All known Chlamydiae only grow by infecting eukaryotic host cells. They are as small or smaller than many viruses. Chlamydiae replicate inside the host cells and are termed intracellular. Most intracellular Chlamydiae are located in an inclusion body or vacuole. Outside of cells they survive only as an extracellular infectious form. Chlamydiae can grow only where their host cells grow. Therefore, Chlamydiae cannot be propagated in bacterial culture media in the clinical laboratory. Chlamydiae are most successfully isolated while still inside their host cell.
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.
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).
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 Screening Programs: A Review of the Literature. Part 1: Issues in the Promotion of Chlamydia Testing of Youth by Primary Care Physicians
Mar 22, 2006; Abstract: Rates of Chlamydia are highest among 15- to 24-year-old females. Often asymptomatic, untreated chlamydia can lead to...
Chlamydia screening among sexually active young female enrollees of health plans--United States, 2000-2007.(Clinical report)
Apr 17, 2009; Chlamydia trachomatis infection is the most common bacterial sexually transmitted disease (STD) in the United States, with more...
Chlamydia Screening Among Sexually Active Young Female Enrollees of Health Plans - United States, 2000-2007
Apr 17, 2009; Chlamydia trachomatis trachomatis infection is the most common bacterial sexually transmitted disease (STD) in the United States,...