A stool sample from a patient with acute watery diarrhea is plated on an agar containing thiosulfate, citrate, bile salts, and sucrose at pH 8.6. After overnight incubation, flat yellow colonies are observed. The yellow colour is produced because the organism:

• A Hydrolyses bile salts, releasing a fluorescent pigment
• B Reduces thiosulfate to hydrogen sulfide, forming black colonies
• C Utilizes citrate as the sole carbon source, alkalinising the medium
• D Ferments sucrose, producing acid that changes bromothymol blue indicator ✓

Explanation

TCBS agar is the selective medium for Vibrio species. The high pH (8.6) and bile salts inhibit most other enteric organisms. Vibrio cholerae ferments sucrose, producing acid that turns the bromothymol blue indicator yellow. Non-sucrose-fermenting vibrios such as V. parahaemolyticus produce green colonies on the same medium, which is the discriminating feature.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.