Vibrio cholerae

Vibrio cholerae is a comma-shaped, Gram-negative, oxidase-positive, facultatively anaerobic bacterium that causes cholera — an acute, profuse, watery, non-inflammatory secretory diarrhoea capable of killing within hours through hypovolaemic shock. It is a perennial NEET PG favourite for its toxin mechanism, culture media, and ORS composition.

Definition & taxonomy

Vibrio cholerae belongs to the family Vibrionaceae. It is a halophilic-tolerant (grows in alkaline conditions), curved Gram-negative rod with a single polar monotrichous flagellum giving it characteristic darting/shooting-star motility on dark-field microscopy. It is the causative agent of epidemic and pandemic cholera.

High-yield: V. cholerae is oxidase positive, ferments sucrose, and grows on alkaline media (pH 8.2–8.6) — alkalinity selectively favours its growth while suppressing commensals.

Serogroup & biotype classification

Classification is based on the somatic O antigen (the H/flagellar antigen is non-specific, shared across the genus, hence not used for typing).

Feature	O1	O139 (Bengal)	Non-O1/Non-O139
Epidemic/pandemic potential	Yes (all pandemics)	Yes (1992 onwards)	No (sporadic gastroenteritis)
Cholera toxin	Produced	Produced	Usually absent
Capsule	Absent	Present (polysaccharide)	Variable
First reported	Classic strains	Madras/Bengal, 1992	—
Agglutinated by O1 antiserum	Yes	No	No

The O1 serogroup is further divided into two biotypes (Classical and El Tor) and three serotypes by combinations of A, B, C antigens.

Serotype	Antigen composition
Ogawa	A + B
Inaba	A + C
Hikojima (rare, unstable)	A + B + C

Mnemonic — serotypes: "Ogawa has B" (Ogawa = A + B), "Inaba = A + C" (think "In-C"). Hikojima has all three.

Classical vs El Tor biotype

This distinction is among the most repeated MCQ tables in microbiology.

Test / Feature	Classical	El Tor
Haemolysis of sheep RBCs	Non-haemolytic	Haemolytic (classically)
Voges–Proskauer (VP)	Negative	Positive
Chick erythrocyte agglutination	Negative	Positive
Polymyxin B (50 U) sensitivity	Sensitive	Resistant
Susceptibility to phage IV	Sensitive	Resistant
Susceptibility to El Tor phage V	Resistant	Sensitive
Carrier state / mild infections	Few	More common
Current dominant biotype	—	El Tor (7th pandemic)

High-yield: The El Tor biotype caused the ongoing 7th pandemic (since 1961). El Tor is hardier, produces more carriers, and is polymyxin B resistant and VP positive — the easiest two tests to remember.

Mnemonic — El Tor properties: "El Tor PHACVP" → Polymyxin resistant, Haemolytic, Agglutinates chick cells, VP positive, Phage IV resistant.

Pandemic history

Cholera has produced seven pandemics:

• 1st–6th pandemics: caused by the Classical biotype, originating from the Gangetic delta of India/Bengal.
• 7th pandemic (1961–present): caused by the El Tor biotype, beginning in Sulawesi (Celebes), Indonesia.
• O139 Bengal (1992): emerged in Chennai/Bengal coast, the first non-O1 strain to cause epidemics, briefly threatening an 8th pandemic.

High-yield: El Tor biotype is named after the El Tor quarantine station in the Sinai (Egypt) where it was first isolated from pilgrims.

Pathogenesis & cholera toxin

Cholera is a classic non-invasive, enterotoxin-mediated disease. The organism does not invade the mucosa or cause bacteraemia; the entire syndrome is due to a secreted toxin.

Sequence of events:

1. Ingestion of contaminated water/food (large infective dose ~10⁸ needed because the organism is acid-labile; achlorhydria/antacids lower the dose dramatically).
2. Colonisation of the small intestine via the toxin-coregulated pilus (TCP) — the principal colonisation factor and receptor for the CTXφ phage.
3. Cholera toxin (CT) secretion.

Mechanism flow: Cholera toxin (A–5B subunit) → B subunits bind GM1 ganglioside on enterocytes → A1 subunit enters cell → ADP-ribosylation of the Gsα subunit → locks adenylate cyclase ON → ↑ cyclic AMP → activation of CFTR/efflux pumps → massive Cl⁻ and HCO₃⁻ secretion + blocked Na⁺ absorption → isotonic fluid pours into the lumen → rice-water stool.

High-yield: Cholera toxin is an A-B₅ toxin. The A1 subunit ADP-ribosylates Gsα, persistently activating adenylate cyclase → ↑ cAMP. (Contrast: heat-labile toxin of ETEC works identically; pertussis toxin ADP-ribosylates Gi.)

