A microbiology laboratory needs to culture fastidious gram-negative diplococci suspected to be Neisseria gonorrhoeae from a urethral specimen. Which of the following is the preferred enriched medium for primary isolation?

Explanation

## Selective Enriched Media for Neisseria gonorrhoeae **Key Point:** Thayer-Martin medium is the gold standard selective enriched medium for primary isolation of N. gonorrhoeae from clinical specimens. ### Composition and Rationale Thayer-Martin medium is a chocolate agar base (provides X and V factors required by Neisseria) supplemented with: - Vancomycin (inhibits gram-positive bacteria) - Colistin (inhibits gram-negative flora) - Nystatin (inhibits fungi) - Amphotericin B (alternative to nystatin) This combination selectively suppresses normal flora while allowing Neisseria species to grow. ### Modified Thayer-Martin (MTM) Contains: - Trimethoprim (replaces nystatin) - Better suppression of Proteus species - Preferred in many laboratories for urogenital specimens **High-Yield:** Thayer-Martin and Modified Thayer-Martin are the ONLY acceptable media for primary isolation of N. gonorrhoeae from urogenital sites. Culture requires 5–10% CO₂ at 35–37°C for 48 hours. **Clinical Pearl:** Oxidase test (purple-black colonies) confirms Neisseria; gram stain shows gram-negative diplococci with kidney bean morphology. | Medium | Base | Selectivity | Use | | --- | --- | --- | --- | | Thayer-Martin | Chocolate agar | Vanc, Col, Nys | Primary isolation | | Modified TM | Chocolate agar | Vanc, Col, TMP | Preferred for urogenital | | Chocolate agar (plain) | Chocolate agar | None | Culture of N. meningitidis | | MacConkey | Lactose fermentation | Non-selective | Not suitable for Neisseria | **Warning:** MacConkey agar is NOT suitable—Neisseria are fastidious and require enriched media. Blood agar lacks selectivity and will be overgrown by normal flora. Sabouraud dextrose agar is for fungi.