Overview: Bacillus cereus is a Gram-positive, spore-forming, rod-shaped, facultative anaerobic bacterium belonging to the Bacillaceae family of the Firmicutes (Figure 1). Most strains of B. cereus are motile, while only 4% are found to be non-motile. This organism is naturally found in soil, but also exist as harmless microflora in the intestinal tract of many soil-dwelling invertebrates. Some strains are harmful to humans and cause foodborne illnesses, while other strains can be beneficial as probiotics for animals. This species has been isolated from a variety of foods, such as meat, eggs, spices, dairy, and rice products. B. cereus bacteria are roughly 5 µm (micrometres) to 10 µm long by 1 µm wide, and arranged singly or in short chains.
Like other members of the genus Bacillus, B. cereus can produce protective endospores. An endospore is a dormant, tough, and non-reproductive structure produced by certain bacteria from the Firmicute phylum. They are typically located at cells' centre and contain hydrophobic appendages. These features help the organism adhere to hydrophobic surfaces and withstand harsh environment during food-making processes. Despite not being part of the natural microflora in the human gut, B. cereus colonies present in food are inevitably consumed due to their ubiquitous distribution and ability to form stable spores (Jesen et al., 2003). Therefore, these organisms sometimes contribute to the microflora community in the human intestinal tract at low number. In fact, fecal samples from 14% of healthy individuals show the presence of B. ceresus. B. cereus also exhibits seasonal preference, as higher populations are detected during the summer than in the winter (Jesen et al., 2003).
Identification: Although colonial morphology varies amongst different strains of the species, colonies generally appear as large spheres with dull surface and undulate margins, surrounded by wide zone of haemolysis (Harjeem, 2006) (Figure 2). B. cereus bacteria are able to ferment glucose, but not mannitol, reduce nitrogen, and synthesize catalase enzyme, yet are unable to split indole from the amino acid tryptophan (Wong et al., 1988). As a result, B. cereus can be identified using common biochemical techniques, such as the nitrate reductase test, Voges–Proskauer test, and catalase test (Wong et al., 1988). Clinically, B. cereus can be detected via plating the sample on the Columbia blood agar base with 5% sheep blood agar spread with either polymycin B or mannitol egg yolk polymycin agar. The threshold number of B. cereus is around 106 colony forming units (CFU) per gram of food ingested (Lee et al., 2006).
Virulence and Pathogenicity: Key virulent factors of B. cereus include the ability to form endospores and produce enterotoxins and emetic toxins. B. cereus endospores are able to withstand relatively high temperatures, broad pH ranges, and a salt concentration of up to a 10%. The vegetative form starts growing when cooked food is exposed to warm temperatures over a prolonged period of time. There are two types of clinical syndromes associated with each toxin, namely, rapid-onset emetic syndrome and slow-onset diarrhoeal syndrome. The diarrheal type is caused by three enterotoxins, namely, haemolysin BL (HBL), a non-hemolytic enterotoxin (NHE), and cytotoxin K (CytK). HBL and NHE proteins exhibit a conformation known as 'beta-barrel' that can insert into cellular membranes due to a hydrophobic exterior, thus creating pores or channels on the cell membranes of target cells with hydrophilic interiors. The effect is loss of cellular membrane potential and eventually cell death. Similarly, CytK is also a pore-forming protein, but is more related to other hemolysins. These enterotoxins can also activate the adenylyl cyclase pathway, leading to intestinal fluid secretion.
The other clinical illness B. cereus triggers is the emetic type. The emetic syndrome is caused by a heat-stabile toxin called cereulide that is found only in emetic strains and is not part of the standard pathogenesis of B. cereus. Cereulide contains three repeats of four amino acids (similar to Valinomycin produced by Streptomyces griseus) produced by non-ribosomal peptide synthesis. Cereulide is believed to either function as a potassium ion channel that alters the cell membrane permeability of nerve cells or activate 5-HT3 (serotonin) receptors, leading to increased afferent vagus nerve stimulation. It was shown independently by two research groups to be encoded on multiple plasmids, namely, 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.
Clinical Infections: Symptoms of the diarrheal-type syndrome include abdominal cramps and diarrhea after an 8 to16 hour incubation period. The emetic syndrome is typically associated with the consumption of rice and rice products, while the enterotoxins that were mentioned earlier are mainly found in dairy products. Severe symptoms of B. cereus food-poisoning are often reported in young adults and the elderly.
Treatment: The use of penicillin to treat infections caused by this pathogen generally has little effect, since most strains of B. cereus produce β-lactamase, an enzyme that chemically degrades penicillin. In contrast, bacterial growth can be inhibited by proper control of the processing and storage of cooked food. In fact, B. cereus growth is inhibited at a pH below 4 and a salt concentration greater than 10%. Temperatures exceeding 100°C also eliminates the majority of spores formed by this species.
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Nitrate Reductase Test: A test to differentiate between bacteria based on their ability or inability to reduce nitrate (NO3−) to nitrite (NO2−) using anaerobic respiration.
Voges–Proskauer test: A test used to detect acetoin in a bacterial broth culture. By adding a concentrated solution of sodium hydroxide a red color appears.
Catalase test: A test that detects whether the species produces catalase enzyme. If the bacteria possess catalase (i.e., are catalase-positive), when a small amount of bacterial isolate is added to hydrogen peroxide, bubbles of oxygen are observed.
Plasmid: A strand or loop of DNA that exists independently of the chromosome in bacteria and yeast and that is capable of genetic replication: used in recombinant DNA procedures as a vehicle of gene transfer.