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Pneumonia caused by Chlamydia pneumoniae in adults

Dori F Zaleznik, MD
Section Editor
John G Bartlett, MD
Deputy Editor
Sheila Bond, MD


The Chlamydia species are obligate, intracellular bacteria. Chlamydia trachomatis is a major cause of genital tract and ocular infections worldwide. The role of Chlamydia (also known as Chlamydophila) pneumoniae and less often Chlamydia psittaci in causing atypical pneumonia has become more appreciated in recent years.

Pneumonia caused by C. pneumoniae in adults will be discussed here. C. psittaci infections (psittacosis) in adults, C. pneumoniae and C. psittaci infections in children, and C. trachomatis infections are discussed separately. (See "Psittacosis" and "Pneumonia caused by Chlamydia species in children" and "Screening for sexually transmitted infections", section on 'Chlamydia and gonorrhea' and "Clinical manifestations and diagnosis of Chlamydia trachomatis infections" and "Treatment of Chlamydia trachomatis infection".)


Chlamydia species are unusual bacteria in a number of ways. The genome of the organisms is only 660 million daltons, smaller than any other prokaryote except Mycoplasma spp. They do not contain a peptidoglycan in the cell wall. The growth cycle of the organism is complex and biphasic, consisting of two distinct entities: elementary (EB) and reticulate bodies (RB) [1].

The form in which the bacterium multiplies is the RB, which is intracellular. Replication occurs within a membrane-bound inclusion. RB are unstable and revert to EB within the inclusion, which then ruptures causing release of EB, the form that survives in the extracellular environment; EB are the infectious particles.

The signal that triggers RB to EB reversion is not known. EB attach to epithelial cells and enter cells via a phagosome. Once inside, EB reorganize to RB, which then replicate.


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Literature review current through: May 2017. | This topic last updated: May 03, 2016.
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