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Bacillus cereus

General characteristics

Bacillus cereus is a Gram-positive, facultatively aerobic sporeformer whose cells are large rods and whose spores do not swell the sporangium. These and other characteristics, including biochemical features, are used to differentiate and confirm the presence B. cereus , although these characteristics are shared with B. cereus var. mycoides , B. thuringiensis and B. anthracis . Differentiation of these organisms depends upon determination of motility (most B. cereus are motile), presence of toxin crystals (B. thuringiensis ), hemolytic activity (B. cereus and others are beta hemolytic whereas B. anthracis is usually nonhemolytic), and rhizoid growth which is characteristic of B. cereus var. mycoides .

Disease symptoms

B. cereus food poisoning is the general description, although two recognized types of illness are caused by two distinct metabolites. The diarrheal type of illness is caused by a large molecular weight protein, while the vomiting (emetic) type of illness is believed to be caused by a low molecular weight, heat-stable peptide.

The symptoms of B. cereus diarrheal type food poisoning mimic those of Clostridium perfringens food poisoning. The onset of watery diarrhea, abdominal cramps, and pain occurs 6-15 hours after consumption of contaminated food. Nausea may accompany diarrhea, but vomiting (emesis) rarely occurs. Symptoms persist for 24 hours in most instances.

The emetic type of food poisoning is characterized by nausea and vomiting within 0.5 to 6 h after consumption of contaminated foods. Occasionally, abdominal cramps and/or diarrhea may also occur. Duration of symptoms is generally less than 24 h. The symptoms of this type of food poisoning parallel those caused by Staphylococcus aureus foodborne intoxication. Some strains of B. subtilis and B. licheniformis have been isolated from lamb and chicken incriminated in food poisoning episodes. These organisms demonstrate the production of a highly heat-stable toxin which may be similar to the vomiting type toxin produced by B. cereus .

The presence of large numbers of B. cereus (greater than 10 6 organisms/g) in a food is indicative of active growth and proliferation of the organism and is consistent with a potential hazard to health.

Diagnosis

Confirmation of B. cereus as the etiologic agent in a foodborne outbreak requires either (1) isolation of strains of the same serotype from the suspect food and feces or vomitus of the patient, (2) isolation of large numbers of a B. cereus serotype known to cause foodborne illness from the suspect food or from the feces or vomitus of the patient, or (3) isolation of B. cereus from suspect foods and determining their enterotoxigenicity by serological (diarrheal toxin) or biological (diarrheal and emetic) tests. The rapid onset time to symptoms in the emetic form of disease, coupled with some food evidence, is often sufficient to diagnose this type of food poisoning.

Associated foods

A wide variety of foods including meats, milk, vegetables, and fish have been associated with the diarrheal type food poisoning. The vomiting-type outbreaks have generally been associated with rice products; however, other starchy foods such as potato, pasta and cheese products have also been implicated. Food mixtures such as sauces, puddings, soups, casseroles, pastries, and salads have frequently been incriminated in food poisoning outbreaks.

Prevention

Total prevention is probably not possible, however properly stored, heated and cooked foods are generally safe for the non-emetic type. The largest risk is cross-contamination, where cooked material comes into contact with raw produce or contaminated materials (cutting boards).

The emetic type is generally associated with improperly stored starchy products (rice, pasta). Proper storage (below 7 °C and for only a few days) will prevent outgrowth and toxin production.

Risk populations

All people are believed to be susceptible to B. cereus food poisoning.

Sources:

The bad bug book : http://www.cfsan.fda.gov/~mow/intro.html