Which selective medium is used for primary isolation of Neisseria species, including Neisseria gonorrhoeae, from clinical specimens?

• A. Mannitol Salt Agar
• B. Potassium Tellurite Agar
• C. Modified Thayer-Martin Agar
• D. Cefsulodin-Irgasan-Novobiocin Agar

Answer: (C)

Neisseria species, including Neisseria gonorrhoeae, require a medium that is both enriched for their growth and selective to suppress competing flora. The Modified Thayer-Martin medium provides an enriched chocolate base (giving Neisseria access to essential growth factors) and adds antibiotics that inhibit many organisms found in clinical specimens: vancomycin blocks Gram-positive bacteria, colistin inhibits many Gram-negative rods, nystatin suppresses fungi, and trimethoprim further reduces flora such as Proteus. This combination creates a selective environment where Neisseria gonorrhoeae and related Neisseria species can grow while contaminants are inhibited, making it the standard choice for primary isolation from sites like urogenital, pharyngeal, and rectal specimens.

Mannitol Salt Agar is designed to select for salt-tolerant Staphylococcus species, not Neisseria. Potassium tellurite agar targets organisms like Corynebacterium diphtheriae and some other gram-positives, not Neisseria. Cefsulodin-Irgasan-Novobiocin agar is used for selective isolation of Yersinia species and other enteric pathogens, not for Neisseria.