Bacillus cereus

A species of bacteria that can cause foodborne illness

Bacillus cereus is a Gram-positive, rod-shaped, aerobic, spore-forming bacterium commonly found in soil and food. It is known for causing foodborne illnesses, particularly in improperly stored starchy foods such as rice and pasta. B. cereus is a member of the Bacillus genus, which includes other notable species such as Bacillus anthracis and Bacillus thuringiensis.

Characteristics

Bacillus cereus is a facultative anaerobe, meaning it can grow in both the presence and absence of oxygen. It is capable of producing endospores, which are resistant to heat, desiccation, and disinfectants, allowing the bacterium to survive in harsh conditions. The spores can germinate into vegetative cells when conditions become favorable.

Bacillus cereus under a scanning electron microscope

B. cereus is motile due to the presence of peritrichous flagella. It is also known for producing a variety of toxins, which can lead to two types of food poisoning: the emetic (vomiting) type and the diarrheal type.

Pathogenicity

Bacillus cereus is an opportunistic pathogen. It can cause two distinct types of foodborne illness:

• Emetic type: This form is associated with the consumption of rice dishes that have been cooked and kept warm for extended periods. The bacterium produces a heat-stable toxin called cereulide, which induces vomiting within 1-6 hours after ingestion.

• Diarrheal type: This form is associated with a wider range of foods, including meats, milk, and vegetables. It is caused by heat-labile enterotoxins produced by the bacteria in the small intestine, leading to diarrhea and abdominal cramps 8-16 hours after consumption.

Laboratory Identification

Bacillus cereus can be identified in the laboratory through various methods:

• Microscopy: Gram staining reveals Gram-positive rods. Endospore staining can highlight the presence of spores.

Bacillus cereus endospore stain

• Culture: B. cereus can be cultured on selective media such as Mannitol Egg Yolk Polymyxin (MYP) agar, where it forms pink colonies due to mannitol fermentation.

Bacillus cereus colonies on indicator media

• Biochemical tests: B. cereus is catalase positive and can hydrolyze starch.

Prevention and Control

To prevent Bacillus cereus food poisoning, it is important to:

• Cook foods thoroughly and maintain them at safe temperatures.
• Refrigerate leftovers promptly to prevent spore germination and bacterial growth.
• Reheat foods to appropriate temperatures to destroy any vegetative cells.

Also see

• Bacillus anthracis
• Bacillus thuringiensis
• Foodborne illness
• Endospore