Stool from a patient with acute watery diarrhea is plated on thiosulfate citrate bile salts sucrose (TCBS) agar. After overnight incubation, smooth yellow colonies are seen. The colony color indicates that the isolate:

• A Does not ferment sucrose, consistent with Vibrio parahaemolyticus
• B Ferments sucrose, consistent with Vibrio cholerae ✓
• C Ferments lactose, consistent with E. coli
• D Produces hydrogen sulfide, consistent with Salmonella

Explanation

TCBS is the selective medium of choice for vibrios, containing high pH, bile salts, citrate and thiosulfate to inhibit other enteric flora. Sucrose-fermenting strains such as V. cholerae produce yellow colonies, while non-sucrose-fermenting vibrios like V. parahaemolyticus produce green colonies. This single color difference is the standard screening step before confirmatory slide agglutination with O1 antiserum. Salmonella does not grow well on TCBS and E. coli is inhibited by the medium.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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