A 28-year-old woman presents to the gynaecology outpatient clinic with a 3-week history of mucopurulent cervical discharge and lower abdominal pain. She is sexually active with a single partner. On examination, cervical motion tenderness is elicited. Gram stain of endocervical swab shows scanty polymorphonuclear leucocytes but no gram-negative diplococci. Culture on standard media yields no growth. Which of the following is the most likely causative organism and the most appropriate initial antibiotic therapy?

Explanation

## Clinical Presentation and Diagnosis **Key Point:** The combination of mucopurulent cervical discharge, lower abdominal pain, and absence of gram-negative diplococci on Gram stain strongly suggests Chlamydia trachomatis infection rather than gonorrhoea. ### Diagnostic Features of Chlamydia trachomatis | Feature | Chlamydia trachomatis | Neisseria gonorrhoeae | |---------|----------------------|----------------------| | **Gram stain finding** | Scanty PMNs, gram-negative intracellular cocci absent | Gram-negative diplococci (kidney-bean shaped) | | **Culture requirement** | Requires special media (McCoy cells, shell vial) or nucleic acid amplification | Grows on Thayer-Martin or Modified Thayer-Martin media | | **Discharge character** | Mucopurulent, often scanty | Purulent, often copious | | **Incubation period** | 7–14 days (can be longer) | 2–5 days | | **First-line treatment** | Doxycycline or azithromycin | Ceftriaxone ± azithromycin | **High-Yield:** Chlamydia trachomatis is an obligate intracellular bacterium that does NOT grow on standard bacterial culture media. The absence of gram-negative diplococci rules out gonorrhoea. ### Treatment Regimen **Clinical Pearl:** Doxycycline 100 mg twice daily for 7 days is the gold-standard first-line therapy for uncomplicated urogenital chlamydial infection in non-pregnant women. Alternative agents include azithromycin 1 g as a single dose or 500 mg daily for 3 days. **Warning:** Doxycycline is contraindicated in pregnancy (risk of teeth discolouration and bone effects in the fetus). In pregnant women, azithromycin or amoxicillin is preferred. ### Why Gram Stain Appears Negative Chlamydia trachomatis is an obligate intracellular pathogen. The organism resides within epithelial cells and does not stain well with Gram stain. The presence of scanty PMNs without visible organisms is characteristic of chlamydial cervicitis. ## Pathophysiology **Mnemonic:** CHLAMYDIA = **C**ellular **H**ost **L**iving **A**nd **M**ultiplying **Y**et **D**ifficult **I**dentification **A**lways 1. Obligate intracellular pathogen 2. Elementary bodies (infectious form) enter epithelial cells 3. Differentiate into reticulate bodies (metabolically active) 4. Replicate within inclusion bodies 5. Release elementary bodies to infect new cells 6. Triggers inflammatory response → mucopurulent discharge ## Complications if Untreated - Pelvic inflammatory disease (PID) - Tubal factor infertility - Ectopic pregnancy - Chronic pelvic pain - Reactive arthritis (Reiter syndrome) in susceptible individuals