Vibrio cholerae
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Direct answer
Vibrio cholerae is a comma-shaped, motile, oxidase-positive Gram-negative bacillus that causes cholera through a secreted enterotoxin. Its AB5 cholera toxin keeps adenylate cyclase switched on in intestinal cells, so chloride, bicarbonate and water pour into the lumen. The result is profuse painless watery diarrhoea that can cause severe dehydration quickly, which makes rehydration the core of treatment.
What you must remember
- Morphology: curved Gram-negative rod with a single polar sheathed flagellum, producing darting motility under dark-field microscopy.
- Culture: grows in alkaline peptone water and on TCBS agar, where sucrose-fermenting colonies appear yellow.
- Serogroups: O1 and O139 cause epidemic cholera, and O139 (Bengal) was first recognised in India and Bangladesh.
- Cholera toxin: an AB5 toxin whose B subunits bind GM1 ganglioside and whose A subunit ADP-ribosylates Gs alpha.
- El Tor versus classical: El Tor is Voges-Proskauer positive, haemolyses sheep red cells and is polymyxin B resistant, while classical strains show the reverse pattern.
- Clinical picture: sudden painless watery stool, often described as rice-water, with effortless vomiting that follows the onset of diarrhoea, while fever and abdominal pain are usually absent.
- Complications: hypovolaemic shock, acidosis and hypokalaemia in severe untreated cases.
- Treatment: oral rehydration salts for most patients, with intravenous Ringer lactate for severe dehydration and single-dose antibiotics as an adjunct.
A rehydration plan matched to severity
Fluid replacement decides survival in cholera, so the plan is chosen according to the degree of dehydration. The steps run from mild to severe, following the three-plan approach used in WHO and national diarrhoea guidance.
- Assess dehydration: the clinician checks thirst, skin turgor, sunken eyes, pulse character and urine output to place the patient in the right plan.
- Plan A, home care: a patient without visible dehydration continues fluids at home with oral rehydration salts and is taught when to return.
- Plan B, some dehydration: oral rehydration salts are given in measured amounts over the first few hours, with the volume calculated from body weight.
- Plan C, severe dehydration: intravenous Ringer lactate is started rapidly, and oral salts follow as soon as the patient can drink safely.
- Zinc and antibiotic: zinc is added for children because it shortens the diarrhoea, and a single dose of doxycycline or azithromycin may be given in moderate to severe disease.
- Monitor and prevent: stool volume, urine output and vital signs are charted, and the source of drinking water is treated to stop further cases.
Where the exam tries to catch you
Cholera questions test the toxin mechanism, the biotypes and the first-line fluid choice.
- Trap: treating rice-water stool as a sign of Shigella dysentery Correct: cholera gives watery stool without blood or mucus, because the organism is non-invasive.
- Trap: believing that O1 is the only serogroup that can cause outbreaks Correct: O139 also causes epidemic cholera, while other non-O1 non-O139 vibrios usually cause sporadic diarrhoea.
- Trap: assuming cholera toxin destroys the epithelium Correct: the toxin drives secretion without killing enterocytes or invading the mucosa.
Frequently asked questions
What is the mechanism of cholera toxin in Vibrio cholerae?
Cholera toxin ADP-ribosylates the Gs alpha subunit, which keeps adenylate cyclase permanently active in enterocytes. The raised cyclic AMP drives chloride and bicarbonate secretion into the intestinal lumen, and water follows osmotically.
How does the El Tor biotype differ from the classical biotype of Vibrio cholerae?
El Tor strains are more hardy in the environment, produce more asymptomatic carriage and cause haemolysis of sheep red cells. El Tor is Voges-Proskauer positive and polymyxin B resistant, whereas classical Vibrio cholerae is Voges-Proskauer negative and polymyxin B sensitive.
Which antibiotic is used in cholera alongside rehydration?
Single-dose doxycycline or azithromycin is commonly used in cholera to shorten diarrhoea and reduce stool volume, per current guidance. Antibiotics never replace fluid replacement, which remains the decisive treatment for Vibrio cholerae infection.
Why is zinc recommended for children with cholera?
Zinc supplementation in children with diarrhoea, including cholera, reduces the duration of illness and the volume of stool loss. National paediatric guidance in India therefore pairs zinc with oral rehydration salts.
What is the role of oral cholera vaccine in India?
The oral cholera vaccine is used in outbreak response and in selected high-risk settings, and it complements water, sanitation and rehydration rather than replacing them. Its protection is partial and time-limited, so it does not remove the need for safe drinking water.