Bacillus cereus

It is famously known for causing food poisoning associated with reheated rice or fried rice. It is now known to cause various gastrointestinal and non gastrointestinal diseases some of them simulating anthrax.

Morphology

It is a gram positive, non-capsulated, motile with peritrichous flagella, spore forming, aerobic, rod shaped bacteria with rounded ends in pairs and short chains. Some strains have an outer surface protein called S layer which plays a role in adhesion and is antiphagocytic. It forms oval, centrally situated endospores. Spores are not seen in clinical specimens.

Classification

Not relevant.

Human Pathology

Pathogenicity is closely related to the production of toxins like hemolysins, enterotoxins, cytotoxins etc. It causes the following diseases in humans:

Food poisoning: It can be caused in two ways. Diarrhoeal type or emetic type.

Once ingested, vegetative bacterial cells multiply in the intestines and produce enterotoxin causing watery diarrhoea. Onset is 8-16 hours after ingestion of food like meat, soups, vegetables, puddings and dairy.

Another way of producing disease is through ingesting preformed, heat stable, emetic toxin (cereulide) in food causing vomiting. Onset is from 30 minutes to 5 hours after ingestion of food like fried rice, pasta, pastry and noodles.

Non-gastrointestinal infections: B.cereus causes both local and systemic infections like endophthalmitis, meningitis, brain abscess, pneumonia, cutaneous infections and bacteremia. They are seen more commonly in neonates, intravenous drug users, indwelling catheters, after traumatic wounds or surgery.

Laboratory diagnosis of B.cereus infections

Characteristic morphology will be seen on clinical specimens like blood, vomit, stool samples, anterior chamber eye aspirates etc. Endospores will be seen in culture but not in host tissue.

On blood agar colonies are beta hemolytic, with a rough, matted surface and show swarming growth. In severe food poisoning cases, toxin assay can be done by PCR or Immunoassays.