Other virulence factors:

• CTXφ (CTX phage): lysogenic filamentous bacteriophage carrying the ctxA/ctxB genes encoding cholera toxin; integrated into the chromosome — explains horizontal toxin transfer.
• TCP: colonisation factor and phage receptor.
• ToxR / ToxT: regulatory cascade coordinating expression of CT and TCP.
• Zonula occludens toxin (Zot), Accessory cholera enterotoxin (Ace), mucinase, neuraminidase.

High-yield: Because the glucose-coupled Na⁺ co-transporter (SGLT1) remains intact during cholera, oral glucose drives sodium and water absorption — the entire rationale for Oral Rehydration Solution (ORS).

Clinical features

• Incubation period: a few hours to 5 days (usually 1–2 days).
• Painless, profuse watery diarrhoea — the hallmark rice-water stool: colourless, odourless (or "fishy/sweetish"), with flecks of mucus and no faecal smell, no blood, no pus (non-inflammatory).
• Vomiting (effortless, early), no fever (or low grade), no tenesmus.
• Fluid loss can reach >1 litre/hour → rapid isotonic dehydration.
• Signs of severe dehydration: sunken eyes, washerwoman's hands, loss of skin turgor, hypotension, tachycardia, anuria, the characteristic husky/aphonic "vox cholerica" voice, and muscle cramps.
• Complications follow from electrolyte loss: hypokalaemia, metabolic acidosis (bicarbonate loss), acute tubular necrosis/renal failure, hypoglycaemia in children.

High-yield: Cholera stool is isotonic with plasma but rich in potassium and bicarbonate → the patient develops hypokalaemia + metabolic acidosis, not hypernatraemia.

Laboratory diagnosis

Specimen: fresh stool or rectal swab; transported in alkaline peptone water (APW) or Venkataraman–Ramakrishnan (VR) medium or Cary–Blair medium if delay is expected.

Microscopy:

• Dark-field / hanging-drop: characteristic darting "shooting-star" motility, abolished on adding specific antiserum (immobilisation test) — a rapid bedside clue.

Enrichment & culture:

• Alkaline peptone water (pH 8.6): enrichment broth; subculture within 6–8 hours (vibrios grow rapidly on the surface as a pellicle).

Medium	Appearance of V. cholerae	Mechanism
TCBS agar (Thiosulphate–Citrate–Bile salts–Sucrose)	Large yellow colonies	Sucrose fermentation turns the bromothymol-blue indicator yellow
TTGA (Taurocholate–Tellurite–Gelatin agar)	Colonies with halo, grey-black centre	Gelatinase + tellurite reduction
MacConkey	Colourless → late pink	Non-lactose fermenter early
Monsur's medium	Translucent colonies, black centre	Tellurite reduction

High-yield: On TCBS agar, V. cholerae and V. alginolyticus form yellow (sucrose-fermenting) colonies, whereas V. parahaemolyticus forms green (non-sucrose) colonies.

Biochemical & confirmatory tests:

• Oxidase positive (key separator from Enterobacteriaceae).
• String test positive: colony emulsified in 0.5% sodium deoxycholate forms a mucoid "string" that can be pulled up with a loop (also positive for Aeromonas).
• Cholera red reaction: nitrosoindole reaction (indole + nitrate reduction in peptone water) → red colour.
• Heiberg fermentation patterns: V. cholerae ferments mannose and sucrose but not arabinose → Heiberg group I.
• Serotyping with O1 (Ogawa/Inaba) and O139 polyvalent antisera (slide agglutination).
• Rapid dipstick antigen tests and PCR (ctxA, tcpA, O1/O139 rfb genes) for field/epidemic confirmation.

Mnemonic — diagnosis combo: "Shooting stars in alkaline water grow yellow on TCBS, give a string, oxidase positive."

Management

Cornerstone = fluid and electrolyte replacement; antibiotics are adjunctive.

Stepwise approach:

1. Assess dehydration (none/some/severe).
2. Severe dehydration: IV Ringer lactate is the fluid of choice (contains potassium and lactate→bicarbonate); give rapidly (e.g. 100 mL/kg in first 3 hours). Normal saline lacks K⁺ and base, so is inferior.
3. Some/mild dehydration: Oral Rehydration Solution (ORS).
4. Add potassium (oral/IV) and monitor; treat hypoglycaemia in children.
5. Antibiotics for moderate–severe cases to shorten duration and reduce shedding.

WHO low-osmolarity ORS composition

Component	Concentration (mmol/L)
Sodium	75
Glucose (anhydrous)	75
Potassium	20
Chloride	65
Citrate	10
Total osmolarity	245 mOsm/L

High-yield: The reduced-osmolarity ORS has total osmolarity 245 mOsm/L with Na⁺ 75 and glucose 75 mmol/L (a roughly 1:1 sodium-to-glucose ratio), replacing the older 311 mOsm/L formula.

