Bacillus cereus

1. "B. cereus: food poisoning" and "Bacillus in blood cultures is most often a contaminant" — direct GC 105 slide points ^[1]. If a vignette gives you a Gram-positive rod in blood culture from a non-neutropenic patient, the answer is contaminant; do not treat.

2. Incubation period < 6 h → preformed toxin (S. aureus, B. cereus) — explicitly taught in paediatric GE notes ^[2] and senior notes ^[3]. This is the single most tested fact about B. cereus. The vignette clue is vomiting after reheated rice with a very short incubation.

3. B. cereus is classified under "food poisoning" (non-inflammatory) GE alongside S. aureus and C. perfringens — producing secretory diarrhoea with NO fecal leukocytes ^[4][5]. This contrasts with inflammatory pathogens (Shigella, Campylobacter) that produce bloody/mucoid stool with fecal WBCs.

4. The diarrhoeal form of B. cereus sits in the "non-inflammatory / small-bowel type" category in the Ryan Ho classification table ^[4], while the emetic form is under "food poisoning" — both are watery, never bloody.

5. B. cereus enterotoxins are heat-labile (destroyed by reheating), but cereulide (emetic toxin) is heat-stable — this is why reheating leftover rice does NOT prevent the emetic syndrome. Exam trap: "why did the patient still vomit despite microwaving the rice?"

• Rice = B. cereus; cream/custard = S. aureus; reheated meat = C. perfringens. All three are classic food-poisoning vignette anchors.
• If the question says incubation < 6 hours with vomiting predominant → B. cereus emetic type or S. aureus. The food item discriminates: rice vs dairy.
• If the question says incubation 8–16 hours with diarrhoea predominant → B. cereus diarrhoeal type or C. perfringens. Food discriminates: rice/vegetables vs meat.
• Blood culture trap: Gram-positive rods growing in blood culture — if the patient is otherwise well with no indwelling lines, think contaminant. Only in neutropenic/line-associated patients should you consider true Bacillus bacteraemia.
• B. cereus is β-lactam resistant (β-lactamase producer). B. anthracis is penicillin sensitive. This is a favourite pharmacology discriminator.

No directly indexed past-paper question specifically about Bacillus cereus was identified in the provided Fourth Summative MCQ, SAQ, or Minicase papers (2016–2025). However, B. cereus knowledge is commonly tested indirectly via:

• Acute GE / food poisoning classification questions (incubation period, organism–food pairing)
• "Identify the organism" MCQs using incubation period and food clues
• Blood culture interpretation questions (contaminant vs true pathogen)

Students should be prepared for B. cereus to appear as a distractor or correct answer in any food-poisoning or acute diarrhoea stem.