C. pneumoniae was isolated in 1986 (3). It was found to be responsible for a variety of respiratory illnesses including 10% of cases of community-acquired pneumonia. Like the more familiar Chlamydia trachomatis, C. pneumoniae is an obligate intracellular pathogen with a unique life cycle (Fig. 1) (4). Generally, C. pneumoniae enters the body through a respiratory route and exists outside of cells in a spore form called the elementary body. Once inside the host cell it makes use ofthe cell's own metabolic machinery and develops into a metabolically active but noninfectious form called the reticulate body. In this form, the bacterium has the ability to divide and differentiate into new elementary bodies, which can then invade other host cells.
C. pneumoniae may also unpredictably convert within the cell to a metabolically inactive form called the persistent body (5). In this state, it may remain within the cell for extended periods essentially undetectable by the immune system and unresponsive to antibiotics that interfere with bacterial metabolism. A proposed association between C. pneumoniae and atherosclerosis is based on serological evidence, detection ofthe organism in the blood, pathological evidence, animal models, and therapeutic trials.
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