{{Short description|Genus of bacteria}} {{For|the disease in humans|Chlamydia infection}} {{Automatic taxobox | image = ChlamydiaTrachomatisEinschlusskörperchen.jpg | image_caption = ''Chlamydia trachomatis'' inclusion bodies (brown) in a McCoy cell culture. | taxon = Chlamydia | authority = Jones, Rake & Stearns 1945 | type_species = ''Chlamydia trachomatis'' | type_species_authority = (Busacca 1935) Rake 1957 | subdivision_ranks = Species | subdivision_ref = <ref name=lpsn1>{{lpsn3|genus/chlamydia|Chlamydia}}</ref><ref name=NCBI>{{cite web |author=Schoch CL |display-authors=et al. |url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?command=show&mode=tree&id=810&lvl=3 |title=Chlamydia |accessdate=2025-06-05 |publisher=National Center for Biotechnology Information (NCBI) taxonomy database}}</ref> | subdivision = See text | synonyms = '''homotypic''' * "''Chlamydozoon''" <small>Moshkovskiy 1945 non Prowazek 1907</small> '''heterotypic''' * ''Chlamydophila'' <small>Everett, Bush & Andersen 1999</small> (type is ''C. psittaci'', not full overlap, see § ''Chlamydophila'')<ref name=LPSN2>{{lpsn3|genus/chlamydophila|Chlamydophila}}</ref> ** "''Bedsonia''" <small>Meyer 1953 ex Levaditi, Roger & Destombes 1964</small> ** "''Microbacterium''" <small>Levinthal 1930 non Orla-Jensen 1919</small> * "''Miyagawanella''" <small>?</small><ref>{{cite journal |last1=Page |first1=L. A. |title=Proposal for the recognition of two species in the genus Chlamydia Jones, Rake, and Stearns, 1945 |journal=International Journal of Systematic Bacteriology |date=1 January 1968 |volume=18 |issue=1 |pages=51–66 |doi=10.1099/00207713-18-1-51}}</ref> }}
'''''Chlamydia''''' is a genus of pathogenic Gram-negative bacteria that are obligate intracellular parasites. ''Chlamydia'' infections are the most common bacterial sexually transmitted diseases in humans and are the leading cause of infectious blindness worldwide.<ref name="Sherris">{{Cite book |author=Drew |first=W. Lawrence |url=https://rlmc.edu.pk/themes/images/gallery/library/books/Microbiology/Ryan%20K.J.,%20Ray%20C.G.%20eds.%20Sherris%20medical%20microbiology%202003.pdf |title=Sherris Medical Microbiology |publisher=McGraw Hill |year=2004 |isbn=978-0-8385-8529-0 |editor-last=Ryan |editor-first=Kenneth |edition=4th |pages=463–470 |chapter=Chlamydia |editor-last2=Ray |editor-first2=C. George}}</ref>
Humans mainly contract ''C. trachomatis'', ''C. pneumoniae'', ''C. abortus'', and ''C. psittaci''.<ref name="Joseph2015">{{Citation |last=Joseph |first=SJ |display-authors=etal |year=2015 |title=Chlamydiaceae genomics reveals interspecies admixture and the recent evolution of Chlamydia abortus infecting lower mammalian species and humans |journal=Genome Biol Evol |volume=7 |issue=11 |pages=3070–3084 |pmid= 26507799|pmc=4994753 |doi=10.1093/gbe/evv201 |postscript=.}}</ref>
==Classification== Because of ''Chlamydia''{{'}}s unique developmental cycle, it was taxonomically classified in a separate order.<ref name="Chlamydia trachomatis">{{cite web|url=http://www.tjclarkinc.com/bacterial_diseases/chlamydia_trachomatis.htm|title=Chlamydia trachomatis|access-date=June 18, 2010|archive-url=https://web.archive.org/web/20100702084907/http://www.tjclarkinc.com/bacterial_diseases/chlamydia_trachomatis.htm|archive-date=July 2, 2010}}</ref> ''Chlamydia'' is part of the order Chlamydiales, family Chlamydiaceae.<ref name=lpsn1/>
=== ''Chlamydophila'' (1999–2009)<span id="Chlamydophila" class="anchor"></span> === Earlier criteria for differentiation of chlamydial species did not always work well. For example, at that time ''C. psittaci'' was distinguished from ''C. trachomatis'' by sulfadiazine resistance, although not all strains identified as ''C. psittaci'' at the time were resistant, and ''C. pneumoniae'' was classified by its appearance under electron microscopy (EM) and its ability to infect humans, although the EM appearance may differ from one research group to the next, and many of these species infected humans.
