Core concept1 exam ›
- Incubation period is the single most discriminating variable - it separates preformed toxin from in-vivo toxin from invasion
| Incubation | Mechanism | Organisms |
|---|---|---|
| 1-6 h, vomiting dominant | PREFORMED heat-stable toxin | Staph aureus (cream, mayonnaise, cold meats), Bacillus cereus emetic (reheated/fried rice) |
| 8-16 h, watery diarrhoea, cramps | Toxin produced in gut | Clostridium perfringens (reheated meat, gravy), Bacillus cereus diarrhoeal |
| 16-72 h, diarrhoea +/- fever | Invasive or enterotoxigenic | Salmonella, Campylobacter, Shigella, ETEC, Vibrio parahaemolyticus |
| 1-8 days, bloody, no/low fever | Shiga toxin | EHEC/STEC O157:H7 (undercooked mince, unpasteurised milk) |
| Days to weeks | Invasive/intracellular | Listeria (soft cheese, pate, deli meats, rockmelon), Yersinia, Brucella |
- "Vomiting within hours of reheated rice" is Bacillus cereus emetic toxin - it is preformed and heat-stable, so reheating does not destroy it
Key detail
Exposure pointers
| Exposure | Organism |
|---|---|
| Reheated fried rice | Bacillus cereus (cereulide) |
| Poultry, eggs, reptiles | Salmonella |
| Chicken, unpasteurised milk | Campylobacter jejuni (commonest bacterial cause in Australia) |
| Undercooked beef mince, unpasteurised milk, petting zoos | STEC/EHEC O157:H7 |
| Raw or undercooked seafood, shellfish | Vibrio parahaemolyticus; Vibrio vulnificus in liver disease/haemochromatosis (fulminant sepsis, bullae) |
| Soft cheese, pate, cold deli meats, rockmelon | Listeria - pregnancy, immunosuppression, elderly |
| Home-canned food, honey (infants) | Clostridium botulinum - descending flaccid paralysis |
| Reef fish (barracuda, coral trout) | Ciguatera - cold allodynia, perioral paraesthesia |
| Scombroid fish (tuna, mackerel) poorly refrigerated | Histamine - flushing, mimics allergy |
| Rice/pork in SE Asia; recent antibiotics | C. difficile |
Complications to know
- STEC -> haemolytic uraemic syndrome in ~5-15% of children
- Triad: microangiopathic haemolytic anaemia + thrombocytopenia + acute kidney injury
- *Antibiotics and antimotility agents increase the risk of HUS - do not give them in bloody diarrhoea until STEC is excluded*
- Campylobacter -> Guillain-Barre (AMAN), reactive arthritis
- Yersinia -> mesenteric adenitis mimicking appendicitis; Salmonella -> osteomyelitis in sickle cell disease, aortitis
- Listeria -> meningitis and bacteraemia; miscarriage and neonatal sepsis - amoxicillin/benzylpenicillin, NOT cephalosporins, to which Listeria is intrinsically resistant
Clinical relevance
- Most bacterial gastroenteritis is self-limiting and needs rehydration, not antibiotics - antibiotics prolong Salmonella carriage and increase HUS risk in STEC
- Treat when: severe or invasive disease, immunocompromise, prosthetic material, extremes of age, systemic sepsis, Shigella, cholera, listeriosis, typhoid, travellers' diarrhoea with severe symptoms
- Campylobacter -> azithromycin (high and rising quinolone resistance)
- Shigella -> azithromycin or ciprofloxacin by susceptibility (extensively drug-resistant Shigella is an emerging problem, especially in men who have sex with men)
- Listeria -> amoxicillin (+/- gentamicin)
- C. difficile -> oral vancomycin first-line; fidaxomicin for recurrence; faecal microbiota transplant for multiply recurrent disease - metronidazole is no longer first-line
- Stool testing when there is blood, fever, severe illness, immunosuppression, recent travel or antibiotics, or an outbreak - multiplex PCR panels now dominate
- Notifiable in Australia: Salmonella, Shigella, Campylobacter, STEC/HUS, listeriosis, botulism, cholera, typhoid
Correlations
- Gram stain classification of common pathogens
- Molecular mimicry (Campylobacter and Guillain-Barre)
- Thrombotic microangiopathy - HUS
- Antimicrobial resistance and stewardship
- Faecal calprotectin as a marker of intestinal inflammation
4 of 4 sections written · drafted 2026-09-04