A laboratory needs to select Neisseria gonorrhoeae from a genital swab that is expected to be contaminated with normal flora. Which selective medium is used, and what components make it selective?
- A MacConkey agar — inhibits gram-positives via bile salts and selects gram-negatives
- B Thayer-Martin medium — a modified chocolate agar with vancomycin (inhibits gram-positives), colistin (inhibits gram-negative rods), nystatin (inhibits fungi) and trimethoprim (inhibits Proteus) ✓
- C TCBS agar with NaCl supplementation — inhibits non-Neisseria flora via alkaline pH
- D Loeffler's serum slope — enhances Neisseria growth and suppresses flora
Explanation
Thayer-Martin (TM) medium is modified chocolate (heat-lysed blood) agar supplemented with the VCN (vancomycin, colistin, nystatin) inhibitor mixture plus trimethoprim (the VCNT formulation also called Modified Thayer-Martin). Vancomycin suppresses gram-positive organisms (streptococci, staphylococci); colistin (polymyxin E) suppresses gram-negative bacilli (E. coli, Klebsiella) by disrupting their outer membrane; nystatin inhibits fungi; trimethoprim inhibits swarming Proteus. Neisseria (fastidious, requiring X and V factors) grows as oxidase-positive grey-white colonies. MacConkey differentiates Enterobacteriaceae, not Neisseria.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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