A clinical microbiology lab uses a selective medium containing bile salts and sodium deoxycholate to selectively isolate Gram-negative enteric bacilli from stool. Lactose fermenters appear pink while non-fermenters are colourless. This medium is:
- A Thiosulphate citrate bile sucrose (TCBS) agar
- B Xylose lysine deoxycholate (XLD) agar
- C MacConkey agar ✓
- D Hektoen enteric agar
Correct answer: C. MacConkey agar
Explanation
MacConkey agar contains bile salts (to inhibit Gram-positive organisms), crystal violet, and neutral red indicator. Lactose-fermenting Gram-negatives produce acid, which turns the neutral red indicator pink/red. Non-fermenters appear colourless. It is the most widely used differential selective medium for Enterobacteriaceae. TCBS is used for Vibrio (yellow colonies for V. cholerae). XLD and Hektoen enteric agars are more selective media for Salmonella/Shigella isolation.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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