Vibrio cholerae

Vibrio cholerae is a gram-negative, comma-shaped (curved) bacillus with a single polar flagellum. It is oxidase-positive, grows well in alkaline media (thiosulfate-citrate-bile salts-sucrose, or TCBS, agar), and is acid-labile — most organisms are killed by stomach acid, so a large inoculum is required and people with reduced gastric acidity are more susceptible. Transmission is fecal-oral, classically through contaminated water or undercooked shellfish in areas with poor sanitation.

Disease is driven by cholera toxin, an AB exotoxin. Its A subunit permanently activates the Gs protein by ADP-ribosylation, locking adenylate cyclase on and flooding intestinal cells with cyclic AMP (cAMP). The result is continuous secretion of chloride and water into the gut lumen. Importantly, the bacterium is non-invasive: it colonizes the small intestine without entering cells or the bloodstream, so the watery diarrhea contains no blood or pus.

The clinical hallmark is voluminous rice-water stools — pale, flecked diarrhea that can exceed a liter per hour, causing rapid dehydration, electrolyte loss, and hypovolemic shock. Death can occur within hours, yet treatment is straightforward: aggressive oral rehydration solution (or IV fluids in severe cases), with antibiotics such as doxycycline playing a secondary role in shortening illness.

USMLE Step 1 tests V. cholerae as a mechanism classic: the Gs/cAMP toxin pathway (contrasted with E. coli heat-labile toxin, which acts the same way), comma-shaped morphology on alkaline media, and rehydration as the cornerstone of therapy.