicative of their potential as a public health

hazard. However. Most A. hydrophila were reduced to non-detectable levels on catfish filets 

cooked to 70oC (Huang et al. 1993).

VIRULENCE FACTORS

The ability of a pathogen to locate, attach to, and subsequently infect a susceptible host is a

primary step in the development of disease. Consequently, factors produced by motile

aeromonads, which can facilitate contagion, are important elements of bacterial virulence. In

fact, motile aeromonads that have been taken from lesions on diseased fish have been shown to 

have a greater chemotactic response to skin mucus than isolates that were obtained as free-living 

organisms from pond water (Razen et al. 1982). Ascencio et al. (1998) have shown that A. 
hydrophila, A. caviae, and A. sobria can actually adhere to animal cell lines that have mucous 

receptors. These workers also determined that the proportion of A. hydrophila strains which

bound various mucins was significantly greater than the proportions of A. caviae and A. sobria 

that acted similarly. Trust et al. (1980) also indicated that A. hydrophila had adhesive

agglutination characteristics which facilitated attachment to eukaryotic cells. Electron 

microscopy further demonstrated that motile aeromonads produce fimbriae (pili) that facilitate

adhesion but these structures were common to cells regardless of their virulence (delCorral 

1990). The results of the latter study, therefore, suggest that attachment may facilitate contagion

but prevented correlation between haemagglutination, yeast cell co-agglutination and virulence. 

In addition to adhesins, Dooley and Trust (1988) characterized a tetragonal surface protein array 

consisting of a 52 kD protein from virulent isolates of A. hydrophila . This S-layer was also 

reported by Ford and Thune (1991) among motile aeromonads that they isolated from clinically

diseased catfish. The existence of such layers structurally permeate bacterial cell membranes

which generally increases cellular hydrophobicity. The increased surface tension enhances

resistance of the bacterium to complement-mediated serum lysis and phagocytosis by leukocytes. 

Indeed, Gado (1998) noted that virulent aeromonad isolates from Nile tilapia share a common

resistance to the killing effect of tilapia serum. Byers et al. (1986) have also shown that A. 

hydrophila can produce siderophores that confer resistance against the ability of serum

transferrin to inhibit bacterial growth. 

Many studies have attempted to further delineate the virulence mechanisms of motile

aeromonads. Kou (1973) found that many of the virulent, avirulent, and attenuated aeromonads 

that he studied possessed hemorrhagic factors and lethal toxins. The virulent bacteria had

quantitatively more toxic potential than did their avirulent or attenuated counterparts. Olivier et

al. (1981) indicated that both A. hydrophila and A. sobria produced enterotoxins, dermonecrotic 

factors, and hemolysins. Although both species produced hemolysis on blood agar plates at

30°C, only A. hydrophila did so at 10°C. Because these researchers were working with salmonid 

fish, they suggested that the hemolysis of red blood cells by A. hydrophila at temperatures

comparable to those of the water in which fish live may at least partially account for the

difference in virulence between A. hydrophila and A. sobria. Enterotoxins, haemolysins,

proteases, haemagglutinins, and endotoxins produced by this complex of bacterial organisms

have been the subject of much research (Cahill 1990). However, the composite nature of

virulence observed both inter- and intra-specifically among the motile aeromonads may be best

defined by looking at synergistic relationships between virulence factors. For example, Rigney 

et al. (1978) found that individual injections of either endotoxin or hemolysin alone did not

produce clinical pathology in frogs. However, when both the endotoxin and hemolysin were

injected in the same inoculum, frogs exhibited pathology that mimicked clinical signs of red leg

disease. Thune et al. (1982a) also found that channel catfish were tolerant to injections of 

endotoxin alone at concentrations up to 400 μg endotoxin per 7.2 g of fish. However, an

extracellular cell-free extract of a spent culture medium had an LD50 value of 15.7 μg protein

within 48 h after its injection into 7.2 g fish. Thune et al. (1982b) later showed that this extract 

contain was proteolytic but not hemolytic. Two proteases were further refined from this extract;

one was heat labile and had an LC50 concentration of 18 μg of protein per gram of fish, and the 

other was heat stable and had an LC50 concentration of 3 μg of protein per gram of fish. Allan 

and Stevenson (1981) also found hemolytic and proteolytic activities in crude extracellular

preparations from A. hydrophila. These researchers indicated that aeration increased growth
rates, cell yield, and the amount of proteolytic activity. Proteolytic activity was reduced,

however, when cultures were incubated at 37°C. Furthermore, extracts from a protease deficient 

mutant were more toxic to fish than were similar extracts from isogenic wild strains. Because the 

extract from the protease deficient mutant had a fivefold increase in hemolytic activity, Allan

and Stevenson concluded that hemolysin, not protease, was the principal virulence factor of A. 

hydrphila. The results of other studies, however, might caution against such conclusions. 

