Which characteristic laboratory test helps identify Vibrio species in stool and what is the typical selective medium?

• A. Oxidase-positive curved Gram-negative rods; TCBS agar
• B. Gram-positive rods; blood agar
• C. Acid-fast bacilli; Lowenstein-Jensen medium
• D. Catalase-positive cocci; mannitol salt agar

Answer: (A)

Vibrio in stool is most reliably suggested by its oxidase-positive result plus its appearance as curved Gram-negative rods. The oxidase test helps separate Vibrio from many other enteric bacteria that are oxidase negative, and the curved, motile Gram-negative rod morphology fits Vibrio rather than straight rods or cocci. To actually isolate and identify Vibrio from stool, a selective medium is used: thiosulfate-citrate-bile-salt-sucrose agar (TCBS). TCBS suppresses many other bacteria with bile salts and high pH, while supporting Vibrio growth; it also helps differentiate by sugar fermentation—Vibrio cholerae typically forms yellow colonies due to sucrose fermentation, whereas other vibrios may form green colonies.