A major re-description of the Chlamydiales order in 1999, using the then-new techniques of DNA analysis split three of the species from the genus ''Chlamydia'' and reclassified them in the then newly created genus '''''Chlamydophila''''' (''Cp.'' hereafter). Five new species were added by splitting from existing species:<ref name="pmid10319462">{{cite journal |vauthors=Everett KD, Bush RM, Andersen AA |title=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 |journal=Int. J. Syst. Bacteriol. |volume=49 |issue=2 |pages=415–40 |date=April 1999 |pmid=10319462 |doi=10.1099/00207713-49-2-415 |doi-access=free }}</ref>
:{| class=wikitable |+ Summary of 1999 classification change<ref name=Ward>{{cite web |vauthors=Ward M |title=Taxonomy diagram |department=Chlamydiales |website=Chlamydiae.com |url=http://www.chlamydiae.com/docs/Chlamydiales/diagram/taxondiag.htm |access-date=2008-10-28 |archive-url=https://web.archive.org/web/20100918212158/http://chlamydiae.com/docs/Chlamydiales/diagram/taxondiag.htm |archive-date=2010-09-18 }}</ref> ! Old name !! Host !! New name |- | rowspan=4| ''C. psittaci'' || Birds || ''Cp. psittaci'' |- | Mammals || ''Cp. abortus'' |- | Cats || ''Cp. felis'' |- | Guinea pig || ''Cp. caviae'' |- | ''C. pecorum'' || Mammals || ''Cp. pecorum'' |- | ''C. pneumoniae'' || Humans || ''Cp. pneumoniae'' |- | rowspan=3 | ''C. trachomatis'' || Humans || ''C. trachomatis'' |- | Swine || ''C. suis'' |- | Mice and hamsters || ''C. muridarum'' |}
According to the authors of the 1999 study, the mean DNA–DNA reassociation difference distinguishing ''Chlamydophila'' from ''Chlamydia'' is 10.1%, an accepted value for genus separation. Although the 16S ribosomal RNA gene sequences of the two are close to 95% identical, unlike the other previously established genera, the authors considered a less than 95% similarity only a guideline for establishing new genera in chlamydial families. In the study, the authors used the similarity of the locations of coding for protein and ribosomal RNA genes in the genome (gene clusters) to help distinguish ''Chlamydophila'' from ''Chlamydia''. Also, the full-length {{nobr|23S ribosomal RNA}} genes of the species of the two genera were less than 95% identical.<ref name=pmid10319462/> Supporting criteria such as antigen detection,<ref>{{cite journal |vauthors=Bush RM, Everett KD |date=January 2001 |title=Molecular evolution of the Chlamydiaceae |journal=International Journal of Systematic and Evolutionary Microbiology |volume=51 |issue=Pt 1 |pages=203–220 |pmid=11211261 |doi=10.1099/00207713-51-1-203 |doi-access=free }}</ref> glycogen staining, host association, and EM morphology were also employed, depending on applicability and availability.
:{| class=wikitable style="text-align:center;" |+ Some molecular criteria distinguishing Chlamydiaceae genera<ref name=pmid10319462/> |- style="vertical-align:bottom;" ! Genus !! Approximate<br/>genome size<br/>(Mega base pairs) !! Detectable<br/>glycogen !! Number of<br/>rDNA operons |- ! ''Chlamydophila'' | 1.2 || No || 1 |- ! ''Chlamydia'' | 1.0 || Yes || 2 |}
In 2001 many bacteriologists strongly objected to the reclassification.<ref name=lpsn1/> Comparative genomic analyses in 2006 identified a number of signature proteins that were uniquely present in species from the genera ''Chlamydia'' and ''Chlamydophila'', which supported the distinctness of ''Chlamydophila'', but did not support an "early separation" scenario as suggested by rRNA.<ref name=pmid16436211>{{cite journal | vauthors = Griffiths E, Ventresca MS, Gupta RS | year = 2006 | title = BLAST screening of chlamydial genomes to identify signature proteins that are unique for the Chlamydiales, Chlamydiaceae, Chlamydophila, and Chlamydia groups of species | journal = BMC Genomics | volume = 7 | page = 14 | pmid = 16436211 | pmc = 1403754 | doi = 10.1186/1471-2164-7-14 | doi-access = free }}</ref>
