Question
A man presents with dysuria and urethral discharge after a history of unprotected sex. The Gram stain of his discharge is shown. What is the best culture medium for isolating the organism responsible?
| A. |
Thayer-Martin agar
|
| B. |
Blood agar
|
| C. |
MacConkey agar
|
| D. |
Chocolate agar
|
Show Answer
|
Correct Answer » A
Explanation
|
|
Image interpretation:
– Gram stain shows neutrophils with intracellular, kidney-shaped gram-negative diplococci → classic for *Neisseria gonorrhoeae*.
Clinical Reasoning for the Correct Answer:
– Man with urethritis post-unprotected sex → likely sexually transmitted infection (STI).
– Gram-negative diplococci inside neutrophils → classic for *Neisseria gonorrhoeae*.
– *Neisseria gonorrhoeae* requires selective medium for isolation as it is fastidious and often outcompeted by normal genital flora.
– Thayer-Martin agar contains antibiotics (vancomycin, colistin, nystatin, trimethoprim) to suppress competing bacteria/fungi and is specifically used for *Neisseria* spp.
Why Option A is Correct (Thayer-Martin agar):
– Selective chocolate agar variant designed for *Neisseria* spp.
– Inhibits contaminating organisms, promotes recovery of *N. gonorrhoeae* and *N. meningitidis*.
Why Option B is Incorrect (Blood agar):
– Non-selective medium; supports many bacteria.
– Does not inhibit competing flora—*Neisseria gonorrhoeae* may be overgrown and missed.
Why Option C is Incorrect (MacConkey agar):
– Selective for gram-negative rods (enterics), not for *Neisseria* (a diplococcus).
– *Neisseria gonorrhoeae* does not grow well or at all on MacConkey.
Why Option D is Incorrect (Chocolate agar):
– Enriched for fastidious organisms (contains lysed red cells), *Neisseria* can grow.
– Not selective—normal flora may outgrow pathogenic *Neisseria*.
– Thayer-Martin is preferred due to added selective antibiotics.
