Formerly grouped with the Rickettsia (see above), these non-motile obligate parasites of birds and mammals are now assigned a separate phylum comprising only five genera, of which Chlamydia is the most important. Like the Rickettsia, members of the Chlamydiae have extremely small cells, and very limited metabolic capacities, and depend on the host cell for energy generation. Unlike that group, however, they are not dependent on an arthropod vector for transmission from host to host.
Chlamydia trachomatis is the causative agent of trachoma, a major cause of blindness in humans. Different strains of this same species are responsible for one of the most common forms of sexually transmitted disease. C. psittaci causes the avian disease psittacosis, and C. pneumoniae causes chlamydial pneumonia in humans as well as being linked to some cases of coronary artery disease.
Representative genus: Chlamydia
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