A purulent aspirate of joint fluid from a young woman shows gram-negative diplococci within neutrophils on Gram stain. Which selective medium is most appropriate for primary isolation of the organism?

• A. Mannitol salt agar
• B. Potassium tellurite agar
• C. Cefsulodin-irgasan-novobiocin
• D. Modified Thayer-Martin agar

Answer: (D)

The main idea is that Neisseria gonorrhoeae is a fastidious organism that grows best on a selective, enriched medium that suppresses competing flora. In a purulent joint fluid with gram-negative diplococci inside neutrophils, you’re looking for a medium that not only supports Neisseria growth but also inhibits other bacteria and fungi commonly present in the specimen.

Modified Thayer-Martin agar is designed for this purpose. It uses a chocolate base enriched medium and includes antibiotics such as vancomycin to inhibit Gram-positive organisms, colistin (or polymyxin) to suppress other Gram-negatives, and nystatin (or another antifungal) to prevent yeasts. This combination creates a selective environment in which Neisseria gonorrhoeae can be isolated from specimens with mixed flora, like joint fluid.

Other media listed are not appropriate for primary gonococcal isolation. Mannitol salt agar selects for Staphylococcus species due to high salt content and is not suitable for Neisseria. Potassium tellurite agar is tailored for organisms like Corynebacterium diphtheriae and some other flora, not Neisseria. CIN agar is selective for Yersinia species and related enteric bacteria, not Neisseria.