Microbe of the Month – Bacillus cereus

The following is from The Bug Bible

Bacillus cereus has been suspected as a cause of foodborne disease since early this century, although the role in foodborne disease was not established until the 1950s.

Bacillus cereus causes two types of illness – one characterized by diarrhoea and the other by vomiting. Illness is caused by enterotoxins. The diarrhoeal enterotoxin is heat sensitive (inactivated by 56ºC for 5 minutes). However the emetic toxin (induces vomiting) is very heat resistant and would not be inactivated by normal cooking or reheating temperatures. Not all strains of B. cereus found in foods are associated with illness

Bacillus species are commonly found in nature particularly in soils and in water. They find their way into plant foods from these sources. Raw materials of plant origin are a major source of Bacillus cereus in foods. B. cereus has been isolated from flour, rice, spices, lentils dried potatoes beans meat and chicken. Rice is a well recognized source of B. cereus with most samples containing low to moderate levels of the organism.

The presence of B. cereus in processed foods is often associated with the ability of the spores to survive heat processes. Many outbreaks of illness due to B. cereus have been associated with boiled and fried rice that has been cooked and stored for extended periods at warm temperatures before consumption. Custards puddings and mousses have also been implicated and powdered milk products reconstituted with warm water and stored at ambient temperatures have been shown to support the growth of B. cereus. Storage of foods below 5ºC have been shown to restrict growth of this organism.

The emetic form of the illness appears to be primarily associated with rice dishes.

Storage of foods for too long at ambient temperatures is almost always the cause of illness due to the growth of B. cereus and toxin production.

Prevention of illness in the home depends on correct storage of cooked foods. Growth of B. cereus must be prevented by storing food below 5⁰C or above 60ºC. Bulk cooked foods should be divided into smaller portions to facilitate rapid cooling and minimize time in the danger zone.