Bacillus cereus is an endemic, soil-dwelling, Gram-positive, rod-shaped, beta hemolytic bacterium. Some are harmful to humans while other strains are beneficial as probiotics for animals. They are aerobes and can produce protective endospores.
B. cereus competes with other microorganisms such as Salmonella and Campylobacter in the gut. In food animals harmless strains of B. cereus are used as a probiotic feed additive to reduce Salmonella in the intestines and cecum. This helps improve animal growth as well as food safety for humans who eat the meat.
It is responsible for a minority of foodborne illnesses (2-5%), causing a severe nausea, vomiting, and diarrhea. Foodborne illness is from bacterial endospores surviving when food is improperly cooked. Improper cooking combined with improper refrigeration can compound the problem and allows endospores to germinate.
Diarrhetic syndromes in patients are thought to stem from the three toxins Hemolysin BL Hbl, Nonhemolytic Enterotoxin Nhe and Cytotoxin K CytK. These toxins are all produced in the small intestine of the host, thus thwarting the issue of digestion by host endogenous enzymes.
It is also known to cause chronic skin infections that are difficult to eradicate though less aggressive than necrotizing fasciitis.