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

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

Answer: (B)

Thiosulfate-citrate-bile salts-sucrose agar is a selective differential medium designed to isolate Vibrio species from clinical or environmental samples. The bile salts and the relatively high pH inhibit many other enteric bacteria, giving Vibrio a growth advantage. The differential part comes from sucrose: vibrios that ferment sucrose produce acid, causing a color change in the indicator and giving yellow colonies, while non–sucrose-fermenting vibrios (and other organisms that don’t ferment the carbohydrate) appear green or colorless. This combination makes TCBS particularly useful for identifying and isolating Vibrio species, such as those seen in cholera and related infections, from stool or seawater samples. Other genera like Shigella, Salmonella, or Campylobacter do not rely on this medium for selective isolation and typically do not produce the same distinctive yellow colonies on TCBS.