Bacillus cereus

Bacillus cereus is a gram-positive, spore-forming, rod-shaped bacterium found widely in soil and on food. It is aerobic (facultatively anaerobic), motile, and beta-hemolytic. Its defining survival trick is the spore: B. cereus spores withstand ordinary cooking temperatures, then germinate when food is left to sit at room temperature.

The organism causes two distinct food poisoning syndromes. The emetic type is the classic "reheated rice" illness: spores survive boiling, germinate as the rice sits out, and the bacteria produce cereulide, a preformed, heat-stable toxin that reheating cannot destroy. Vomiting begins quickly, typically within 1 to 6 hours of eating. The diarrheal type is caused by heat-labile enterotoxins produced in the gut after ingestion of contaminated meats, vegetables, or sauces, producing watery diarrhea and cramps roughly 8 to 18 hours later.

Both syndromes are self-limited, usually resolving within about 24 hours, and management is supportive with fluids rather than antibiotics — the emetic form in particular is an intoxication, not an infection. The rapid onset after a suspicious meal is the key diagnostic clue, and the short incubation of the emetic form groups it with Staphylococcus aureus food poisoning, which also results from a preformed toxin.

The USMLE Step 1 tests Bacillus cereus in its gram-positive bacilli and gastrointestinal pathology material. The high-yield associations: fried or reheated rice, preformed heat-stable cereulide toxin, vomiting within hours, spore formation, and the contrast between the fast emetic and slower diarrheal syndromes.