Bacillus cereus

Bacillus (B.) cereus is a spore forming bacterium which is found worldwide. As a result of contamination with spore containing soil particles or dust, B. cereus can be easily transmitted to foods. Moreover, B. cereus is capable of forming biofilms, which also enables the organism to persist in food producing environments. Common heat treatments inactivate vegetative cells but the more resistant spores may survive.

The complete prevention of food contamination is difficult due to the high resistance of the spores but low concentrations of bacteria are generally not problematic for consumers. Poorly temperature controlled storage conditions can, however, result in the germination of spores and multiplication of vegetative cells in foods. When consuming foods contaminated by B. cereus, bacteria and/or toxins are ingested which can result in gastro-intestinal disease in humans. The germination of spores can be prevented by rapid cold or hot storage of heated foods.

Characteristics

Initial B. cereus contaminations on foods are mostly low. Usually, cell growth to bacterial counts of 10⁵ to 10⁸ colony-forming units per gram (CFU/g) of food is required to generate disease-relevant amounts of toxins in foods or in the small intestine.

The vegetative form of B. cereus grows in a range of 10 to 50 °C, with a temperature optimum between 30 and 40 °C. However, individual cold-tolerant strains can also multiply at temperatures of 4 to 6 °C, though with considerably longer generation times. In dry or acidic foods B. cereus is not able to grow. The heat resistance of the spores varies between strains and is strongly dependent on the food matrix. Common heat treatments, such as cooking or pasteurization, inactivate vegetative cells but are not sufficient to completely inactivate spores. The reduction of the competitive microbiota by heat treatment supports both the germination of the spores and the growth of vegetative cells. Thus, a sufficient and rapid cooling (≤ 7 °C) or hot storage (≥ 60 °C at all points of the product) after heat treatment of foods is necessary in order to prevent the germination of spores.

Two types of foodborne disease are caused by B. cereus:

1. Emetic disease: Here, a toxin (cereulide) is generated in the food during growth of the vegetative cells. Cereulide is extremely resistant to heat, pH and proteolysis. Ingestion of the toxin (intoxication) causes nausea and vomiting within a few hours (0.5 to 6 hours). The symptoms are mostly self-limiting within few days. Very rarely severe clinical cases are reported due to liver failure and brain edema. Cereulide intoxication is frequently associated with the consumption of starchy, cooked food such as rice or pasta dishes. Only a minority of B. cereus strains is capable of producing cereulide.
2. Diarrheal disease: Here, high numbers of cells and/or spores of B. cereus are ingested with the food and, after germination of spores in the small intestine, the vegetative cells begin producing enterotoxins. These enterotoxins are sensitive to heat, acid and proteolysis and typically cause abdominal pain and diarrhea (toxicoinfection). The incubation time of the diarrheal disease is 6 to 24 hours. In contrast to the spores, most ingested cells as well as potentially present enterotoxins are inactivated during gastro-intestinal passage. It is assumed that only vegetative cells that rapidly reach the mucus layer of the small intestine can contribute to enterotoxin production.

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