Foodborne diseases / Bacillus cereus

Bacillus cereus

Many Bacillus cereus strains produce enterotoxin and are capable of causing food poisoning. The bacterium is a poor competitor, however, and plays little or no part in spoilage of meat and meat products.

1. Bacteriological characteristics

Bacillus cereus is a Gram-positive, spore-forming, rod of the family Bacillaceae. The spores survive for only a few minutes at 100°C. However, sublethal heat treatment causes spore activation and can lead to rapid outgrowth at ca. 45°C during cooling of, for example, meat pâté and Bologna-type sausage.

2. Disease in humans

B. cereus causes two main kinds of food poisoning, a diarrhoeal type, associated with toxin formed in the intestinal tract, and an emetic type which follows the ingestion of preformed toxin. In the first case, diarrhoea usually develops 10-16 hours after ingestion of the organisms and a wide variety of foods have been implicated, including meats, soups, vegetables and puddings. In some countries B. cereus food poisoning is particularly common, being associated with the consumption of highly spiced meat dishes. Raw foods are not involved in outbreaks. The second type of illness is characterised by vomiting 1-5 hours after ingestion and is largely confined to rice dishes. Both forms of the disease are self-limiting and usually last for 1-2 days or less.

3. Sources of infection and epidemiology

B. cereus is common in soil and in many dry products, e.g. spices, grain and especially rice. It is also regularly present in evaporated foods like dried milk or in meat products. The presence of the organism in raw meats is of little significance, but low numbers in heat treated meat products, originating from spices, flour and other ingredients, may result in substantial growth if temperature abuse occurs.

4. Growth properties in relation to meat and meat products

There is a wide variation in the growth capabilities of B. cereus. Some strains are cold tolerant and can grow at 4-5°C, but not at 37°C, while others are mesophilic and can grow at 15-50°C or even 55°C. Optimum temperatures vary from 30-40°C. B. cereus survives the pasteurization process used for egg products.
In general growth does not occur at a pH value of <4.8, but acid tolerance varies between strains. Growth is inhibited at a water activity value of 0.92.


5. Isolation

The organism can be isolated by surface inoculation of mannitol egg yolk polymyxin agar, with incubation at 30 C for 48 hours. Another useful medium is 5% blood agar.


6. Control measures in slaughterhouses and cutting plants

Although the organism is of no real significance in raw meat, experience from the dairy industry shows that the bacterium readily becomes established in pipelines and on the surfaces of equipment, even stainless steel plates. This is because the spores are covered in appendages that promote adhesion and make them less amendable to normal cleaning. Thus particular attention should be given to the efficacy of cleaning and disinfection procedures and the need to increase sanitiser concentrations at appropriate intervals.
The most common sources of meat contamination are spices, cereals, dried foods, like casein and milk powder, and meat extract. It is recommended that microbiological investigation of these raw materials is included in Hazard Analysis Critical Control Points (HACCP) programmes for the production of cooked sausages and other ready-to-eat meat dishes. Ideally, the organisms should be absent from the final product since any temperature abuse would allow rapid growth, with no competition from other bacteria that had been killed by the heat treatment. Only very low numbers are acceptable in the ingredients used. For this reason, the use of decontaminated spices is to be recommended. Cooling of the end-product should be checked regularly as part of the HACCP programme. Cooling from 50 to 5°C should take <3 hours.