Bacillus cereus

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**Description and significance**

*Bacillus cereus* is a large, 1 x 3-4 µm, Gram-positive,
rod-shaped, endospore forming, facultative aerobic bacterium [2]. It was first
successfully isolated in 1969 from a case of fatal pneumonia in a male patient
and was cultured from the blood and pleural fluid [5]. 16s rRNA comparison
reveals *Bacillus cereus* to be most related to *Bacillus*
anthracis*, the cause of anthrax, and*Bacillus thuringiensis*, an*
insect pathogen used as pesticide [3]. Although they have similar
characteristics, they are distinguishable as *B. cereus* is most
motile, *B. thuringiensis* produces crystal toxins, and *B.*
anthracis*is non-hemolytic [4].*

*B. cereus* is mesophilic, growing optimally at
temperatures between 20°C and 40°C, and is capable of adapting to a wide range
of environmental conditions. It is distributed widely in nature and is commonly
found in the soil as a saprophytic organism [2]. *B. cereus* is
also a contributor to the microflora of insects, deriving nutrients from its
host, and is found in the rhizosphere of some plants [2].

As a soil bacterium, *B. cereus* can spread
easily to many types of foods such as plants, eggs, meat, and dairy products,
and is known for causing 2-5 % of food-borne intoxications due to its
secretion of emetic toxins and enterotoxins [4]. Food poisoning occurs when
food is left without refrigeration for several hours before it is served.
Remaining spores of contaminated food from heat treatment grow well after
cooling and are the source of food poisoning.

In addition, *Bacillus cereus* is an
opportunistic human pathogen and is occasionally associated with infections,
causing periodontal diseases and other more serious infections [5].
Immunocompromised patients are susceptible to bacteremia, endocarditis,
meningitis, pneumonia, and endophthalmitis [6]. Its potential to cause systemic
infections are of current public health and biomedical concerns. Thus, the
genome sequence of *Bacillus cereus* is significant in order to
establish genetic background information for future investigations. Sequencing
its genome is vital to expand understanding of its pathogenicity for treatment
and for the development of antimicrobial drugs. Additionally, since *Bacillus*
cereus*strains are so genetically closely related to*B.
anthracis*, genomic comparisons between the two species are important to the*
study of *B. anthracis* virulence.

*Bacillus cereus* is a 1 x 3-4 µm, rod shaped,
Gram- positive bacterium. Its cell structure consists of an inner membrane and
a thick peptidoglycan which functions to maintain cell shape [10]. The polysaccharide
portion makes up 50% percent of the cell wall and consists of a neutral
polysaccharide composed of N-acetylglucosamine, N-acetylmannosamine (ManNac),
N-acetylgalactosamine and glucose in a molar ratio of 4: 1: 1: 1 [11]. The
acidic portion of the cell wall is characteristic in having a repeating
tetrasaccharide unit [11]. 5% of the cell wall is made up of teichoic acids
consisting of N-acetylglucosamine, galactose, glycerol, and phosphorus in a
molar ratio of 1: 1.4: 1: 1 [11]. The linkage between the polysaccharide and
peptidoglycan is a muramic acid 6-phosphate [11]. The peptidoglycan of
some *B. cereus* strains are unique with only a few oligomers
present, the cross-linked muropeptides are dimmers, and many of the muropeptide
lack the N-acetly group [12]. These distinguishing features affect cell surface
charge which contributes to the attachment of an outer capsule or an S-layer in
pathogenic strains.

Clinical isolates of *B. cereus* have a
glycoprotein S-layer over its peptidoglycan which consists of proteinaceous
paracrystalline arrays and covers the cell surface. The S-layer is involved in
the virulence of *B. cereus* and functions to promote
interactions with human polymorphonuclear leucocytes [13]. It also
enables *B. cereus* to adhere to laminin, type I collagen,
fibronectin, and fibrinogen of the epithelium, and thus has a role in
increasing interaction between *B. cereus* and its host [13]. In
addition, this proteinaceous layer enhances its resistance to radiation.

