Fecal cultures are inoculated on thiosulfate-citrate-bile salts-sucrose agar specifically for isolation of which organism?

• A. Shigella
• B. Vibrio
• C. Campylobacter
• D. Salmonella

Answer: (B)

Thiosulfate-citrate-bile salts-sucrose agar is a selective-differential medium used to isolate Vibrio species from fecal samples. The bile salts and the alkaline (high pH) environment suppress many other gut bacteria, allowing Vibrio to grow. The sucrose in the medium lets you differentiate Vibrio: those that ferment sucrose produce acid and turn the colonies yellow (and often the surrounding medium also yellow), while non-sucrose fermenters stay green. This combination makes it especially useful when Vibrio infection (such as cholera) is suspected. Other organisms listed are typically isolated on different media and conditions (for example, Shigella and Salmonella on XLD or MacConkey, Campylobacter on Campy agar under microaerophilic conditions), so TCBS is the specific choice for Vibrio isolation.