Although Rogulska et al. (1994) found that haemolytic activity was high in 93% of very

pathogenic strains of A. hydrophila and the activity was also also high in 87.5% of pathogenic A. 

sobria. Measurement of hemolytic activity alone was not the decisive characteristic in terms of

isolate virulence because even some (15%) avirulent A. hydrophila isolates were hemolytic. 

Additional measurements showed that a combination of high hemolytic and high proteolytic

activity, which was detected in 90% of virulent A. hydrophila strains and 87.5% of the A. sobria

strains, may be a better measure of total virulence. Only one non-pathogenic strain of A. 

hydrophila had high levels of both activities. Rodriguez et al. (1992) have purified a

metalloprotease, a serine protease and a haemolysin from culture supernatants of A. hydrophila. 

Each of these factors have lethal activity for rainbow trout. The metalloprotease had a molecular 

weight of 38 kd, was stable at 56oC for 10 min, had no cytotoxic activity but produced an LC50

value of 150 ng/g of trout. The serine protease had a molecular weight of 22 kD, was also stable 

at 56oC for 10 min, possessed cytotoxic activity and also had an LC50 value of 150 ng/g of trout. 

The haemolysin was alpha -hemolytic and had a molecular weight of 68 kD and produced an

LC50 value of 2 μg/g of trout. The haemolysin was stable at 56oC deg for 20 min and at 60oC for 

10 min. Because the haemolysin possessed esterase activity on beta -naphthyl acetate,

Rodriguez et al. suggested that the compound may be distinct from either alpha - or beta –

haemolysin. Casc’on et al. (2000) have cloned a gene encoding elastolytic activity, ahyB, from

Aeromonas hydrophila. AhyB is synthesized as a pre-proprotein which is further processed into 

a mature protease and a C-terminal propeptide. The protease hydrolyzed casein and elastin and 

showed a high sequence similarity to other metalloproteases. Results of a recent molecular

analysis, which was conducted by Zhang et al. (2000) to underscore genetic differences between

virulent and avirulent isolates, confirms that virulence among the motile aeromonads depends

upon a multiplicity of factors. Their results showed that 22 DNA fragments were present in most 

of the virulent strains and that these genes encoded for five known virulence factors of A. 

hydrophila including haemolysin (hlyA), protease (oligopeptidase A), outer-membrane protein

(Omp), multidrug-resistance protein and histone-like protein (HU-2). These same fragments

were mostly absent in the avirulent isolates that were examined. 

IDENTIFICATION AND DIAGNOSIS

Presumptive diagnosis of A. hydrophila may be based on the species of fish affected, the past 

disease status of those fish, and the presence of clinical signs of disease. However, bacteria must 

be isolated and identified biochemically to provide a definitive diagnosis. Either tryptic soy agar

(TSA) or brain-heart infusion agar (BRIA) is a suitable medium for the primary isolation of

motile aeromonads from diseased fish. Because mixed bacterial infections are common in fish

affected with hemorrhagic septicemias, it is often difficult to isolate pure cultures of a single

species of motile aeromonads from clinically diseased tissues. In order to facilitate the recovery

of motile aeromonads upon primary isolation, Shotts and Rimler (1973) designed a differential
medium for selective isolation of motile aeromonads. This medium, termed R-S agar, is prepared 

by dissolving the following ingredients (grams) in distilled water to a volume of 1 liter: L-lysine 

hydrochloride (5.0), L-orninthine hydrochloride (6.5), L-cystine hydrochloride (0.3), maltose

(3.5), sodium thiosulfate (6.8), bromothymol blue (0.03), ferric ammonium citrate (0.8), sodium

deoxycholate (1.0), novobiocin (0.005), yeast extract (3.0), sodium chloride (5.0), and agar

(13.5). The mixture is constantly stirred, heated to boiling for 1 min, and brought to pH 7.0. The 

medium is cooled to 45°C, dispensed into sterile petri dishes, and can be refrigerated in plastic 

sleeves until used. 

