Description
Bacillus cereus are Gram-positive, facultative aerobic sporeforming rod. They can be differentiated from other Bacillus species by their cell position and biochemical tests. The optimum growth temperatures range from 30 to 50°C, although some psychrotrophic strains can grow down to 4 to 5°C. They can grow at pH values of between 4.3 and 9.3, and can grow at water activity values down to 0.912. The organism produces heat resistant spores and these may germinate if cooling is too slow.
Nature of acute disease
There are two illness associated with B. cereus, emetic and diarrhoeal illness. The emetic illness is caused by the ingestion of a heat-stable toxin produced by the microorganisms in the food. The diarrhoeal illness is caused by the ingestion of moderate to high number of B. cereus and their subsequent production of toxin in the stomach.
Characteristics of Illness
Symptoms: The emetic type of food poisoning, with symptoms similar to that caused by Staphylococcus aureus, is characterised by nausea and vomiting. The symptoms of B. cereus diarrhoeal illness, similar to Clostridium perfringens food poisoning, include watery diarrhoea, abdominal cramps, and pain. Nausea may sometimes occur, and vomiting rarely occurs. Other Bacillus species will cause vomiting and diarrhoea.
Onset of Illness: Onset times for the emetic illness is usually ½ to 6 hours after consumption of contaminated foods. For the diarrhoeal illness onset times are usually 6 to 15 hours, and for other Bacillus species varies from ½ hour to 14 hours.
Duration of Illness: The emetic illness usually lasts less than 24 hours, while the diarrhoeal illness usually persist for 24 hours in most instances. As with the emetic illness, illnesses caused by other Bacillus species usually last less than 24 hours.
Infective Dose: Foods implicated in foodborne illness caused by B. cereus and other Bacillus species usually contain at least 105 microorganisms per gram of food.
Diagnosis of human illness
Confirmation of B. cereus as the cause in a foodborne outbreak requires either:
- Isolating the same serotype from the suspect food and faeces or vomitus of the patient
- Isolating large numbers of a B. cereus serotype known to cause foodborne illness
- Isolating B. cereus from suspect foods and determining their enterotoxigenicity.
The rapid onset time for the emetic illness, coupled with some food evidence, is often sufficient to diagnose this type of food poisoning.
Complications
No specific complications have been associated with the foodborne disease causes by B. cereus and other Bacillus species, although dehydration may occur from the diarrhoeal illness in the elderly and sick.
Susceptible individuals
All people are believed to be susceptible to B. cereus food poisoning.
Associated foods
A wide variety of foods including meats, milk, vegetables, and fish have been associated with the diarrhoeal type food poisoning. The emetic illness has generally been associated with rice products and other starchy foods such as potato, pasta and cheese products. Food such as sauces, puddings, soups, casseroles, pastries, and salads have frequently been incriminated in food poisoning outbreaks.
Food analysis
Bacillus species can be isolated from foods and identified using traditional microbiological methods. Serological methods can be used to identify the toxin produced in the diarrhoeal illness, while cell culture techniques can be used to detect the toxin produced in the emetic illness. See Hocking et al (1997) for more details.
Outbreaks
Many outbreaks in Australia and other industrialised countries have been associated with the consumption of rice, especially fried rice. There is a tendency to store boiled rice at ambient temperature a long periods of time before frying and consumption. This allows B. cereus spores, which survived the boiling to germinate, grow and produce toxin. The heat-stable toxin is not inactivated during frying. In the USA in 1985, 11 people became ill after dining in a Japanese restaurant, some while they were still in the restaurant. The onset time for the illness ranged from 30 minutes to 5 hours. All reported symptoms of nausea and vomiting, with some also experiencing diarrhoea. The illness lasted 5 hours to several days, with two individuals requiring hospitalisation from rehydration. No specific food could be associated with the illness since all persons consumed the some food items. Although there was no food evidence, it was suspected that the emetic form of B. cereus food poisoning caused the illness. Due to the short illness duration associated with Bacillus cereus many people recover before seeking medical advice, and it is thought that many cases go unreported.
Prevention
Cool hot foods quickly. Do not stored perishable foods within the Temperature Danger Zone (5°C to 60°C). Always evenly reheat food to above 75°C. When reheating in a microwave, periodically stir the food to ensure even heating. Do not store precooked foods under refrigeration for excessive periods of time
Further Information
Web sites:
Communicable Diseases - Australia
FDA Bad Bug Book
Literature:
Hocking, A.D. et al. (1997). Foodborne Microorganisms of Public Health Significance. 5th ed. North Sydney. AIFST NSW Branch Food Microbiology Group.
Doyle, M.P. (1989). Foodborne Bacterial Pathogens. Marcel Dekker; New York.