A purulent aspirate of joint fluid from a 28-year-old female with joint pain showed many polymorphonuclear cells with intracellular and extracellular gram-negative diplococci. The appropriate selective medium for primary isolation would be

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

Answer: (C)

Neisseria gonorrhoeae is a primary suspect when purulent joint fluid shows many neutrophils with intracellular and extracellular gram-negative diplococci. To recover this fastidious organism from sterile sites, you use a medium that both supports Neisseria growth and suppresses competing flora. Modified Thayer-Martin agar is a chocolate-based, antibiotic-containing medium designed for this purpose. It includes agents that inhibit most Gram-positive bacteria, many Gram-negative rods, and fungi, while allowing Neisseria species to grow. That selective environment makes it the best choice for isolating the pathogen from this joint fluid.

Mannitol salt agar would favor Staphylococcus and isn’t selective for Neisseria. Potassium tellurite agar targets organisms like Corynebacterium diphtheriae and other tellurite-resistance organisms, not Neisseria. The cefsulodin-irgasan-novobiocin medium is used for Yersinia species (and some enteric pathogens), not Neisseria gonorrhoeae.