Figure 5: Growth of yellow colonies that are

charcacteristic of Aeromonas hydrophila after

incubation on Rimler-Shotts agar.

After inoculation, R-S agar should be incubated at 37°C for 24 to 48 h, to ensure optimal

differentiation of bacteria. Colonies of motile aeromonads are yellow; those of Pseudomonas, 

Escherichia, and Enterobacter are green; and those of Edwardsiella are green with black centers. 

Although Proteus vulgaris and some Citrobacter are also yellow on R-S agar, these colonies 

have black centers that indicate the bacteria are producing hydrogen sulfide gas. 

Although R-S agar has facilitated primary isolation of motile aeromonads, yellow colonies

should not be accepted as a basis for definitive diagnosis. Aeromonas salmonicida will also 

produce yellow colonies on R-S agar, but unlike the motile aeromonads growth of this bacterium 

is inhibited at 37°C. Davis and Sizemore (1981) also indicated that R-S agar supported the

growth of a limited number of yellow colonies whose DNA homology ratios were inconsistent

for the genus Aeromonas. Many of these isolates, especially those obtained from low-salinity 

sites, were also identified by the API 20E system as A. hydrophila. Therefore, additional

biochemical tests should be performed on isolated colonies that have been cloned from purified 

colonies on either TSA or BHIA. It is important to note that reactions inconsistent for the species 

can occur if one uses colonies picked directly from from the differential R-S agar medium. For 

example, Overman et al. (1979) found that 8% of the isolates they examined were cytochrome 

oxidase negative when taken from differential media. 

Glucose fermentation is a critical reaction that differentiates the motile aeromonads from species

of Pseudomonas (Bullock 1961). Bacteria are inoculated into two tubes of oxidation

fermentation (OF) basal medium supplemented with 1% glucose. The medium in one tube is

overlaid with a plug of sterile petrolatum and both tubes are incubated at 25°C for 24 to 48 h. 
Results are interpreted as follows: yellow coloration in both tubes indicates acidic fermentation

of glucose typical of Aeromonas, whereas yellow coloration only in the tube without petrolatum

indicates oxidation of glucose characteristic of Pseudomonas. A single-tube modification of this 

test was described by Walters and Plumb (1978), in which a tube is prepared with twice the

amount of glucose medium contained in the two-tube test. After incubation, a yellow coloration

throughout the tube indicates fermentation, whereas a yellowish coloration only at the top of the 

medium indicates oxidation. Motility can also be determined by examination for diffused

bacterial growth away from the origin of inoculation. Production of gas is evidenced in either test 

by the formation of bubbles in the medium. Although most strains of A. hydrophila produce gas 

during the fermentation of glucose, some motile aeromonads isolated from diseased fish are

anaerogenic--that is, they do not generate gas (Ross 1962). 

Because motile aeromonads are ubiquitous and have considerable antigenic diversity,

serodiagnostic identification is not reliable. Agglutination procedures, fluorescent antibody tests

(Eurel1 et al. 1978, Soltani and Rabani Khorasgani 1999), and immunoenzyme procedures

(Lewis 1981, Mishra 1998) have been adapted for use within homologous antigen/antibody

systems. These assays may accurately detect strains of motile aeromonads against which

homologous detection antibodies were developed, but detection of heterologous strains makes

serodiagnostic identification impractical. Fliermans and Hazen (1980) found that a total of three

different antisera to A. hydrophila gave a positive fluorescent antibody reaction with only 27.5% 

of the A. hydrophila isolates tested. Therefore, the lack of an effective polyvalent antiserum

specific for A. hydrophila still limits reliable serodiagnostic detection of this pathogen. Calabrez

and Lintermans (1993) have also developed a polymerase chain reaction (PCR) technique which

has been used to detect strains of A. sobria in water, sediments, and fish. Genetic analysis

suggests that there are presently 14 genomospecies recognized within the aeromonads (Joseph 

and Carnahan 1994) and, consequently a number of species-specific PCR tests would need to be 

developed and run on individual clinical samples in order to achieve accurate diagnosis. 

