A patient returning from coastal Odisha presents with profuse rice-water diarrhea. Stool culture on a medium containing thiosulfate, citrate, bile salts and sucrose yields yellow colonies. The reason the colonies are yellow is:

• A Fermentation of lactose producing acid that changes neutral red
• B Utilization of citrate raising the pH and shifting phenol red
• C Reduction of thiosulfate to hydrogen sulfide detected by ferric ammonium citrate
• D Fermentation of sucrose producing acid that changes bromothymol blue ✓

Explanation

TCBS agar contains sucrose as the fermentable carbohydrate and bromothymol blue as indicator. Vibrio cholerae ferments sucrose, producing acid that turns the surrounding medium and colonies yellow, while non-sucrose-fermenting vibrios such as V. parahaemolyticus form green colonies. Lactose with neutral red is the MacConkey system, and hydrogen sulfide detection is not part of TCBS, making those mechanisms incorrect for this observation.

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.