*B. cereus* is motile by means of flagella and
exhibits two types of motility including swimming and swarming, depending on
the environment. Single cells exhibit swimming motility by means of short
flagellated rods [14]. On the other hand, swarming is a collective movement of
swarm cells with flagellum that is observed to be three to four times longer,
and also forty times more flagellated than single swimming cells [14]. **4.2**
Spore Structure

*B. cereus* spore formation occurs when nutrients
are scarce within the environment and germinate into vegetative cells once they
are available [6]. Therefore, spore structure is important to the survival of
this bacterium. *B. cereus* spores consist of an inner core
surrounded by the inner membrane, and outer cortex surrounded by the outer
membrane with an additional exterior coat [16]. The spore coat is made of
proteins and small amounts of lipids and carbohydrates which contribute to its
resistance to oxidizing agents and chemicals by blocking toxic molecules [15].
In addition, the outer spore structure allows them to be heat and γ-radiation
resistant [15]. Spore germination is commonly in response to L-alanine which
stimulates germination events including hydrations of spores, loss of Ca2+ and
dipicolinic acid, and metabolism [15].

**Metabolism**

*B. cereus* is a facultative aerobe so it can
utilize oxygen as a terminal electron accepter, but also has methods of
anaerobic respiration as a mechanism of energy release. Whole genome sequencing
revealed genes encoding for metabolic enzymes such as NADH dehydrogenases,
succinate dehydrogenase, complex III, non-proton-pumping cytochrome bd quinol
oxidases, and proton-pumping oxidases such as cytochrome c oxidase and
cytochrome aa3 quinol oxidase [17].

In aerobic respiration, reducing equivalents produced from
glycolysis and the Krebs cycle are reoxidized by the electron transport chain,
creating a proton motive force and ATP by ATP synthase [17]. In anaerobic
respiration, *B. cereus* utilizes fermentation to generate
energy. Fermentation recycles NAD+ by reducing pyruvate and produces lactate
and ethanol [17]. ATP is generated by substrate level phosphorylation.

*B. cereus* can metabolize a variety of compounds
including carbohydrates, proteins, peptides and amino acids for growth and
energy. Some of the major products produced from carbon sources such as sucrose
or glucose during anaerobic respiration include L-lactate, acetate, formate,
succinate, ethanol, and carbon dioxide [18]. During nitrate respiration,
nitrate reductase converts nitrate into nitrite which is converted to ammonium
by nitrite reductase [18].

### **Ecology**

*B. cereus* interacts with other microorganisms in
the rhizosphere, the region surrounding plant roots. Plants benefit from the
presence of *B. cereus* since it is capable of inhibiting plant
disease caused by protest pathogens and also enhances plant growth [23]. It
naturally produces the antibiotics zwittermicin A. and Kanosamine which inhibit
the growth of plant pathogen, oomycetes, certain fungi, and a few bacterial
species [19]. The presence of *B. cereus* in the rhizosphere
also increases nodulation of soybean plants by *Bradyrhizobium japonicum* [22].
In addition, *Cytophaga-Flavobacterium* group (CF) benefit
from *B. cereus* by utilizing its peptidoglycan as a carbon and
energy source. CF bacteria attain *B. cereus* peptidoglycan by
hydrolyzing the outer layer [21]. The relationship between the two organisms is
commensal since the growth of *B.cereus* is not affected by the
presence of CF bacteria.

*B. cereus* is a also found in the gut microflora
of invertebrates, and is an intestinal symbiont of arthropods where it exhibits
filamentous growth in sow bugs, roaches, and termites [22]. Arthropods consume
feces or soil with spores or cells of *B. cereus*. This intestinal
stage of *B. cereus*, also known as the *Arthromitus* stage,
involves the attachment of endospore fibers released from a parent cell [22].
After attachment of spores, they begin motile and filament growth stages and
attach to the epithelium [22]. *B. cereus* cells are then
defecated back into the soil where they can continue to grow.