SEROLOGY AND VACCINATION

Motile aeromonads are one of the most taxonomically and antigenically diverse groups of

bacteria pathogenic to fish. The amount of antigenic diversity inherent within this group is

especially expressed within H and O somatic antigens. Ewing et al. (1961) described 12 Oantigen groups and 9 H-antigen groups. Each group was further divided into a number of

additional serotypes. Chodyniecki (1965) also found a high degree of antigenic diversity among

strains of motile aeromonads obtained from the same population of fish and even from different

organs of the same fish. Monovalent bacterins were initially prepared against A. hydrophila, but 

these vaccines only provided acceptable levels of protection against challenge with a

homologous bacterium. Fish were not immune to infection by heterologous strains of A. 

hydrophila (Post 1966; Schaperclaus 1967). Although some strains of motile aeromonads have 

common somatic antigens (Rao and Foster 1977; Lallier et al. 1981), it has been consistently

demonstrated that a monovalent antiserum could agglutinate only a small percentage of the total

isolates examined. Kingma (1978) produced seven rabbit antisera to heat stable antigens of seven 

isolates. Collectively, these antisera agglutinated only 19.5% of the total number of motile
aeromonad isolates studied. Despite the tremendous degree of serological diversity that exists

within this taxon, Mittal et al. (1980) found that strains of A. hydrophila, which were highly

virulent for rainbow trout, shared a common O-antigen. Furthermore, these bacteria did not

agglutinate in acriflavine, settled after boiling, and resisted the bactericidal activity of normal

mammalian sera. In contrast, low-virulence strains of A. sobria did not share the common Oantigen; these bacteria settled after boiling and were lysed by normal mammalian sera. 

Schachte (1978) also found that the delivery of vaccine by injection or immersion or per os

stimulated differential kinetics of antibody production. However, no significant protection

against natural exposure to heterologous A. hydrophila was afforded to channel catfish

vaccinated by any of the methods. Anbarasu et al. (1998) found that formalin inactivated

vaccines were superior to heat killed preparations, especially when the bacterins were injected

with adjuvants. Thune and Plumb (1982) found that both sac fry and swim-up fry vaccinated by 

immersion in sonicated polyvalent bacterin were protected against challenge with homologous 

bacteria, indicating an early onset of immunocompetence in channel catfish. This study was

important because A. hydrophila causes a severe problem in channel catfish in spring and early

summer, when fry are abundant. The early onset of immunocompetence among channel catfish

fry indicates that immunization, if effective, could be used to reduce outbreaks of A. hydrophila 

infection. Regardless of whether whole cells, freeze-thawed cells, or cell sonicates were used,

Thune and Plumb (1982) further indicated that injection was superior to either immersion or

spray vaccination for developing humoral antibodies. However, cell sonicates evoked the best

antibody response. Sonication may disrupt the cell and allow better processing of certain somatic 

antigens (e.g. – bacterial lipopolysaccharide). Consequently, Baba et al. (1988) were able to

evoke better protection against A. hydrophila among carp that were vaccinated with crude

lipopolysaccharide (LPS) rather than whole-cell, formalin killed vaccine. Immersion of fish in

the LPS vaccine for 2 h at 25oC was more efficacious and less stressful than injection, but

vaccination with crude LPS did not invoke an observed humoral immune response as measured 

by bacterial agglutination, passive haemagglutination and agar gel diffusion tests.

Because of the antigenic diversity that exists among this complex group of organisms, additional

vaccination strategies included research on development of polyvalent vaccines, immunization

against inactivated extracellular toxins (toxoids), and the development of vaccines consisting of

cellular antigens plus toxoid. Liu (1961) noted that the biological activity of extracellular toxins

from motile aeromonads was neutralized by a single antiserum prepared against an isolate of A. 

liquefaciens (synonym for A. hydrophila). He therefore concluded that the motile aeromonads

shared extracellular antigens. Bullock et al. (1972) also indicated that aerogenic and

anaerogenicstrains of A. hydrophila possessed common extracellular antigens. Schaperclaus

