Vibrio
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Direct answer
Vibrio cholerae is the comma-shaped, polar-flagellated, oxidase-positive gram-negative bacillus that grows best in alkaline media — alkaline peptone water at pH 8.6 for enrichment and thiosulphate citrate bile salts sucrose agar for plating, where it ferments sucrose into yellow colonies — and produces rice-water stool through one elegant toxin: cholera toxin enters enterocytes and ADP-ribosylates the Gs protein, keeping adenylate cyclase switched on so that cyclic AMP drives chloride out through CFTR with water following. Serogroups O1 (classical and El Tor biotypes; Ogawa, Inaba and Hikojima serotypes) and O139 cause epidemic cholera; non-O1, non-O139 strains cause sporadic mild diarrhoea.
What you must remember
- Laboratory card: darting motility on hanging drop, oxidase positive (the family Enterobacteriaceae is negative), growth at pH 8.6–9.0, yellow sucrose-fermenting colonies on TCBS, and a positive string test with 0.5 per cent sodium deoxycholate.
- Cholera toxin structure: one A subunit (the enzymatic ADP-ribosyltransferase) and five B subunits binding the GM1 ganglioside receptor — the same architecture as the heat-labile E. coli toxin, encoded by CTX phi phage.
- El Tor biotypes cause milder disease with more asymptomatic carriers and have largely replaced classical biotypes globally, including in Indian outbreaks.
- Stool picture: no pus, no blood, flecks of mucus in watery "rice-water" stool, and a field diagnostic of rapid fluid loss with sunken eyes, reduced skin turgor and absent urine output in severe cases.
- Rehydration arithmetic governs survival: WHO plan A for no dehydration, plan B (75 millilitres per kilogram over four hours) for some, plan C (100 millilitres per kilogram, half in the first 30 minutes) with rapid intravenous Ringer's lactate when severe.
- Antibiotics are adjuncts — azithromycin as a single dose or tetracycline for sensitive strains shorten the illness by about a day and cut shedding; they never substitute for fluids.
- V. parahaemolyticus, from seafood, is a common cause of food-borne diarrhoea in coastal Japan and India (Kanagawa haemolysis phenomenon); V. vulnificus causes septicaemia and bullous lesions in liver disease after oysters or seawater exposure.
- Outbreak control rests on water, sanitation and notification — chlorination of water sources and hand hygiene — not on chemoprophylaxis of contacts.
Handling a suspected cholera case from door to report
A man from a flood-relief camp passes painless voluminous watery stool with mucus flecks and vomits; the tense abdomen and absent radial pulse say severe dehydration. Resuscitation starts before any test: intravenous Ringer's lactate, 100 millilitres per kilogram, half within the first half-hour, reassessed continuously. The specimen goes into alkaline peptone water via Cary–Blair; a hanging drop examined immediately shows the darting motility that stops dead on adding a drop of antisera — immobilisation is both a bedside and a rapid laboratory clue. After enrichment, TCBS gives yellow colonies; oxidase positivity and the string test complete confirmation. The patient recovers with fluids alone, receives a single dose of azithromycin to shorten shedding, and is counselled about hand hygiene; the public-health team chlorinates the camp's water point and reports the case — in India a notifiable, outbreak-prone disease under Integrated Disease Surveillance Programme criteria.
Where students slip
The oxidase point is under-used: candidates forget that this one positive test lifts vibrios out of the entire oxidase-negative Enterobacteriaceae crowd, which is exactly how the exam frames "which organism differs". Second, the toxin mechanism gets half-remembered — it is permanent activation of Gs (up), not Gi, raising cyclic AMP and opening CFTR chloride channels; sodium and water follow into the lumen. Third, the medium-and-pH trivia invert: alkaline peptone water enriches, TCBS selects and differentiates by sucrose, and pH 8.6 is the number the stem tests. Finally, the therapy question baits "start antibiotics": the mark goes to aggressive rehydration, with azithromycin or tetracycline as the adjunct that shortens shedding.
Frequently asked questions
Why does cholera toxin cause secretory diarrhoea?
Its A subunit ADP-ribosylates and permanently activates the Gs regulatory protein, keeping adenylate cyclase active; sustained cyclic AMP opens CFTR chloride channels, and water and electrolytes pour into the lumen.
Which media isolate Vibrio cholerae from stool?
Alkaline peptone water at pH 8.6 for enrichment, then TCBS agar where V. cholerae ferments sucrose to yellow colonies; the hanging drop shows characteristic darting motility.
What distinguishes El Tor from classical biotypes?
El Tor causes milder illness with more carriers and survives longer in the environment; bench markers include a positive Voges–Proskauer test and polymyxin resistance, and it has largely displaced classical strains.
What is the priority in treating severe cholera?
Rapid rehydration — intravenous Ringer's lactate at 100 millilitres per kilogram with half given in 30 minutes for severe dehydration — with oral rehydration solution continuing recovery; antibiotics only shorten shedding.
Which vibrio causes seafood-associated watery diarrhoea with Kanagawa haemolysis?
Vibrio parahaemolyticus from seafood, associated with the Kanagawa haemolysin; V. vulnificus is the separate seafood organism causing septicaemia in chronic liver disease.