Chlamydia and Chlamydophila
Also known as: chlamydiae
Chlamydia and Chlamydophila are genera of obligate intracellular bacteria that cannot make their own ATP and must replicate inside host cells. They cause genital and ocular infections, atypical pneumonia, and psittacosis.
These organisms are bacteria, but unusual ones. They are obligate intracellular pathogens that depend on the host cell for ATP, and their cell wall lacks the muramic acid found in classic peptidoglycan. That structural difference has two consequences worth remembering: they do not stain reliably on Gram stain, and beta-lactam antibiotics are ineffective against them because there is no conventional peptidoglycan target.
Replication follows a two-form developmental cycle. The elementary body is the small, metabolically inert, infectious form that survives outside cells and enters a new host cell by endocytosis. Once inside, it converts to the reticulate body, the larger, metabolically active form that replicates by binary fission but is not infectious. Reticulate bodies accumulate in a cytoplasmic inclusion, convert back to elementary bodies, and are released to infect neighboring cells. Giemsa staining or direct fluorescent antibody testing can demonstrate these inclusions, although nucleic acid amplification is the standard diagnostic method for genital infection.
Species differ by disease. Chlamydia trachomatis is divided by serotype: A through C cause trachoma, a chronic conjunctival infection and a leading infectious cause of blindness worldwide; D through K cause urethritis, cervicitis, pelvic inflammatory disease, and neonatal conjunctivitis and pneumonia acquired during passage through the birth canal; L1 through L3 cause lymphogranuloma venereum. Chlamydophila pneumoniae causes atypical pneumonia spread person to person by respiratory droplets, and Chlamydophila psittaci causes psittacosis, an atypical pneumonia acquired from birds. Because they are intracellular, treatment relies on agents that penetrate cells, principally macrolides and tetracyclines.
On USMLE Step 1, chlamydiae are covered with the other important bacteria. High-yield details are the elementary versus reticulate body distinction, the serotype-to-disease mapping for C. trachomatis, the absence of a beta-lactam target, and the bird exposure history that points to C. psittaci.
Key takeaways
- Chlamydiae are obligate intracellular bacteria that cannot generate their own ATP.
- Their cell wall lacks muramic acid, so beta-lactams do not work and Gram staining is unreliable.
- The elementary body is the infectious extracellular form; the reticulate body is the replicating intracellular form.
- C. trachomatis serotypes A–C cause trachoma, D–K cause genital and neonatal infections, and L1–L3 cause lymphogranuloma venereum.
- Chlamydophila pneumoniae causes atypical pneumonia, and C. psittaci causes psittacosis after bird exposure.