Chlamydiae is a phylum of bacteria that is naturally found living only inside the cells of animals (including humans), insects, or protozoa. 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. Chlamydiae has a life cycle with an intracellular replicative form and an extracellular infectious form, which cannot be cultured in typical biological culture media. Isolation of Chlamydiae typically requires the isolation of an infected host cell. These bacteria are as small or smaller than many viruses.

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 (Moulder, 1966; Page 1966). 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 (Second Edition Release 1.0, April 2001). 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) (Everett et al., 1999; Rurangirwa et al., 1999; Corsaro et al, 1006).

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 (McCoy and Maurelli, 2006). 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 (Dautry-Varsat et al., 2005 ). 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 (Greub and Raoult, 2003; Horn et al., 2004). Reports have varied as to whether Chlamydiae is related to Planctomycetales or Spirochaetes (Ward et al., 2000; Teeling et al., 2004). 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 (Horn et al., 2004). 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 (Corsaro and Greub, 2006).

RECENT REFERENCES

• Dautry-Varsat A, Subtil A, Hackstadt T. 2005. Recent insights into the mechanisms of Chlamydia entry. Cell Microbiol. 7: 1714-1722.

• Everett KDE, Bush RM, and Andersen AA. 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. 49:415-440.

• Corsaro D, and Greub G. 2006. Pathogenic potential of novel Chlamydiae and diagnostic approaches to infections due to these obligate intracellular bacteria. Clin Microbiol Rev. 19:283-297.

• Corsaro D, Thomas V, Goy G, Venditti D, Radek R, and Greub G. 2006. Rhabdochlamydia crassificans, sp. nov., comb. nov., an intracellular bacterial pathogen of the cockroach Blatta orientalis (Insecta: Blattodea), and proposal for Rhabdochlamydiaceae, fam. nov., a new family of Chlamydiae infecting arthropods. Syst Appl Microbiol. Apr 16; ahead of print

• Greub G, and D Raoult. 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. 69:5530-5535.

• Horn M, Collingro A, Schmitz-Esser S, Beier CL, Purkhold U, Fartmann B, Brandt P, Nyakatura GJ, Droege M, Frishman D, Rattei T, Mewes HW, and Wagner M. 2004. Illuminating the evolutionary history of chlamydiae. Science. 304:728-730.

• McCoy AJ, and Maurelli AT. 2006. Building the invisible wall: updating the chlamydial peptidoglycan anomaly. Trends Microbiol. 14:70-77.

• Rurangirwa FR, Dilbeck PM, Crawford TB, McGuire TC, and McElwain TF. 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. 49:577-581.

• Taxonomic Outline of the Procaryotes, Bergey's Manual of Systematic Bacteriology, Second Edition Release 1.0, April 2001 ( ^*)

• Teeling H, Lombardot T, Bauer M, Ludwig W, and Glöckner FO. 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. 54:791-801.

• Ward NL, FA Rainey, BP Hedlund, JT Staley, W Ludwig, and E Stackebrandt. 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. 50:1965-1972.

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