As a ubiquitous bacterium, small amounts are consumed by
humans from foods. Therefore, it is a contributor to the human intestinal
microflora [23]. In addition *B cereus* is widely known to
affect humans by causing food poisoning and infections as an opportunistic
pathogen. Refer to pathology section for further elaboration.

### **Pathology**

*Bacillus cereus* causes two types of food
poisoning in humans including diarrhoeal syndrome and emetic syndrome. Food
poisoning results from its production of enterotoxins in the gastrointestinal
tract. The dosage of ingested *B. cereus* spores leading to
diarrhoeal syndrome is 105–107 g 1 of ingested food, and 105–108 g 1 of
ingested food for emetic syndrome [6]. Enterotoxins associated with diarrhoeal
syndrome are unresistant to the acidic conditions of the stomach. However, the
cereulide peptide toxin associated with emetic syndrome is more resistant to
acidic conditions and remains active at 121 °C [6].

Virulence factors associated with diarrhoeal syndrome
involve three enterotoxins including hemolysin BL (HBL), non-hemolytic
enterotoxin (NHE), and cytotoxin K [24]. The main virulence factor of *B.*
cereus*is HBL which is made of the three proteins B, L1, and L2 [24].*
Symptoms of diarrhoeal syndrome include watery diarrhea, abdominal cramps, and
pain occurring 6-15 hours after ingestion which may last for twenty-four hours
[4]. The emetic syndrome is caused by cereulide peptide toxin which is secreted
during stationary phase [25]. This toxin has a ring structure,
dodecadepsipeptide, which consists of four amino acids, repeating three times,
and oxy acids [25]. Symptoms associated with emetic syndrome include nausea and
vomiting within half an hour to six hours after ingestion of food and also lasts
for about twenty-four hours [6].

Although *B. cereus* is commonly known to
cause food-borne intoxications, it has been reported to cause local and
systemic infections, as an opportunistic pathogen, especially among
immunocompromised patients, newborns, and patients with surgical wounds
[24]. *B. cereus* can cause ocular infections such as keratitis,
endophthalmitis, and panophthalmitis [24]. The main virulence factor in *B.*
cereus*endophthalmitis is HBL which can result in the detachment of*
the retina and blindness. In addition, *B. cereus* can cause
gangrene, bovine mastitis, pyogenic infections, cellulitis, infant death,
septic meningitis, periodontal disease, lung abscesses, and endocarditis [6].
However, these infections are less common. Virulence factors associated with
non gastrointestinal infections include hemolysins and phospholipase C.
Hemolysin III causes erythrocyte lysis [6]. Phospholipase C causes tissue
damage by stimulating degranulation of human neutrophil, and breaks down the
subepithelial matrix affecting the healing of tissue in infections [24].

### Genome structure

**3.1 Genome**

*B. cereus* has a circular chromosome measuring
5,411,809 nt in length and was completely sequenced using the shotgun
sequencing method [7]. The genome structure of *B. cereus* consists
of 5481 genes, 5234 protein coding, 147 structural RNAs, and 5, 366 RNA operons
[1]. An interesting gene cluster found within its genome encodes for arginine
deiminase metabolic pathway. This cluster is predicted to have a role in its
survival, enabling it to be resistant to acidic conditions in a similar manner
as *Streptococcus pyogenes* [8]. Additionally, *B. cereus* has
a nine gene urease gene cluster that encodes for proteins, blasticidin S
deaminase, and an S-layer protein [8]. The urease enzyme increases its vigor in
acidic conditions and is similar to the urease found in other bacteria that is
required for colonization of the human stomach [8].