(1970) recognized that common carp (Cyprinus carpio) vaccinated against A. hydrophila

developed both agglutinating serum antibodies against cellular antigens and antitoxic activity

against extracellular antigens. In a later study, he found that common carp vaccinated by

intraperitoneal injection of bacteria produced more circulating antibodies than did carp

vaccinated by the oral route (Schaperclaus 1972). Furthermore, vaccination with soluble

extracellular antigens was more efficacious and provided a wider base of protection against

heterologous serotypes than did vaccination with whole-cell antigen. Shieh (1987) found that

Atlantic salmon immunized by intramuscular injections of extracellular protease from A. 

hydrophila were protected from challenge with the homologous and some heterologous isolates
of A. hydrophila. Shieh (1990) repeated these studies and produced similar results when A. 

sobria extracellular protease was used as the immunogen and challenges were conducted with

homologous and heterologous strains of the same bacterium. However, no report was made on

the ability of extracellular protease derived from one of these species to cross protect against the 

other motile aeromonad species. Despite the great amount of research that has been expended to 

develop a safe and efficacious vaccine, there is still no product that has been licensed for use

against the motile aeromonads within the United States.

OCCURRENCE AND RANGE

Motile aeromonads cause diseases wherever bait fishes or warm water or ornamental fishes are 

propagated. To a lesser extent, these bacteria also initiate disease in cold-water species. 

Although diseases associated with motile aeromonads are most severe among fish that are

propagated under conditions of intensive culture, these bacteria may also affect feral fish and are 

common in the intestinal flora of apparently healthy fish (Trust et al. 1974). The bacterium is

ubiquitous and occurs in most fresh water environments. It can be found both in the water

column and in the top centimeter of sediment (Hazen 1979). Motile aeromonads are adapted to 

environments that have a wide range of conductivity, turbidity, pH, salinity, and temperature

(Hazen et al. 1978a). Temperature optimums may depend upon the particular strain under

investigation, but generally range from 25oC to 35°C. Consequently, most epizootics among

warm water fishes in the southeastern United States are generally reported in spring and early

summer (Meyer 1970). 

Pond water, diseased fish, and diseased frogs, as well as convalescent frogs and fishes, may

become reservoirs of infection. Certain algae (Kawakami and Hashimoto 1978) and other

protozoa (Chang and Huang 1981) that are grazed upon by fish, can harbor motile aeromonads. 

In the latter study, Tetrahymena pyriformis was experimentally shown to graze on populations of 

Aeromonas hydrophila. The bacterium, at concentrations of 1x106 cells/mL co-existed with the 

protozoan. If a second bacterial species was introduced, predation by the protozoan increased. In 

reservoir studies, Hazen (1979) found high densities of Aeromonas hydrophila in mats of

decomposing Mypiophyllum spiaatum and, enterically, within largemouth bass, several other

species of fish, turtles, alligators, and snails. The densities of A. hydrophila were highest in the 

water from March to June, although a second peak occurred in October. Mean monthly densities 

of A. hydrophila were positively correlated with incidence of incubation in largemouth bass.

Largemouth bass from thermally altered parts of the reservoir had a 

significantly higher incidence of infection. Kawakami and Hashimoto (1978) found greater quantities 

of A. hydrophila (as many as 103 cells) in algae fed upon by ayu (Plecoglossus altivelis) than 

were present in the water itself where the density of A. hydrophila was generally less than 10 

cells/mL. Osborne et al. (1989) found high densities of motile aeromonads within the

environment during midsummer when sedimentary chlorophyll sub(a) and water temperature

were highest. This also correlated temporally with the highest prevalence of dermally ulcerated

striped mullet (Mugil cephalus) that also contain large concentrations of the bacteria within their

stomachs and on their skin. The authors suggest that mullet graze on bacteria-laden sediment for 

algae and consequently bioaccumulate the pathogen within their guts and on their skin which, in

turn, enhances disease. Thus, the intestinal tract or epidermal abrasions are likely portals of
bacterial entry. Under conditions of stress, it is even likely that some strains of motile

aeromonads that are ordinarily part of the normal gut flora become pathogenic. It is believed that 

infection occurs in winter, when fish are relatively inactive, and that the disease breaks out in

spring. Aquarium fish, which are usually maintained at constant water temperature, can develop

this disease at any time. Rainbow trout appear to be among the most susceptible of the salmonids 

that develop motile aeromonad septicemias. 

