Chlamydiae

# Overview
Chlamydiae is a bacterial phylum whose members are obligate intracellular pathogens. Many Chlamydiae coexist in an asymptomatic state within specific hosts, and it is widely believed that these hosts provide a natural reservoir for these species.
All Chlamydiae grow by infecting eukaryotic host cells.  They are as small or smaller than many viruses.  Inside the cells Chlamydiae take on an intracellular replicative form; outside of cells they survive only in an extracellular infectious form. Chlamydiae can be grown only where their host cells grow, leading to two consequences in the laboratory: Chlamydiae cannot be propagated in bacterial culture media, and Chlamydiae are most successfully isolated while still inside the host cell.
Cavalier-Smith has postulated that the Chlamydiae fall into the clade Planctobacteria in the larger clade Gracilicutes.

# History
Chlamydia-like disease affecting the eyes of people was first described in ancient Chinese and Egyptian manuscripts.  A modern description of Chlamydia-like organisms was provided by Halberstaedter and von Prowazek in 1907.  Chlamydial isolates cultured in the yolk sacs of embryonating eggs were obtained from a human pneumonitis outbreak in the late 1920s and early 1930s, and by the mid-20th Century isolates had been obtained from dozens of vertebrate species.  The term Chlamydia (a cloak) appeared in the literature in 1945, although other names continued to be used, including Bedsonia, Miyagawanella, ornithosis-, TRIC-, and PLT-agents.

# Nomenclature
In 1966, Chlamydiae were recognized as bacteria and the genus Chlamydia was validated. The Order Chlamydiales was created by Storz and Page in 1971.  Between 1989 and 1999, new families, genera, and species were recognized.  The phylum Chlamydiae was established in Bergey's Manual of Systematic Bacteriology.
By 2006, genetic data for over 350 chlamydial lineages had been reported (), four chlamydial families recognized (Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, and Waddliaceae), and another family proposed (Rhabdochlamydiaceae).

# Structure
Chlamydiae can be either parasites or endosymbionts, depending on the eukaryotic host and chlamydial species.  The infectious, extracellular form is an elementary body (EB, a term borrowed from virologists) is electron-dense, typically 0.2-0.6 m in diameter.  The EB wall is held together with disulfide bonds.  EBs that have been endocytosed by eukaryotic cells typically remain in vacuolar inclusions (a virology term), where the disulfide bonds are reduced and EBs transform into reticulate bodies, (RBs; the contents of each RB were reticulated, i.e., homogeneous).  RBs range up to 1.5 m, take up nutrients from the host cell, and undergo multiple rounds of binary division. Binary division may involve ring structures comprised of a transitory type of peptidoglycan. Using electron microscopy, both EBs and RBs can be seen during replication in the inclusion.  Inclusions do not undergo acidification or lysosomal fusion and do not correspond to canonical endocytic vesicles, being essentially dissociated from the endocytic pathway and having some similarities with recycling endosomes. After several days of replication, the RBs transform back into metabolically inactive EBs that are released through host cell rupture or fusion of the inclusion/plasma membranes. Chlamydiae are spread by aerosol or by contact and require no alternate vector.

# Genomics
Chlamydiae is a unique bacterial evolutionary group that separated from other bacteria approximately a billion years ago.  Reports have varied as to whether Chlamydiae is related to Planctomycetales or Spirochaetes. Genome sequencing, however, indicates that 11% of the genes in Candidatus Protochlamydia amoebophila UWE25 and 4% in Chlamydiaceae are most similar to chloroplast, plant, and cyanobacterial genes. Comparison of ribosomal RNA genes has provided a phylogeny of known strains within Chlamydiae. The unique status of Chlamydiae has enabled the use of DNA analysis for chlamydial diagnostics.
There are three described species of chlamydiae that commonly infect humans:
- Chlamydia trachomatis, which causes the eye-disease trachoma and the sexually transmitted infection chlamydia;
- Chlamydophila pneumoniae, which causes a form of pneumonia;
- Chlamydophila psittaci, which causes psittacosis.