- Clinical science
Chlamydiaceae is a family of gram-negative, obligate intracellular bacteria that includes 3 organisms pathogenic to humans: Chlamydia trachomatis, Chlamydophila pneumoniae, and Chlamydophila psittaci. C. trachomatis can be differentiated into serotypes A–C, D–K (the most common STI), and L1–L3. Serotypes A–C mainly affect the eyes and cause trachoma. An infection with serotypes D–K can result in genitourinary infections (e.g., vaginitis, , urethritis), conjunctivitis, and infant pneumonia. Serotypes L1–L3, in turn, lead to sexually transmitted lymphogranuloma venereum. While both C. pneumoniae and C. psittaci primarily affect the respiratory system, C. psittaci also causes psittacosis. Chlamydial infections are mostly diagnosed based on clinical presentation and are treated with doxycycline or macrolides. In all cases of sexually transmitted chlamydial infection, expedited partner therapy should also be initiated as soon as possible. All ocular manifestations are discussed in more detail in the conjunctivitis learning card.
- Gram-negative bacterium, but does not gram stain well because:
- Characteristic life cycle consisting of two phases
- First phase: elementary bodies
Second phase: reticulate bodies
- Multiplication and aggregation of various reticulate bodies in the endosome, which take over most of the infected cell, at which point they are called inclusion bodies. Inclusion bodies are visible under light microscopy.
- Transformation of reticulate bodies to elementary bodies
- Lysis of endosomes
- Release of newly formed elementary bodies and exit from cell
- New start of cycle
- Cultivation very difficult
|Chlamydia trachomatis||A–C|| || |
|D–K|| || |
|L1–L3|| || || |
|Chlamydophila pneumoniae|| || |
|Chlamydophila psittaci|| || |
Infant pneumonia due to Chlamydia trachomatis (serotypes D–K) 
- Transmission: perinatal transmission during delivery through contact with an infected mother's genital flora
- Incubation period: 4–12 weeks after delivery
- Clinical features: pneumonia
- Prevention: maternal screening and treatment before birth
- Complications: respiratory failure
- Treatment: oral erythromycin (best choice), azithromycin
- Transmission: respiratory droplets
- Incubation period: 3–4 weeks
- Clinical features
Chlamydophila psittaci → psittacosis ("parrot fever") , ornithosis )
- Transmission: : airborne → pathogens from feces and dander of infected birds
Transmission by direct contact with infected individuals is rare.
- Mainly affects individuals in contact with free-ranging birds or pets, or occurs as an occupational disease
- Incubation period: 1–3 weeks
- Clinical features: Symptoms can vary greatly.
- Notifiable disease: There is an obligation to report the condition in most states.
- Pathogen: Chlamydia trachomatis serotypes D–K (also see the learning card on )
- Men: ,
- Women: , ,
- Both men and women: , 
- The majority of infected individuals are asymptomatic, which leads to a delay in seeking treatment, and thus increases the risk of disease transmission.
- (Muco)purulent vaginal discharge; and/or intermenstrual or postcoital bleeding
- May present with dysuria, polyuria, dyspareunia
- See “Clinical features” of
Nucleic acid amplification tests (NAAT): the gold standard
- Polymerase chain reaction (PCR) detects Chlamydia trachomatis RNA or DNA from vaginal swabs (women) or first-catch urine (men)
- Helps differentiate between C. trachomatis and N. gonorrhea
- NAAT may also be done on rectal, urethral, and endocervical swabs.
- Rapid test: XPert CT/NG assay (special NAAT with results in 90 minutes)
- Uses an enzyme immunoassay
- The specimen is collected with a swab from the cervix or urethra.
- The sensitivity is 80–95%.
- 100% specificity, but pathogens are hard to cultivate (obligate intracellular) and the process is expensive
- Results available only after a minimum of 4 days
- Typically only used in research, but also utilized in (potential) legal cases (e.g., suspected rape, sexual abuse)
- Nucleic acid amplification tests (NAAT): the gold standard
- Azithromycin or doxycycline 
- Expedited partner therapy is recommended in most cases of STIs, particularly chlamydia and gonorrhea.
- Asymptomatic patients should also be treated to prevent serious complications (i.e., PID and infertility) and further spreading
- CDC recommendations
Lymphogranuloma venereum (LVG) causes painless genital ulcers, which heal spontaneously within a few days. After a few weeks, swelling of the lymph nodes occurs, with possible abscess formation and discharge of pus. Without antibiotic treatment, this can lead to chronic lymphedema. (Also see the learning card on s)
- Pathogen: Chlamydia trachomatis serotypes L1–L3
- Found mostly in tropical and subtropical countries
- Increasing incidence among MSMs
- Primary infection after about a week: painless, herpetiform ulcers
- Secondary infection after about three weeks: painful lymph nodes in the inguinal region (buboes); with formation of abscesses (pus); , systemic symptoms
- Treatment: doxycycline; alternatively, erythromycin