Bacillus cereus is a Gram-positive, rod-shaped, aerobic, facultatively anaerobic, motile, beta hemolytic bacterium commonly found in soil and food. Some strains are harmful to humans and cause foodborne illness, while other strains can be beneficial as probiotics for animals. It is the cause of "fried rice syndrome", as the bacteria are classically contracted from fried rice dishes that have been sitting at room temperature for hours. B. cereus bacteria are facultative anaerobes, and like other members of the genus Bacillus, can produce protective endospores. Its virulence factors include cereolysin and phospholipase C.
The Bacillus cereus group comprises seven closely related species: B. cereus sensu stricto (referred to herein as B. cereus), B. anthracis, B. thuringiensis, B. mycoides, B. pseudomycoides, B. weihenstephanensis, and B. cytotoxicus.
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History
Colonies of Bacillus cereus were originally isolated from an agar plate left exposed to the air in a cow shed. In the 2010s, examination of warning letters issued by the US Food and Drug Administration issued to pharmaceutical manufacturing facilities addressing facility microbial contamination revealed that the most common contaminant was B. cereus.
Several new enzymes have been discovered in B. cereus, e.g. AlkC and AlkD, both of which are involved in DNA repair.
Ecology
B. cereus competes with other microorganisms such as Salmonella and Campylobacter in the gut, so its presence reduces the numbers of those microorganisms. In food animals such as chickens, rabbits and pigs, some harmless strains of B. cereus are used as a probiotic feed additive to reduce Salmonella in the intestines and cecum. This improves the animals' growth as well as food safety for humans who eat them.They parasitize codling moth larvae.
Bacillus cereus and other members of Bacillus are not easily killed by alcohol; in fact, they have been known to colonize distilled liquors and alcohol-soaked swabs and pads in numbers sufficient to cause infection.
Some strains of B. cereus produce cereins, bacteriocins active against different B. cereus strains or other Gram-positive bacteria.
Reproduction
At 30 °C (86 °F), a population of B. cereus can double in as little as 20 minutes or as long as 3 hours, depending on the food product.
Pathogenesis
B. cereus is responsible for a minority of foodborne illnesses (2-5%), causing severe nausea, vomiting, and diarrhea. Bacillus foodborne illnesses occur due to survival of the bacterial endospores when food is improperly cooked. Cooking temperatures less than or equal to 100 °C (212 °F) allow some B. cereus spores to survive. This problem is compounded when food is then improperly refrigerated, allowing the endospores to germinate. Cooked foods not meant for either immediate consumption or rapid cooling and refrigeration should be kept at temperatures below 10 °C (50 °F) or above 50 °C (122 °F). Germination and growth generally occur between 10 °C and 50 °C, though some strains are psychrotrophic. Bacterial growth results in production of enterotoxins, one of which is highly resistant to heat and acids (pH levels between 2 and 11); ingestion leads to two types of illness, diarrheal and emetic (vomiting) syndrome.
The diarrhetic syndromes observed in patients are thought to stem from the three toxins: hemolysin BL (Hbl), nonhemolytic enterotoxin (Nhe) and cytotoxin K (CytK). The nhe/hbl/cytK genes are located on the chromosome of the bacteria. Transcription of these genes is controlled by PlcR. These genes occur in the taxonomically related B. thuringiensis and B. anthracis, as well. These enterotoxins are all produced in the small intestine of the host, thus thwarting digestion by host endogenous enzymes. The Hbl and Nhe toxins are pore-forming toxins closely related to ClyA of E. coli. The proteins exhibit a conformation known as "beta-barrel" that can insert into cellular membranes due to a hydrophobic exterior, thus creating pores with hydrophilic interiors. The effect is loss of cellular membrane potential and eventually cell death. CytK is a pore-forming protein more related to other hemolysins.
The timing of the toxin production was previously thought to be possibly responsible for the two different courses of disease, but in fact the emetic syndrome is caused by a toxin, cereulide, found only in emetic strains and is not part of the "standard toolbox" of B. cereus. Cereulide is a cyclic polypeptide containing three repeats of four amino acids: D-oxy-Leu--D-Ala--L-oxy-Val--L-Val (similar to valinomycin produced by Streptomyces griseus) produced by nonribosomal peptide synthesis. Cereulide is believed to bind to 5-hydroxytryptamine 3 (5-HT3) serotonin receptors, activating them and leading to increased afferent vagus nerve stimulation. It was shown independently by two research groups to be encoded on multiple plasmids: pCERE01 or pBCE4810. Plasmid pBCE4810 shares homology with the Bacillus anthracis virulence plasmid pXO1, which encodes the anthrax toxin. Periodontal isolates of B. cereus also possess distinct pXO1-like plasmids. Like most of cyclic peptides containing nonproteogenic amino acids, cereulid is resistant to heat, proteolysis, and acid conditions.
B. cereus is also known to cause difficult-to-eradicate chronic skin infections, though less aggressive than necrotizing fasciitis. B. cereus can also cause keratitis.
Diagnosis
In case of foodborne illness, the diagnosis of B. cereus can be confirmed by the isolation of more than 105 B. cereus organisms per gram from epidemiologically implicated food, but such testing is often not done because the illness is relatively harmless and usually self-limiting.
Identification through testing
Below is a list of differential techniques and results that can help to identify Bacillus cereus from other bacteria and Bacillus species.
- Anaerobic Growth: Positive
- Voges Proskauer test: Positive
- Acid produced from
- D-glucose: Positive
- L-arabinose: Negative
- D-xylose: Negative
- D-mannitol: Negative
- Starch hydrolysis: Positive
- Nitrate reduction: Positive
- Degradation of tyrosine: Positive
- Growth at
- >50 °C: Negative
- Utilization of citrate: Positive
Prognosis
Most emetic patients recover within six to 24 hours, but in some cases, the toxin can be fatal. In 2014, 23 neonates receiving total parenteral nutrition contaminated with B. cereus developed septicaemia, with three of the infants later dying as a result of infection.
Bacteriophage
Bacteria of the B. cereus group are infected by bacteriophage belonging to the family Tectiviridae. This family includes tail-less phages that have a lipid membrane/vesicle beneath the icosahedral protein shell and that are formed of approximately equal amounts of virus-encoded proteins and lipids derived from the host cell plasma membrane. Upon infection, the lipid membrane becomes a tail-like structure used in genome delivery. The genome is composed of ~15-kb linear double-stranded DNA (dsDNA) with long inverted terminal repeat sequences (?100 bp). GIL01, Bam35, GIL16, AP50, and Wip1 are examples of temperate tectiviruses infecting the B. cereus group.
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