Chlamydiae

Chlamydiae is a bacterial phylum whose members are obligate intracellular pathogens.^[1] 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.^[2]

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.

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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.^[3] The Order Chlamydiales was created by Storz and Page in 1971.^[4] Between 1989 and 1999, new families, genera, and species were recognized. The phylum Chlamydiae was established in Bergey's Manual of Systematic Bacteriology.^[5]

By 2006, genetic data for over 350 chlamydial lineages had been reported ([1]), four chlamydial families recognized (Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, and Waddliaceae), and another family proposed (Rhabdochlamydiaceae).^[6][7]

Genomics

Chlamydiae is a unique bacterial evolutionary group that separated from other bacteria approximately a billion years ago.^[8][9] Reports have varied as to whether Chlamydiae is related to Planctomycetales or Spirochaetes.^[10][11] 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.^[9] Comparison of ribosomal RNA genes has provided a phylogeny of known strains within Chlamydiae.^[12] The unique status of Chlamydiae has enabled the use of DNA analysis for chlamydial diagnostics.^[13]

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.

References