Chlamydiales

Chlamydiales is an order of Gram-negative, obligate intracellular bacteria within the phylum Chlamydota. It comprises several families, including Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, Criblamydiaceae, and Waddliaceae, and contains the well-known human pathogens of the genus Chlamydia. Members infect a wide range of hosts, from humans and other mammals to birds and amoebae.

Like other members of their phylum, Chlamydiales display a biphasic developmental cycle consisting of infectious elementary bodies (EBs) that enter host cells and metabolically active reticulate bodies (RBs) that replicate inside a membrane-bound inclusion. The cycle alternates as RBs convert back to EBs for dissemination. This intracellular lifestyle underlies many of their distinctive biological features, including their reliance on the host for nutrients and energy.

Chlamydial infections are significant in medicine and veterinary science. In humans, Chlamydia trachomatis causes urogenital infections and trachoma; Chlamydia pneumoniae causes respiratory illness; and Chlamydia psittaci causes psittacosis. Other Chlamydiales species infect animals and can be zoonotic, spreading between animals and humans in certain contexts.

Genome and biology: Chlamydiales have small, reduced genomes and rely on their hosts for many essential processes. They encode secretory systems such as the type III secretion system and possess a limited, often atypical cell wall. They are generally susceptible to macrolides, tetracyclines, and some fluoroquinolones, while beta-lactam antibiotics are typically ineffective. Diagnosis commonly relies on nucleic acid amplification tests, with culture being difficult and infrequently performed.