A microbiology quality control officer finds that a batch of culture media for selective isolation of Neisseria gonorrhoeae (Modified Thayer-Martin medium) is contaminated with Proteus mirabilis (swarming over the plates). Which component of the medium is responsible for inhibiting Proteus, and at what concentration is it included?

• A Nystatin 12,500 IU/L — antifungal used to suppress yeast, also inhibits Proteus
• B Vancomycin 3 mg/L — gram-positive inhibitor that secondarily blocks Proteus via unknown mechanism
• C Trimethoprim 5 mg/L — inhibits gram-negative swarming bacteria including Proteus ✓
• D Colistin (polymyxin E) 7.5 mg/L — inhibits gram-negative bacteria including Proteus while N. gonorrhoeae is intrinsically resistant to polymyxins

Explanation

Modified Thayer-Martin (MTM) medium contains the VCAT (or VCNT) supplement: Vancomycin (inhibits gram-positive bacteria), Colistin/polymyxin B (inhibits most gram-negative rods), Anisomycin or nystatin (inhibits fungi), and crucially Trimethoprim (5 mg/L) specifically to suppress swarming Proteus mirabilis, which is intrinsically resistant to colistin. N. gonorrhoeae is inherently resistant to trimethoprim at this low concentration. Without trimethoprim, Proteus would overgrow and mask gonococci. Colistin (polymyxin) is the component that inhibits other gram-negative rods but not Proteus.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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