Antibiotics (adjunct):

• Doxycycline single dose is the drug of choice in adults (a single 300 mg dose).
• Azithromycin is preferred in children and pregnancy (and where doxycycline resistance exists).
• Alternatives: tetracycline, ciprofloxacin (rising resistance), cotrimoxazole.

High-yield: Single-dose doxycycline for adults; azithromycin for pregnant women and children. Antibiotics reduce stool volume and duration of shedding but are not life-saving — rehydration is.

Prevention & vaccines

• Safe water, sanitation, hand hygiene, and food safety remain primary.
• Oral killed whole-cell vaccines: Dukoral (WC + recombinant B subunit, O1 only) and Shanchol / Euvichol (bivalent O1 + O139, no B subunit — does not need a buffer, used in mass campaigns).
• The older parenteral killed vaccine is obsolete (poor, short-lived protection).

Complications

• Hypovolaemic shock and death (the major killer).
• Hypokalaemia → arrhythmias, ileus, cardiac arrest.
• Metabolic acidosis from bicarbonate loss.
• Acute kidney injury / acute tubular necrosis from prolonged hypoperfusion.
• Hypoglycaemia (especially children — a cause of seizures and death).
• Pulmonary oedema from over-vigorous IV rehydration without correcting acidosis.

Key differentials

Feature	V. cholerae	ETEC	V. parahaemolyticus	Rotavirus
Stool	Rice-water, profuse	Watery	Watery, sometimes bloody	Watery
Mechanism	Cholera toxin (↑cAMP)	LT/ST toxins	TDH (Kanagawa), invasive-like	Enterotoxin NSP4
Source	Water	Travellers' diarrhoea	Seafood (shellfish)	Children
TCBS colour	Yellow	—	Green	—
Fever/blood	Absent	Absent	May be present	Mild

Other secretory diarrhoeas to consider: other Vibrio spp. (V. vulnificus — septicaemia in liver disease/iron overload, raw oysters), and toxin-mediated Bacillus cereus / Staph aureus food poisoning (short incubation, vomiting predominant).

Recently asked / exam angle

• Mechanism of cholera toxin: "ADP-ribosylation of Gsα → ↑ cAMP" — repeatedly tested; distinguish from pertussis (Gi) and diphtheria (EF-2).
• Receptor for cholera toxin: GM1 ganglioside (B subunit binding).
• Biotype of the 7th pandemic: El Tor.
• TCBS colony colour: V. cholerae = yellow (sucrose fermenter).
• Reduced-osmolarity ORS: total 245 mOsm/L, Na 75 / glucose 75.
• Fluid of choice in severe cholera: Ringer lactate.
• Single-dose antibiotic of choice (adult): doxycycline.
• O139 distinguishing feature: it is capsulated and not agglutinated by O1 antiserum.
• Dark-field motility: "shooting-star / darting" — abolished by specific antiserum.
• Transport medium of choice: VR fluid / alkaline peptone water; Cary–Blair for delay.
• Toxin-coregulated pilus (TCP): colonisation factor + receptor for CTXφ phage.

Rapid revision

1. V. cholerae — curved Gram-negative rod, oxidase positive, single polar monotrichous flagellum, darting motility.
2. O1 and O139 serogroups cause epidemics; non-O1/O139 cause sporadic gastroenteritis only.
3. O1 serotypes: Ogawa (A+B), Inaba (A+C), Hikojima (A+B+C).
4. El Tor biotype (7th pandemic): VP positive, haemolytic, polymyxin-B resistant, agglutinates chick RBCs.
5. Cholera toxin = A-B₅; A1 ADP-ribosylates Gsα → ↑cAMP → Cl⁻/HCO₃⁻ secretion → rice-water stool.
6. B subunit binds GM1 ganglioside; toxin genes (ctxA/ctxB) carried by the CTXφ phage.
7. TCP is the colonisation factor and phage receptor; ToxR/ToxT regulate virulence.
8. Disease is non-invasive, no fever, no blood/pus; loss is isotonic → hypokalaemia + metabolic acidosis.
9. Culture: alkaline peptone water enrichment → TCBS = yellow colonies; string test positive, oxidase positive.
10. ORS works because SGLT1 (Na-glucose co-transport) is intact; low-osmolarity ORS = 245 mOsm/L.
11. Severe dehydration → IV Ringer lactate; mild → ORS; add potassium.
12. Antibiotic: single-dose doxycycline (adults), azithromycin (children/pregnancy); vaccines — Dukoral, Shanchol.