Bacillus cereus

Bacillus cereus is an aerobic, spore forming, rod-shaped, exotoxins producing gram positive bacteria. The toxins of Bacillus cereus can produce two forms of food poisoning: one is diarrheal form, resembling clostridial gastroenteritis and other is emetic (vomiting) form, similar to staphylococcal food poisoning.

Source of infection:

The diarrheal form is associated with a wide variety of foods including fish, meat, desserts, pastas, cheese and dairy products.  In turn, the emetic form is typically associated with contaminated rice product. The organism is common in uncooked rice. The spores of Bacillus cereus are heat-resistant and survive after boiling. If cooked rice is not refrigerated, the spores of Bacillus cereus can germinate and produce exotoxin. Frying before serving may not destroy the heat-stable preformed toxin that produces food poisoning.

How Bacillus cereus produces food poisoning?

Bacillus cereus produces two enterotoxins. One act by binding to a ganglioside receptor on surface of the cell in the small intestine then enters into the cytoplasm of cell, where it adds adenosine diphosphate (ADP)-ribose to the stimulatory G protein. This locks the G protein causes the persistant stimulation of adenylate cyclase. The resulting overproduction of cyclic adenosine monophosphate (AMP) within the enterocyte, stimulates secretion of water and chloride ions, leading to watery diarrhea.

Other enterotoxin of Bacillus cereus acts as a superantigen within the gastrointestinal tract. It acts locally on the lymphoid cells lining the small intestine to stimulate the release of cytokines. Cytokines released from lymphoid cells stimulate the enteric nervous system to activate the vomiting center in the brain causes prominent vomiting.

Symptoms:

The symptoms of B. cereus food poisoning differ according to which of the two distinct forms present.

(1) Diarrheal form

If viable Bacillus cereus are ingested with food, and toxin production takes place within the gastrointestinal lumen, then the diarrheal form of food poisoning occurs. It has a longer incubation period and the symptoms appear within 12-24 hours of contaminated food consumption. Predominant symptoms are watery diarrhea, abdominal cramps and pain. It is self-limiting but can be quite severe.

(2) Emetic (vomiting) form

Ingestion of preformed toxins of Bacillus cereus causes emetic form of food poisoning.  It has a short incubation period. This type of food poisoning is characterized by rapid onset of nausea and vomiting within hours of contaminated food consumption. Occasionally, some watery diarrhea may also occur. It resolves within 24 hours.

Diagnosis:
Usually, Bacillus cereus food poisoning is diagnosed clinically. Diagnosis can be confirmed by isolation of bacteria from implicated food, or detection of toxin from stool and uneaten food.

Treatment:
Bacillus cereus food poisoning is usually self-limiting. Only symptomatic treatment is given. Plenty of electrolytes containing fluids should be taken. In severe cases, patients should be admitted to a hospital and judicious intravenous fluids should be given if severe dehydration developed.

Prevention:
Prevention of Bacillus cereus food poisoning is completely not possible. There is no vaccine is available. However, proper food safety precautions including proper cooking and storage can reduce the risk of infection.