Chlamydia (genus)

Chlamydia is a genus of pathogenic Gram-negative bacteria that are obligate intracellular parasites. Chlamydia infections are the most common bacterial sexually transmitted diseases in humans and are the leading cause of infectious blindness worldwide. Species include Chlamydia trachomatis (a human pathogen), Ch. suis (affects only swine), and Ch. muridarum (affects only mice and hamsters). Humans mainly contract Ch. trachomatis, Ch. pneumoniae, Ch. abortus, and Ch. psittaci. Because of Chlamydias unique developmental cycle, it was taxonomically classified in a separate order. Chlamydia is part of the order Chlamydiales, family Chlamydiaceae. In the early 1990s six species of Chlamydia were known. A major re-description of the Chlamydiales order in 1999, using the then new techniques of DNA analysis, split three of the species from the genus Chlamydia and reclassified them in the then newly created genus Chlamydophila, and also added three new species to this genus. In 2001 many bacteriologists strongly objected to the reclassification, although in 2006 some scientists still supported the distinctness of Chlamydophila. In 2009 the validity of Chlamydophila was challenged by newer DNA analysis techniques, leading to a proposal to "reunite the Chlamydiaceae into a single genus, Chlamydia". This appears to have been accepted by the community, bringing the number of (valid) Chlamydia species up to 9. Many probable species were subsequently isolated, but no one bothered to name them. In 2013 a 10th species was added, Ch. ibidis, known only from feral sacred ibis in France. Two more species were added in 2014 (but validated 2015): Ch. avium which infects pigeons and parrots, and Ch. gallinacea infecting chickens, guinea fowl and turkeys. Ch. abortus was added in 2015, and the Chlamydophila species reclassified. A number of new species were originally classified as aberrant strains of Ch. psittaci Chlamydia species have genomes around 1.0 to 1.3 megabases in length. Most encode ~900 to 1050 proteins.