Genes present within the chromosome associated with *B.*
cereus*virulence include genes encoding for non-hemolytic*
enterotoxins, channel-forming type III hemolysins, phospholipase C, a
perfringolysin O (listeriolysin O), and extracellular proteases [6]. The *hbl* operon,
an RNA transcript of 5.5 kb, transcribes all three proteins of the hemolysin BL
enterotoxins associated with food poisoning. These genes along with other genes
encoding for metabolic enzymes, proteins involved in motility and chemotaxis,
proteins involved in sporulation, and cellular transporters are all regulated
by the *plcR* gene [9]. The *plcR* gene is also
required for full virulence of *B. cereus*, and is often the target
of antimicrobial drugs. Another gene found on its chromosome is the *gerA* gene
which is essential for sporulation when nutrients are depleted, and is
responsible for spore germination stimulated by L-alanine and ribosides [9]. It
also has 18-23 genes that encode for peptide and amino acid ABC transporter-ATP
binding proteins which suggests that amino acids, proteins, and peptids are
preferred nutrient sources [9].

**3.2 Plasmids**

*B. cereus* has a diverse range of plasmids that
vary in size from 5 to 500 kb and is known to have more than one plasmid with
only a few that are associated with pathogenesis. *B. cereus* ATCC
has a pXO1 plasmid that is found in *B. anthracis* [7]. It is
avirulent because it does not have the portion that encodes for the toxin and
regulatory proteins. *B. cereus* G9241 has a plasmid that is
99.6% identical to pXO1 plasmid from *B. anthracis*, but does not
have the pXO2 plasmid which is required for full virulence. It also has a
second plasmid that encodes for a capsule biosynthesis operon [7]. *B.*
cereus ZK*, a pathogenic strain, has five plasmids. Transposase genes are*
found in the two large plasmids which function in gene exchange between
plasmids and chromosome [7]. The three smaller plasmids of the five function in
identifying replication and mobilization proteins [7].

### Application to Biotechnology

**7.1 Biological Control Agent**

Biological control agents are alternatives to chemical
pesticides that are capable of suppressing plant pests and can also enhance
plant growth. Most strains of *Bacillus cereus* produce toxins
known to cause food-borne illnesses. However, there are strains that do not
produce the HBL enterotoxin. Thus, antifungal compounds of *Bacillus*
cereus*strains have been developed as a useful biological control*
agent in the suppression of fungi and crop disease.

**7.2 Bacillus cereus Strains**

***Bacillus cereus* B4** is used as a
pesticide to deter fungi from rotting seedling plants. This strain produces
three types of metabolites that enable it to suppress certain plant diseases
and promote plant growth. The metabolites include Kanosamine, 3, 4-dihydroxy
benzoate, and 2 keto-4 methylthiobutyrate. The metabolite 3,4-DOHB improves
crop strength and results in a healthier and larger root system [27].

***Bacillus cereus* DGA34** was isolated
from the environment and is useful in fighting off fungal damping disease in
crops. It naturally produces the antibiotic zwittermicin A. by fermentation
which is found in the supernantant fluid of its culture medium. This antibiotic
is effective in fighting a wide range of fungi and bacteria, and reduces
symptoms of damping-off disease and root rot [26].

***Bacillus cereus* UW85** produces two
antibiotic toxins including zwittermicin A. and antibiotic B. *Bacillus*
cereus*UW85 is used to protect alfalfa seeds from dampening off caused*
by *Phytophthora medicaginis*, cucumber fruits from *Pythium*
aphanidermatum*, peanuts from*Sclerotinia minor*, and tobacco*
seedlings from *Phytophthora nicotianae* [28].

These are just a few *B. cereus* strains with
current United States patents as biocontrol agents for crop diseases. There are
currently many *Bacillus cereus* strains that are still under
review.

Current Research

**8.1 Biofilms of *B. cereus***

The ability of *B. cereus* to form biofilms
on surfaces can cause potential contamination problems within the food
industry. Therefore, biofilm formation of several *B. cereus* strains
are currently being studied to prevent potential food contamination and to
ensure safety during production. In a recent study, microtiter assay and assays
on stainless steel were completely or partially submerged in liquid in order to
observe *B. cereus* biofilm formation. Since stainless steel is
commonly used for pipes and tanks in the food industry, additional tests were
conducted to investigate *B. cereus* biofilm formation from
spores on stainless steal coupons. The results from both tests were similar. It
appears that *B. cereus* biofilms preferentially form within the
air-liquid interface. This tendency is due to the availability of oxygen in
this region, causing bacterial movement toward oxygen. In addition, spore
formation was more rapid in the suspension phase of biofilm formation
suggesting that biofilms are a cavity for sporulation. The results show that *B.*
cereus*biofilms may develop within storage and piping systems when*
either partially filled or when liquid residues remain during production. In
addition, increase in spore formation by *B. cereus* within
biofilms can potentially cause recontamination and equipment failure during
food production [29].

