Microbiology · Intracellular and atypical bacteria
Chlamydia trachomatis !
- Bacterium
- Gram-negative
- Exam priority: high
The most common sexually transmitted bacterium: often silent but can cause infertility; it also causes trachoma in the eyes.
Updated 2026-11-01
Also called: chlamydia
At a glance
- Gram
- Gram-negative
- Shape
- Small coccobacillus
- Oxygen
- Obligate intracellular (depends on host-cell ATP)
- Family
- Chlamydiaceae
- Stain
- Barely seen on Gram; Giemsa or iodine show cytoplasmic inclusions
Morphology
- Two forms: elementary body (infectious, extracellular) and reticulate body (divides inside the cell)
- Gram-negative-type envelope with almost no peptidoglycan
- Glycogen-rich inclusions that stain with iodine
Transmission
- Vaginal, anal or oral sex without a condom
- Mother to baby during vaginal birth
- Trachoma: contact with eye and nose secretions, hands, cloths and flies
Reservoir: Humans only
Isolation precautions
What the care team adds to standard precautions.
- Standard
Life cycle
- The small, tough elementary body binds an epithelial cell and enters by endocytosis
- Inside a vacuole (inclusion) it blocks fusion with the lysosome
- It turns into a larger, metabolically active reticulate body
- The reticulate body divides by binary fission using host-cell ATP
- Reticulate bodies condense back into elementary bodies
- The cell bursts or extrudes the inclusion and elementary bodies infect new cells
Virulence factors
- Intracellular life: escapes antibodies and beta-lactams
- Repeated chronic inflammation scars fallopian tubes and eyelids
- Serovars A–C: trachoma; D–K: genital and neonatal; L1–L3: lymphogranuloma venereum
Diseases it causes
- Cervicitis, urethritis, proctitis and epididymitis Chlamydia infection →
- Pelvic inflammatory disease: infertility, ectopic pregnancy, chronic pelvic pain Pelvic inflammatory disease →
- Inclusion conjunctivitis in adults and newborns Conjunctivitis →
- Trachoma: chronic keratoconjunctivitis and preventable blindness Trachoma →
- Lymphogranuloma venereum; infant pneumonia; reactive arthritis
Clinical picture
- Most genital infections cause no symptoms, especially in women
- Discharge, post-coital bleeding, dysuria; in men, scant urethral discharge
- Perihepatitis (Fitz-Hugh–Curtis) in PID
- Infant: conjunctivitis and afebrile pneumonia with staccato cough
Diagnosis
- Nucleic acid amplification tests (NAAT) on urine or vaginal, endocervical, rectal or throat swabs
- Gram stain shows neutrophils without bacteria (non-gonococcal urethritis)
- Does not grow on agar: culture needs living cells
- Lymphogranuloma venereum: a specific molecular test
Treatment
Doxycycline first; azithromycin in pregnancy and for neonatal conjunctivitis. Sexual partners are treated, and if gonorrhoea is suspected, ceftriaxone is added.
Drug class only; doses and regimens are set by the clinician from the current guideline.
In Pharmacology:
Prevention
- Condoms and screening of sexually active young women
- Partner treatment and retesting because reinfection is common
- Prenatal screening to prevent newborn infection
- Trachoma: SAFE strategy (surgery, antibiotics, facial cleanliness, environmental improvement)
Test yourself
Think of the answer, then tap the question to see it.
Which Chlamydia form is infectious and which replicates?
The elementary body is the infectious extracellular form; the reticulate body divides inside the cell.
Which test is preferred for genital C. trachomatis infection?
Nucleic acid amplification tests (NAAT) on urine or a swab.
Which serovars cause lymphogranuloma venereum?
Serovars L1, L2 and L3, which invade lymph nodes.
Why is penicillin not useful against Chlamydia?
Because it lives inside cells and has almost no peptidoglycan in its envelope.
Related
Related conditions
← Back to the group: Intracellular and atypical bacteria
Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Chlamydia (World Health Organization (WHO), 2025); About Chlamydia | Chlamydia | CDC (Centers for Disease Control and Prevention (CDC), 2025); Trachoma (World Health Organization (WHO), 2025).
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Representative model, for educational purposes only.