B. cereus is widespread in the environment and commonly found in the soil. It is able to produce spores that are resistant to heat and desiccation, therefore it is not uncommon to isolate it from both raw and cooked foods. These spores will germinate into the vegetative form of B. cereus and grow if the food is held under favorable conditions of pH (>4.8) and temperature (between 8°C and 55°C) for a sufficient time. Although the presence of vegetative forms of B. cereus in food is always necessary for foodborne disease to occur, not all the B. cereus strains can produce the toxins that cause the emetic or diarrhoeal syndromes. In addition, the conditions leading to each of the syndromes differ slightly.
The emetic syndrome will affect consumers of food contaminated with the emetic toxin cereulide, therefore the food needs to be contaminated with B. cereus strains that are able produce this toxin and be handled in a way that allows bacterial growth and subsequent toxin formation. It is estimated that, in order to produce sufficient cereulide to induce vomiting, levels of B. cereus should be greater than 10,000 per gram of food, but several publications have documented illnesses, including hospitalizations with lower numbers. The toxin is produced in the food and is resistant to heat; therefore it will not be eliminated by most cooking methods, even when the vegetative cells are inactivated. This syndrome is frequently associated with starchy food such as pasta or rice dishes.
The diarrhoeal syndrome occurs when a large number of vegetative cells of B. cereus (at least 10,000 per gram of food) are ingested and produce enterotoxin in the small intestine. A wider range of foods have been linked with the diarrhoeal syndrome, such as meat products, stews, soups, sauces, vegetables and milk products.