**8.2 Effects of Porcine Bile on *B. cereus***

Resistance to bile is important to the survival of *B.*
cereus*within the small intestine where it can proliferate and release*
enterotoxins. On going research projects are being conducted to test *B.*
cereus*and factors that may affect its growth and release of*
enterotoxins once ingested. A recent study tested the effects of porcine bile
(PB) on *B. cereus* and on its HBL enterotoxin in the small
intestine by using intestine media with different food types. Different
concentrations of porcine bile were added to the gastric media to simulate acid
stress. Results show that the growth of *B. cereus* was affected
by the type of food in the small intestine media which may be explained by the
protective effect of different types of food against porcine bile. For
instance, components of food such as fiber can bind to bile salts reducing
their toxic effects on *B. cereus*. In addition, bile salts may be
sequestered by food components that lower cholesterol levels. It was concluded
that the tolerance of *B. cereus* to porcine bile and its
ability to grow and produce toxins is dependent on the type of food and on bile
concentrations in the small intestine. Additional studies are being conducted
to test the effects of bile on *B. cereus*enterotoxin as well
as the effects of different carbon sources on enterotoxin release [32].

**8.3 Virulence Factors of *B. cereus***

*B. cereus* has been a growing and established
opportunistic human pathogen. Therefore, current research is being conducted to
understand its pathogenicity and virulence factors in order to find potential
targets for antimicrobial drugs. In a recent study conducted, *B. cereus* *fur*gene,
a transcriptional regulator that is responsible for bacterial iron uptake and
metabolism, was shown to reduce virulence in the pathogenic *fur*mutant.
Results in this investigation revealed a decrease in the regulation of iron
uptake with three times more intercellular iron in the *fur* mutant
than in the wild-type, resulting in a greater amount of oxidizing free
radicals. The virulence of *B. cereus fur* mutant was measured
in an insect infection which revealed the significance of iron metabolism
regulation in bacterial pathogens by its reduced virulence. This recent study
gives insight to the importance of the *fur* gene in the
regulation of iron concentrations for cell growth, survival, and pathogenesis.
The reduced virulence of *B. cereus fur* mutants in this
experiment demonstrates the potential of the *fur* gene to be a
good target for antimicrobial drugs as a conserved protein among pathogenic
bacteria [30].

**8.4 *B. cereus* Endophthalmitis**

*B. cereus* causes endophthalmitis which can lead
to blindness. There is currently no universal treatment for this disease.
Various research projects are being conducted by the Callegan Lab to understand
its pathogenicity and virulence in order to develop better treatment and
antibiotics. Link
to Current Research Website

One current research project is studying the role of *B.*
cereus*proteases in endophthalmitis. It appears that strains without*
metalloproteases InhA and InhA2, immune inhibitors, were more virulent than the
wild-type strain. Further studies are being conducted to determine why [31].

Another ongoing research project involves the study of
the *plcR* gene which regulates toxin production and has an
important role in *B. cereus* endophthalmitis pathogenisis.
The *plcR* mutant strain exhibited a reduction in virulence
which resulted in 20% retained retinal function. The effects of blocking the
peptide *PapR* as a potential form of treatment is being
investigated [31].

Cell wall structures of *B. cereus* including
peptidoglycan, S-layer, capsules, and techoic acids are being studied for their
ability to cause intraocular inflammation. Recognition of these cell wall
structures by retinal cells are being researched [31].

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Edited by Jacqueline Nguyen student of Rachel Larsen

Edited by KLB