Chlamydia

Excerpt

The chlamydiae are a small group of nonmotile coccoid bacteria that are obligate intracellular parasites of eukaryotic cells. Chlamydial cells are unable to carry out energy metabolism and lack many biosynthetic pathways; therefore they are entirely dependent on the host cell to supply them with ATP and other intermediates. Because of their dependence on host biosynthetic machinery, the chlamydiae were originally thought to be viruses; however, they have a cell wall and contain DNA, RNA, and ribosomes and therefore are now classified as bacteria. The group consists of a single genus, Chlamydia (order Chlamydiales, class Chlamydiaceae). This genus contains the species C trachomatis and C psittaci, as well as a new organism, the TWAR organism, which has recently been proposed as a third species (C pneumoniae). All three species cause disease in humans. Chlamydia psittaci infects a wide variety of birds and a number of mammals, whereas C trachomatis is limited largely to humans. Chlamydia pneumoniae (TWAR organism) has been found only in humans.