Biology of Chlamydia.

The genus Chlamydia consists of two species, Chlamydia trachomatis and C. psittaci. The former includes (a) the trachoma/inclusion conjunctivitis (TRIC) agents, subdivided into the serotypes A-K; (b) the lymphogranuloma venereum (LGV) agents, subdivided into the serogroups (L 1-3); and (c) the mouse penumonitis agent. The major characteristics differentiating the two species are sulfonamide susceptibility and the formation of distinct inclusion granula in host cells, in the case of C. trachomatis, whereas C. psittaci is resistant to sulfonamides and forms less dense inclusions. Chlamydiae are characterized by a special growth cycle. The infective form of Chlamydia is the elementary body (EB), which stimulates its own phagocytosis by the host cell, an event followed by the EB's transformation into the reticulate body (RB). The RB forms small buds which subdivide by simple fission to produce several RB within the host cell cytoplasm. The growth cycle is completed by transformation of the RB into the infective EB from before liberation of the latter from the host cell. The chlamydiae are deficient in independent energy metabolism. Thus, their supply of ATP and essential building blocks must be obtained from the host cell cytoplasm. The TRIC agents have neuraminidase localized to the surface structures. Receptors of the microbe are temperature-sensitive, whereas the host cell is trypsin-sensitive.