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Bacillus cereus

Bacillus (B.) cereus is a sporogenous bacterium which is present all over the world. As a result of contamination with sporophorous soil particles or dust, B. cereus can be easily transmitted to foods. Through heat treatment the germ is killed but its spores survive.

The complete avoidance of contamination of foods is difficult because of the worldwide occurrence of the bacterium. A low bacterial count is normally no problem for consumers. However, certain deficient storage conditions can result in the germination of the spores and / or multiplication of germs on foods. When consuming foods contaminated by B. cereus, toxins and / or germs are taken up which can result in food poisoning and / or gastro-intestinal infections in humans. A germination of the spores can be prevented by rapid cold storage of the heated foods. For that reason the emphasis must be placed on the maintenance of the cold chain.

Characteristics

The initial contamination dosage on foods with B. cereus is in most cases very low (<102-103 CFU/g). As a rule multiplication in foods to a total bacterial count of 105 to 108 colony-forming units of the germ per gram (CFU/g) is required to generate 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, the individual cold-tolerating strains can also multiply at temperatures of 4 to 6°C; in these cases the generation times are, however, considerably longer. Below a pH value of 4.8 B. cereus strains cannot multiply. However, the acid tolerance between the strains varies considerably. The minimum water activity value (aw value), which allows for a multiplication, is at approximately 0.92. The heat resistance of the spores is strongly dependent on the food matrix. In general, the heat resistance of the spores corresponds to that of other mesophilic spore-forming bacteria growing optimally at mean temperatures but strain variability is very high. Usual heat treatments kill vegetative cells. Temperatures below 100°C allow the survival of individual spores; a sufficient and rapid cooling after heat treatment is, therefore, necessary in order to prevent the germination of spores. The destruction of the competitive flora by heat treatment supports the germination of the spores and the transition to the vegetative phase. The surviving spores are usually causing a B. cereus disease.

Two types of disease can be differentiated:

(i) an emetic intoxication. In this case a heat, pH and proteolysis-stable toxin (cereulide) preformed in the food during the multiplication of the vegetative cells is taken up.

(ii) the diarrhoea type. In this case vegetative cells or spores of B. cereus are taken up via the food. The spores form a heat-labile enterotoxin in the small intestine – preformed toxins are taken up more rarely; this requires, however, a very high initial B. cereus bacterial count in the food.

The incubation time and intoxication by preformed toxin occurs almost exclusively after the consumption of starch-containing foods such as rice and noodles. Meat and meat products play a subordinate role for the emetic type. However, contaminated meat and contaminated meat products are among the possible causes of the diarrhoea type. In the event of an insufficient cooling of meat and meat products and a previous contamination of the products with B. cereus there can be a critical multiplication of the germ on food. A similarly problematic cause are long warming phases of contaminated foods in a temperature range between 15 and 65°C. This might result under certain circumstances in diarrhoea.

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