Question
Etiology and laboratory diagnosis of Cholera. (1+5)
Answer
Etiology: Cholera is caused by Vibrio cholerae, a curved, comma-shaped, motile, Gram-negative bacillus. Only serogroups O1 and O139 produce cholera toxin and cause epidemic cholera; O1 further splits into biotypes classical and El Tor (the biotype behind the ongoing seventh, current pandemic since 1961), with serotypes Ogawa and Inaba.
Laboratory diagnosis:
- Stool microscopy: a hanging-drop or dark-field preparation of fresh stool shows characteristic darting (“shooting star”) motility; adding specific anti-O1/O139 antiserum and observing that this motility stops (immobilization test) provides rapid presumptive confirmation.
- Culture: TCBS (Thiosulfate-Citrate-Bile-Salts-Sucrose) agar, the standard selective medium, yields large, yellow (sucrose-fermenting) colonies; alkaline peptone water serves as an effective enrichment broth, exploiting the organism’s alkaline tolerance and rapid growth relative to competing flora.
- Confirmatory identification: biochemical tests (the organism is oxidase-positive, unlike most Enterobacteriaceae) and slide agglutination with O1/O139-specific antisera.
- Rapid diagnostic tests: dipstick immunochromatographic assays detecting O1/O139 antigen directly in stool, increasingly used in field/outbreak settings for speed, though culture remains necessary for definitive confirmation and antimicrobial susceptibility testing.
Treatment note (context): fluid and electrolyte replacement (oral rehydration solution, or IV fluids — Ringer’s lactate preferred — for severe dehydration) is the single most important, life-saving intervention; antibiotics (doxycycline, azithromycin, or ciprofloxacin) are a useful adjunct that shortens illness duration and reduces vibrio excretion, but are secondary to rehydration.