Chlamydiales

Chla·my·di·a·les (klə-midґe-a″lēz) an order of bacteria of the class Chlamydiae, phylum Chlamydiae, consisting of coccoid, gram-negative, parasitic microorganisms that have a unique, obligately intracellular developmental cycle and are incapable of synthesizing ATP. Infection occurs when the small, rigid-walled extracellular form (elementary body) enters the cell and changes into a larger, thin-walled form (reticulate body) that divides by fission. The daughter cells thus formed reorganize and condense to become elementary bodies that are released and infect other cells. The organisms can produce disease in vertebrates and arthropods. Species of medical interest are in the families Chlamydiaceae, Parachlamydiaceae, and Simkaniaceae.