- Obligate intracellular bacteria:
- Cannot produce their own adenosine triphosphate (ATP)
- Capable of synthesizing their own macromolecules
- Visualized using Giemsa stain: nucleic acid stain
- Classified as gram-negative bacteria, but exhibit poor Gram staining
- Cell wall shows some characteristics of gram-negative bacteria, but lacks peptidoglycans.
- Lack of peptidoglycans makes them insensitive to β-lactam antibiotics.
- Iodine stain: to visualize inclusion bodies (replicating intracellular forms)
- Life cycle:
- Elementary body (EB): infectious, metabolically inactive, spore-like
- Reticulate body (RB): non-infectious, metabolically active, replicative form (seen only within host cells)
- Pathogenic species:
- C. trachomatis
- C. psittaci
- C. pneumoniae
To help recall the characteristics of the life cycle, remember the 3 Es and 2 Rs:
- Elementary bodies Enter the cells and are the “Enfectious” form.
- Reticular bodies Replicate.
Reservoirs, transmission, and risk factors
|C. trachomatis||C. psittaci||C. pneumoniae|
|Host range||Humans primarily||Humans only|
|Transmission||Inhalation of contaminated, dried bird feces||Aerosolized droplets|
|Risk factors||Exposure to birds||Crowded settings (schools, nursing homes); the elderly are at higher risk for severe disease.|
Clinical Relevance (C. trachomatis)
- Most common bacterial cause of sexually transmitted genital infections in the United States
- More prevalent in young adults (< 26 years of age)
- Higher incidence in African Americans, Alaskans, and Native Americans
- Co-infection with other sexually transmitted pathogens is common.
- Trachoma (ocular infection) is endemic in some areas of Africa, Asia, Latin America, the Middle East, and the Pacific Rim.
- Leading infectious cause of blindness worldwide
- 1 in 20 sexually active young women aged 14–24 years has chlamydia.
- Many infections are asymptomatic, especially in women.
- Rates of transmission between symptomatic and asymptomatic individuals are unknown.
- Person-to-person through ocular and nasal secretions
- Eye-seeking flies
- Elementary bodies invade the host cell through endocytosis.
- Inside the cell, they convert to the metabolically active reticulate bodies.
- Reticulate bodies replicate inside the host cell via fission.
- They then reorganize into elementary bodies that are released from the cell.
- Clusters of replicating reticulate bodies are known as inclusion bodies and can be visualized using Giemsa, Pap, or iodine staining.
- There are multiple serovars of C. trachomatis:
- Serovars A–C: cause trachoma
- Serovars D–K:
- Most common sexually transmitted infection in the United States
- Also associated with neonatal conjunctivitis and pneumonia (acquired during passage through the birth canal)
- Serovars L1–L3:
- Also sexually transmitted
- Cause lymphogranuloma venereum (LGV)
- Urogenital infections (serovars D–K):
- In women:
- Often asymptomatic
- Pelvic inflammatory disease (PID)
- Symptoms include mucopurulent discharge, dysuria, and pyuria
- In men:
- In women:
- Systemic disease:
- Usually follows a sexually transmitted infection
- Reactive arthritis (formerly known as Reiter’s syndrome); reactive arthritis triad (RAT):
- Arthritis, urethritis, and uveitis
- More common in men
- Lymphogranuloma venereum (serovars L1–L3):
- Primary infection: genital ulcer, usually small and painless
- Secondary infection:
- Involvement of regional lymph nodes (inguinal, perianal)
- Presents with severe inflammation and scarring
Neonatal infections (serovars D–K):
- Neonatal conjunctivitis:
- Occurs 2–30 days following birth
- Causes eyelid swelling, hyperemia, and purulent discharge
- Can lead to conjunctival scarring and corneal vascularization
- Prevention: routine topical erythromycin after birth
- Infant pneumonia:
- Occurs 2–3 weeks after birth
- Causes diffuse interstitial pneumonia if untreated
Trachoma (serovars A–C):
- Early-stage (active trachoma):
- Redness, light sensitivity, mucopurulent discharge
- Late-stage (cicatrical trachoma):
- Conjunctival scarring
- Repeated infections lead to eyelid scarring, entropion, and eventually corneal damage and blindness.
- Nucleic acid amplification testing (NAAT): gold standard
- Other tests mostly used for research purposes:
- Culture (needs to be grown in tissue culture)
- Antigen detection
- Genetic probe methods
Clinical Relevance (C. psittaci and C. pneumoniae)
- Causes about 1% of community-acquired pneumonia (psittacosis or “parrot fever”)
- Transmitted from birds, usually by inhalation of dried feces
- Human-to-human transmission is possible.
- Incubation period 5–14 days
- Systemic symptoms (may be more prominent than respiratory):
- Fever, myalgias, sweats, rigors
- Headache (may be severe), photophobia
- Respiratory symptoms: cough, dyspnea, chest pain, hemoptysis
- Complications (renal, neurological, hematological) are uncommon, but may occur.
- Responsible for 1%–20% cases of community-acquired pneumonia
- Human-to-human transmission through respiratory droplets or contact
- Presents with fever, cough, shortness of breath
- Varies in severity from mild to lethal
- Can cause pharyngitis and upper respiratory symptoms
- Chronic infection may contribute to the pathogenesis of atherosclerosis (nucleic acid of other bacteria and viruses have also been found in atherosclerotic plaques).
- Hammerschlag M. R. (2019). Pneumonia caused by Chlamydia pneumoniae in adults. Retrieved 05 January 2021, from https://www.uptodate.com/contents/pneumonia-caused-by-chlamydia-pneumoniae-in-adults
- Hsu K. (2019). Clinical manifestations and diagnosis of Chlamydia trachomatis infections. Retrieved 05 January 2021, from https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-chlamydia-trachomatis-infections
- Hsu K. (2019). Epidemiology of Chlamydia trachomatis infections. Retrieved 05 January 2021, from https://www.uptodate.com/contents/epidemiology-of-chlamydia-trachomatis-infections
- Richards M. R. (2020). Psittacosis. Retrieved 05 January 2021, from https://www.uptodate.com/contents/psittacosissearch=chlamydia%20psittaci
- Zhao, Xue-Qiao. (2020). Pathogenesis of atherosclerosis. Retrieved January 12, 2021, from https://www.uptodate.com/contents/pathogenesis-of-atherosclerosis