ABSTRACT
This chapter describes the definitive properties of the genus Chlamydia, how its species and biovars are distinguished one from the other, and shows how advances in chlamydial biology and bacterial phylogeny allows construction of a plausible evolutionary history of chlamydiae. It considers ways in which the familiar Chlamydia trachomatis and Chlamydia psittaci and the as yet poorly characterized chlamydia-like inhabitants of invertebrates may all be accommodated within the order Chlamydiales, perhaps by the creation of new families and genera. The orthodox view of the natural host range of Chlamydia is that C. trachomatis, with the exception of the mouse biovar, is a uniquely human pathogen, whereas C. psittaci is a parasite of birds and nonhuman mammals. A casual search of the literature unearthed nine reports of chlamydia-like organisms living intracellularly in invertebrate hosts. The characteristics of a few strains of Chlamydia that cause disease in humans, farm animals, and birds have been reasonably well established.
