# Vibrio Cholerae

> Vibrio cholerae for MBBS Microbiology: comma-shaped bacilli, cholera toxin action, TCBS culture, serogroups O1 and O139, rice-water stool and treatment points.

- Canonical URL: https://prepelephant.com/topics/mbbs/microbiology/vibrio-cholerae
- Exam / course: MBBS · Subject: Microbiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Vibrio Cholerae", PrepElephant, https://prepelephant.com/topics/mbbs/microbiology/vibrio-cholerae

## Direct answer

Vibrio cholerae is a comma-shaped, gram-negative, highly motile bacillus with a single polar flagellum, oxidase positive, growing best in alkaline media and causing cholera — painless profuse watery diarrhoea with rice-water stools and rapid dehydration. The El Tor biotype of serogroup O1 and the O139 serogroup cause epidemic cholera in India through the cholera toxin, which raises cyclic adenosine monophosphate in enterocytes and drives massive chloride and water secretion. Treatment is dominated by rehydration, with a single dose of doxycycline or azithromycin as an adjunct.

## What you must remember

- **Morphology:** comma-shaped gram-negative bacilli with darting, shooting-star motility on hanging drop, immobilised by specific antiserum in the immobilisation test.
- **Culture:** grows at alkaline pH of about 8.6 in alkaline peptone water and on thiosulphate citrate bile salts sucrose agar, forming yellow sucrose-fermenting colonies; it does not grow on MacConkey agar.
- **Classification:** serogroup O1 with classical and El Tor biotypes causes most epidemics — El Tor is identified by haemolysin production and chicken red cell agglutination; serogroup O139, which emerged in India in the 1990s, causes similar disease.
- **Toxin:** the B subunit of cholera toxin binds the GM1 ganglioside while the A subunit activates adenylate cyclase; the raised cyclic adenosine monophosphate opens chloride channels, and water follows into the lumen.
- **Clinical picture:** painless voluminous rice-water stool with mucus flecks, no fever, and dehydration with hypokalaemia and acidosis causing death unless fluids are replaced.
- **Diagnosis:** hanging drop for motility, alkaline peptone water enrichment with TCBS subculture, agglutination with O1 and O139 antisera, and rapid dipsticks in outbreaks.
- **Treatment and prevention:** oral rehydration solution or intravenous Ringer's lactate is life-saving; doxycycline once in adults, azithromycin in children and pregnancy, shortens diarrhoea; killed oral vaccines help in endemic outbreaks.

## Common confusion

Vibrio cholerae is non-halophilic, unlike the halophilic Vibrio parahaemolyticus from seafood — students often present them as identical. The toxin mechanism is garbled: remember binding, adenylate cyclase activation, raised cyclic adenosine monophosphate and chloride secretion — secretion, not invasion — so the stool has no pus cells and there is no fever, in contrast to bacillary dysentery.

## Exam-focused takeaway

In theory, describe the morphology with hanging drop, the alkaline media with TCBS colonies, biotypes and serogroups, the toxin's mechanism and rehydration-first management. In viva, expect the reason for alkaline media, the difference between El Tor and classical biotypes, and why the stool has no pus. At the practical, darting motility on hanging drop, yellow colonies on TCBS and agglutination in O1 antiserum complete the identification.

## Frequently asked questions

### Why is alkaline peptone water used?

The organism grows best around pH 8.6 while acidic media inhibit it, so alkaline peptone water enriches vibrios from stool before TCBS subculture.

### How does cholera toxin cause diarrhoea?

Its B subunit binds GM1 on enterocytes and the A subunit activates adenylate cyclase; the rise in cyclic adenosine monophosphate drives chloride and water secretion into the lumen.

### What distinguishes El Tor from classical biotypes?

El Tor strains produce haemolysin, agglutinate chicken red cells, survive longer in the environment and usually cause milder but more persistent infection.

### What is the importance of the O139 serogroup?

It emerged in southern India in the 1990s, causes classic cholera, and immunity to O1 does not protect against it, allowing new epidemics.

### Why is there no fever or pus in cholera stool?

The illness is toxin-mediated without mucosal invasion, so inflammation is minimal, unlike the invasive bacillary dysentery.

### Which single-dose antibiotics are used?

Doxycycline in adults and azithromycin in children and pregnancy reduce stool volume and duration; rehydration remains the life-saving measure.