Vibrio cholerae

1. V. cholerae is the prototypical organism for non-inflammatory (secretory / small-bowel type) diarrhoea — contrasted with inflammatory (large-bowel type: Shigella, EHEC, Campylobacter) and invasive/penetrating (enteric fever: S. typhi). This classification framework appears in GC 142 (A child with loose stool) and multiple senior notes ^[4][5][6].

2. Cholera toxin ADP-ribosylates Gsα → constitutive adenylyl cyclase activation → ↑cAMP → massive chloride and water secretion — this molecular mechanism is a favourite MCQ target. Know that E. coli heat-labile toxin (LT) shares the identical mechanism, and pertussis toxin also ADP-ribosylates Gsα (but inhibits Giα in some formulations of the question) ^[1][2].

3. The table "Non-inflammatory vs Inflammatory vs Penetrating diarrhoea" is a core exam framework: V. cholerae sits firmly in the non-inflammatory column alongside ETEC, Rotavirus, Norovirus, Giardia, C. perfringens, and B. cereus. Stool shows NO fecal leukocytes ^[4][5].

4. TCBS agar → yellow colonies is the laboratory discriminator for V. cholerae (sucrose fermenter) vs V. parahaemolyticus (green colonies, non-sucrose fermenter). Both are oxidase +ve curved G−ve rods.

5. Treatment is primarily rehydration — antibiotics are adjunctive. ORS is the single most important intervention in cholera management. This is a key paediatric and public health exam point.

6. Notifiable disease in HK — any suspected cholera case must be reported to the Centre for Health Protection. Outbreak / infection control questions may reference GC 104 (Infection outbreak/infection control) ^[1].

• Rice-water stool = cholera until proven otherwise. If a vignette describes profuse watery diarrhoea after travel to an endemic area with rapid dehydration and NO blood/mucus, think V. cholerae.
• Bloody diarrhoea with fecal leukocytes → NOT cholera. Think Shigella, EHEC, Campylobacter, V. parahaemolyticus, Entamoeba histolytica.
• Seafood exposure + diarrhoea: V. cholerae (watery) vs V. parahaemolyticus (bloody/inflammatory) vs V. vulnificus (wound infection / septicaemia in liver cirrhosis patients — different clinical syndrome entirely).
• Toxin mechanism trap: Cholera toxin and E. coli LT toxin both → ↑cAMP. E. coli ST toxin → ↑cGMP (activates guanylyl cyclase). Don't confuse them.
• El Tor biotype now predominates globally; causes milder disease but higher carrier rate than Classical biotype — may appear in epidemiology questions.
• Achlorhydria / PPI use lowers the infective dose — a risk factor vignette clue.

No specific past paper questions directly examining Vibrio cholerae as a named organism or primary diagnosis were identified in the indexed Fourth Summative past papers (2016–2025). However, the non-inflammatory vs inflammatory diarrhoea classification framework (in which V. cholerae is the index organism) is heavily tested:

• The 2019 Fourth Summative Mini Case Case 1 ^[7] presents a traveller from Bangladesh with fever, constipation then diarrhoea, relative bradycardia, and rose spots — this is typhoid fever (S. typhi), the "penetrating/invasive" column organism. V. cholerae is a key differential to exclude (no blood, afebrile, rice-water stool would point to cholera instead). Understanding the three-column diarrhoea framework is essential to score on such questions.

• The diarrhoea classification table from senior notes (Ryan Ho Fundamentals ^[5], Felix Lai Paeds ^[4]) mirrors GC 142 lecture content and is the basis for MCQ distractors that test whether students can correctly categorise organisms into non-inflammatory / inflammatory / invasive.

Students should be prepared for an MCQ asking "Which organism causes non-inflammatory watery diarrhoea?" with V. cholerae and ETEC as correct-answer options, or "Which toxin mechanism increases intracellular cAMP?" with cholera toxin as the answer.