A 22-year-old male presents with urethral discharge. Gram stain shows no organisms. NAAT (Nucleic acid amplification test) on urethral swab is positive for Chlamydia trachomatis. The serovar responsible, and why culture is not the first-line diagnostic, are:
- A Serovar L1–L3 (LGV); culture in McCoy cells is insensitive
- B Serovar A–C (trachoma); NAAT is not approved for genital specimens
- C Serovars D–K cause urogenital infection; CT is an obligate intracellular organism requiring cell culture which is technically demanding, time-consuming, and less sensitive than NAAT ✓
- D Mycoplasma genitalium which cannot be cultured; NAAT is the only option
Correct answer: C. Serovars D–K cause urogenital infection; CT is an obligate intracellular organism requiring cell culture which is technically demanding, time-consuming, and less sensitive than NAAT
Explanation
Serovars D–K of Chlamydia trachomatis are responsible for urogenital tract infections (urethritis, cervicitis, PID, epididymitis); as an obligate intracellular pathogen, culture requires inoculation into living cell lines (McCoy or HeLa-229 cells), centrifuge-enhanced shell vial technique, and detection by IF or Giemsa staining of intracytoplasmic inclusions — technically complex with lower sensitivity (~70–85%) compared to NAAT (~95%). NAAT on urethral swabs/urine is the current gold standard. Serovars L1–L3 cause LGV. Serovars A–C cause trachoma.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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