Chlamydia

General Characteristics

General characteristics

• Obligate intracellular bacteria:
  • Cannot produce their own adenosine triphosphate (ATP)
  • Capable of synthesizing their own macromolecules
• Staining:
  • 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

Mnemonic

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

Clinical Relevance (C. trachomatis)

Epidemiology

• 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

Transmission

• 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.
• Trachoma:
  • Person-to-person through ocular and nasal secretions
  • Eye-seeking flies

Pathogenesis

• 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)

Clinical presentation

Sexually transmitted:

• Urogenital infections (serovars D–K):
  • In women: 
    • Often asymptomatic 
    • Cervicitis
    • Urethritis
    • Salpingitis
    • Pelvic inflammatory disease (PID)
    • Symptoms include mucopurulent discharge, dysuria, and pyuria
  • In men: 
    • Urethritis
    • Epididymitis
    • Proctitis
• Systemic disease:
  • Usually follows a sexually transmitted infection
  • Arthritis
  • Dermatitis
  • Reactive arthritis; 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): 
  • Conjunctivitis
  • Redness, light sensitivity, mucopurulent discharge
• Late-stage (cicatrical trachoma):
  • Conjunctival scarring
  • Repeated infections lead to eyelid scarring, entropion, and eventually corneal damage and blindness.

Identification

• Nucleic acid amplification testing (NAAT): gold standard
• Other tests mostly used for research purposes:
  • Serology
  • Culture (needs to be grown in tissue culture)
  • Antigen detection
  • Genetic probe methods

Clinical Relevance (C. psittaci and C. pneumoniae)

C. psittaci

• 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.

C. pneumoniae

• 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).