Laboratory diagnosis of cholera (Vibrio cholerae) relies on rapid direct demonstration and culture, since prompt confirmation supports timely outbreak response:
Direct microscopy — hanging-drop preparation of fresh stool shows characteristic rapid “darting/shooting-star” motility, which is specifically immobilized on addition of V. cholerae O1/O139 antiserum, providing a rapid presumptive diagnosis within minutes.
String test — a loopful of growth/stool mixed in 0.5% sodium deoxycholate forms a mucoid string when the loop is lifted — a quick presumptive bedside/lab test.
Culture — stool is first enriched in alkaline peptone water (pH 8.6, which selectively favours V. cholerae growth over other gut flora) for 6–8 hours, then subcultured onto TCBS (thiosulfate-citrate-bile salts-sucrose) agar, the selective/differential medium of choice, on which V. cholerae produces characteristic yellow, sucrose-fermenting colonies.
Biochemical confirmation and serotyping — oxidase-positive Gram-negative curved bacilli, confirmed biochemically, then serogrouped by slide agglutination with specific O1 and O139 antisera; O1 strains are further subtyped into Ogawa, Inaba, or Hikojima serotypes, and biotyped as classical or El Tor.
Sample transport: stool/rectal swab should be sent promptly in Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium (alkaline transport media specific for V. cholerae, avoiding glycerol-based media which are toxic to the organism).
