Stool from a patient with suspected cholera is plated on thiosulfate citrate bile salts sucrose (TCBS) agar. After overnight incubation, smooth yellow colonies are seen. The yellow colour indicates that the organism:

• A Produces indole from tryptophan
• B Reduces thiosulfate to hydrogen sulfide
• C Hydrolyses bile salts
• D Ferments sucrose producing acid ✓

Explanation

TCBS is a selective medium for Vibrio species. It contains sucrose and the pH indicator bromothymol blue. Vibrio cholerae ferments sucrose, lowering the pH locally so colonies appear yellow. Non-sucrose-fermenting halophilic vibrios such as Vibrio parahaemolyticus form green colonies on the same medium, which kills that tempting alternative. Thiosulfate reduction gives blackening only in media containing iron such as XLD or SS agar, not a yellow colour.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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