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Scientific classification
Kingdom: Bacteria
Phylum: Chlamydiae
Class: Chlamydiae
Order: Chlamydiales


Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

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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.


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.[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 ([2]), four chlamydial families recognized (Chlamydiaceae, Parachlamydiaceae, Simkaniaceae, and Waddliaceae), and another family proposed (Rhabdochlamydiaceae).[6][7]


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 μm 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 μm, 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.[8] 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.[9] 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.


Chlamydiae is a unique bacterial evolutionary group that separated from other bacteria approximately a billion years ago.[10][11] Reports have varied as to whether Chlamydiae is related to Planctomycetales or Spirochaetes.[12][13] 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.[11] Comparison of ribosomal RNA genes has provided a phylogeny of known strains within Chlamydiae.[14] The unique status of Chlamydiae has enabled the use of DNA analysis for chlamydial diagnostics.[15]

There are three described species of chlamydiae that commonly infect humans:


  1. Wyrick P (2000). "Intracellular survival by Chlamydia". Cell Microbiol. 2 (4): 275–82. PMID 11207584.
  2. Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed. ed.). McGraw Hill. ISBN 0838585299.
  3. Moulder J (1966). "The relation of the psittacosis group (Chlamydiae) to bacteria and viruses". Annu Rev Microbiol. 20: 107–30. PMID 5330228.
  4. Storz J, Page LA (1971). "Taxonomy of the Chlamydiae: reasons for classifying organisms of the genus Chlamydia, family Chlamydiaceae, in a separate order, Chlamydiales ord. nov". International Journal of Systematic Bacteriology. 21: 332–334.
  5. Garrity GM, Boone DR (editors) (2001). Bergey's Manual of Systematic Bacteriology Volume 1: The Archaea and the Deeply Branching and Phototrophic Bacteria (2nd ed.). Springer. ISBN 0387987711.
  6. Everett K, Bush R, Andersen A (1999). "Emended description of the order Chlamydiales, proposal of Parachlamydiaceae fam. nov. and Simkaniaceae fam. nov., each containing one monotypic genus, revised taxonomy of the family Chlamydiaceae, including a new genus and five new species, and standards for the identification of organisms". Int J Syst Bacteriol. 49 Pt 2: 415–40. PMID 10319462.
  7. Rurangirwa F, Dilbeck P, Crawford T, McGuire T, McElwain T (1999). "Analysis of the 16S rRNA gene of micro-organism WSU 86-1044 from an aborted bovine foetus reveals that it is a member of the order Chlamydiales: proposal of Waddliaceae fam. nov., Waddlia chondrophila gen. nov., sp. nov". Int J Syst Bacteriol. 49 Pt 2: 577–81. PMID 10319478.
  8. McCoy A, Maurelli A (2006). "Building the invisible wall: updating the chlamydial peptidoglycan anomaly". Trends Microbiol. 14 (2): 70–7. PMID 16413190.
  9. Dautry-Varsat A, Subtil A, Hackstadt T (2005). "Recent insights into the mechanisms of Chlamydia entry". Cell Microbiol. 7 (12): 1714–22. PMID 16309458.
  10. Greub G, Raoult D (2003). "History of the ADP/ATP-translocase-encoding gene, a parasitism gene transferred from a Chlamydiales ancestor to plants 1 billion years ago". Appl Environ Microbiol. 69 (9): 5530–5. PMID 12957942.
  11. 11.0 11.1 Horn M, Collingro A, Schmitz-Esser S, Beier C, Purkhold U, Fartmann B, Brandt P, Nyakatura G, Droege M, Frishman D, Rattei T, Mewes H, Wagner M (2004). "Illuminating the evolutionary history of chlamydiae". Science. 304 (5671): 728–30. PMID 15073324.
  12. Ward N, Rainey F, Hedlund B, Staley J, Ludwig W, Stackebrandt E (2000). "Comparative phylogenetic analyses of members of the order Planctomycetales and the division Verrucomicrobia: 23S rRNA gene sequence analysis supports the 16S rRNA gene sequence-derived phylogeny". Int J Syst Evol Microbiol. 50 Pt 6: 1965–72. PMID 11155969.
  13. Teeling H, Lombardot T, Bauer M, Ludwig W, Glöckner F (2004). "Evaluation of the phylogenetic position of the planctomycete 'Rhodopirellula baltica' SH 1 by means of concatenated ribosomal protein sequences, DNA-directed RNA polymerase subunit sequences and whole genome trees". Int J Syst Evol Microbiol. 54 (Pt 3): 791–801. PMID 15143026.
  14. Everett K, Thao M, Horn M, Dyszynski G, Baumann P (2005). "Novel chlamydiae in whiteflies and scale insects: endosymbionts 'Candidatus Fritschea bemisiae' strain Falk and 'Candidatus Fritschea eriococci' strain Elm". Int J Syst Evol Microbiol. 55 (Pt 4): 1581–7. PMID 16014485.
  15. Corsaro D, Greub G (2006). "Pathogenic potential of novel Chlamydiae and diagnostic approaches to infections due to these obligate intracellular bacteria". Clin Microbiol Rev. 19 (2): 283–97. PMID 16614250.

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