METHODS OF CONTROL

Prevention 

Effective hatchery management is the best approach to avoid infections and subsequent

epizootics caused by these bacteria. In water reuse hatcheries, both ozonation (Colberg and

Lingg 1978) and filtration combined with ultraviolet irradiation (Bullock and Stuckey 1977)

effectively eliminate the threat of A. hydrophila. Colberg and Lingg (1978) showed that a

specific microbial oxygen demand was exerted during batch ozonation that caused greater than a 

99% mortality of bacteria within a 60-second contact during continuous flow exposure at 0.1-1.0 

mg ozone per liter. Calesante et al. (1981) eliminated recurrent motile aeromonad mortality

among muskellunge (Esox masquinongy) fry by ultraviolet irradiation of lake and well water

supplies during egg incubation and yolk absorption. 

Motile aeromonad septicemias are generally mediated by stress. Elevated water temperature

(Esch and Hazen 1980), a decrease in dissolved oxygen concentration, or increases in ammonia 

and carbon dioxide concentrations have been shown to promote stress in fish and trigger motile 

aeromonad infections (Walters and Plumb 1980). The monitoring of environmental variables can

therefore enable one to forecast stressful situations and possibly avoid problems before they

arise. Wherever this disease occurs frequently, pond fish should not be handled but transferred

only after water temperature is high enough for fish to be active and feeding normally (Rychlicki

and Zarnecki 1957). Mortalities were reduced dramatically (80-90%) when fish, at the time of

spring transfer, were injected intraperitoneally with 10-20 mg of chloromycetin or by dissolving

10-20 mg chloromycetin in water per kilogram of fish (5-10 mg/lb). In the United States,

however, this drug is not registered for use on food fish nor is it legal to treat fish

prophylactically (in the absence of disease).

It is prudent that managers should avoid introducing fish into their hatcheries that have recently

undergone infection. Shipments of new eggs should be surface disinfected to prevent

contamination of facilities and stocks. Wright and Snow (1975) found that either acriflavine

(500-700 mg/L for 15 min) or iodine as Betadine (100-150 mg/L iodine for 15 min) successfully 

disinfected eggs of largemouth bass (Micropterus salmoides), but neither Roccal nor formalin

was effective. When warm water fishes are held in tanks or hauled in trucks or plastic bags, the 

value of adding disinfectants or antibiotics should be examined. The most promising compounds

include chloramphenicol, oxytetracycline, chlortetracycline, and a mixture of penicillin and

streptomycin added to water at a rate of 10-15 mg/L. Again, it is important to note that the use 

and administration of such prophylaxis must be in compliance with local regulations. 
Treatment 

Oxytetracycline (Terramycin) has been the drug of choice for treating motile aeromonad

septicemias in fishes. The drug is approved for use with pond fishes, channel catfish, and

salmonids. It is administered in feed at a daily rate of 50 to 75 mg/kg of fish for 10 days. Fish 

must be withdrawn from treatment for 21 days before they are stocked or eaten. This treatment 

sometimes produces dramatic results when it is administered for even 2 or 3 days, and is

particularly effective when fish become infected after they have been handled, crowded, or held 

under stress for short periods of time (Meyer 1964; Meyer and Collar 1964).

Furanace, though not registered for use in the United States, is extremely effective against motile 

aeromonads if the affected fish are immersed for 5 to 10 minutes in water containing 1-2 mg/L 

furanace, or by maintaining fish for 1 week in water containing 0.1 mg/L drug. However,

furanace can be toxic to fishes if used improperly (Mitchell and Plumb 1980). 

Chloramphenicol (chloromycetin) was successfully used to treat frogs with red leg disease by

gastric intubation of 3 to 5 mg per 100g of frog for 5 days, twice daily. Chloromycetin, like

oxytetracycline, was effective in treating fish when it was administered orally. However, its use

is prohibited in food fishes and discouraged in other fishes because it is the drug of last resort in 

certain human diseases--e.g., typhoid fever. Indiscriminant use of chloramphenicol can result in

drug resistance and thus reduce the value of the antibiotic in human medicine. Against strains of

motile aeromonads that show multiple drug resistance, piromidic acid administered orally has

been experimentally shown to be more effective than either chloromycetin or oxytetracycline

(Katae et al. 1979). However, this drug is not registered for use on food fishes in the United 

States. 

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