In 2009 the validity of ''Chlamydophila'' was challenged by newer DNA analysis techniques (using 100 concatenated proteins instead of 16S rRNA), leading to a proposal to "reunite the ''Chlamydiaceae'' into a single genus, ''Chlamydia''". The authors pointed to the poor bootstrap support of the {{nobr|1999 rRNA tree,}} which demonstrated a split in only 68% of the sampled trees, and argued that the 2006 study did not provide sufficiently strong support for the separation.<ref name=pmid19281563>{{cite journal | vauthors = Stephens RS, Myers G, Eppinger M, Bavoil PM | date=March 2009 | title = Divergence without difference: Phylogenetics and taxonomy of Chlamydia resolved | journal = FEMS Immunol. Med. Microbiol. | volume = 55 | issue = 2 | pages = 115–119 | pmid = 19281563 | doi = 10.1111/j.1574-695X.2008.00516.x | doi-access = free }}</ref> This reversion appears to have been accepted by the community<ref>{{cite journal |last=Greub |first=Gilbert |date=November 2010 |title=International Committee on Systematics of Prokaryotes Subcommittee on the taxonomy of the Chlamydiae Minutes of the inaugural closed meeting, 21 March 2009, Little Rock, AR, USA |journal=International Journal of Systematic and Evolutionary Microbiology |volume=60 |issue=11 |pages=2691–2693 |doi=10.1099/ijs.0.028225-0 |doi-access=free |pmid=21048221 }}</ref> and was formally validated in 2015,<ref>{{cite journal |vauthors=Sachse K, Bavoil PM, Kaltenboeck B, Stephens RS, Kuo CC, Rosselló-Móra R, Horn M |date=March 2015 |title=Emendation of the family Chlamydiaceae: Proposal of a single genus, Chlamydia, to include all currently recognized species |journal=Systematic and Applied Microbiology |volume=38 |issue=2 |pages=99–103 |doi=10.1016/j.syapm.2014.12.004 |pmid=25618261 |bibcode=2015SyApM..38...99S }}</ref><ref>{{cite journal |last1=Garrity |first1=George M. |last2=Oren |first2=Aharon |date=July 2015 |title=Notification of changes in taxonomic opinion previously published outside the IJSEM |journal=International Journal of Systematic and Evolutionary Microbiology |volume=65 |issue=7 |pages=2028–2029 |doi=10.1099/ijs.0.000286 }}</ref> bringing the number of (valid) ''Chlamydia'' species up to 9 as of 2017.<ref name=Balsamo2017>{{cite journal |vauthors=Balsamo G, Maxted AM, Midla JW, Murphy JM, Wohrle R, Edling TM, Fish PH, Flammer K, Hyde D, Kutty PK, Kobayashi M, Helm B, Oiulfstad B, Ritchie BW, Stobierski MG, Ehnert K, Tully TN Jr |display-authors=6 |date=September 2017 |title=Compendium of measures to control Chlamydia psittaci infection among humans (psittacosis) and pet birds (avian chlamydiosis), 2017 |journal=Journal of Avian Medicine and Surgery |volume=31 |issue=3 |pages=262–282 |issn=1082-6742 |pmid=28891690 |doi=10.1647/217-265 |doi-access=free |s2cid=26000244 |url=http://www.nasphv.org/Documents/PsittacosisCompendium.pdf |via=nasphv.org }}</ref> The merger of the genus ''Chlamydophila'' back into the genus ''Chlamydia'' is, by 2018, generally accepted.<ref>{{cite journal |vauthors=Bavoil P, Kaltenboeck B, Greub G |date=March 2013 |title=In Chlamydia veritas |journal=Pathogens and Disease |volume=67 |issue=2 |pages=89–90 |pmid=23620152 |doi=10.1111/2049-632X.12026 |doi-access=free |s2cid=31730015 |url=https://serval.unil.ch/notice/serval:BIB_998F8DDA064D }}</ref><ref>{{cite journal |vauthors=Bachmann NL, Polkinghorne A, Timms P |date=August 2014 |title=Chlamydia genomics: Providing novel insights into chlamydial biology |journal=Trends in Microbiology |volume=22 |issue=8 |pages=464–472 |pmid=24882432 |doi=10.1016/j.tim.2014.04.013 }}</ref><ref>{{cite journal |vauthors=Sachse K, Bavoil PM, Kaltenboeck B, Stephens RS, Kuo CC, Rosselló-Móra R, Horn M |date=March 2015 |title=Emendation of the family Chlamydiaceae: Proposal of a single genus, Chlamydia, to include all currently recognized species |journal=Systematic and Applied Microbiology |volume=38 |issue=2 |pages=99–103 |pmid=25618261 |doi=10.1016/j.syapm.2014.12.004 |bibcode=2015SyApM..38...99S |hdl=10261/123714 }}</ref><ref>{{cite journal |vauthors=Borel N, Polkinghorne A, Pospischil A |date=May 2018 |title=A review on Chlamydial diseases in animals: Still a challenge for pathologists? |journal=Veterinary Pathology |volume=55 |issue=3 |pages=374–390 |pmid=29310550 |doi=10.1177/0300985817751218 |doi-access=free}}</ref>
However, the much newer analyses of Genome Taxonomy Database using 120 concatenated proteins again show a split of those two genera to be valid (see {{section link||Phylogeny}} below), and has led to the resurrection of the genus in the GTDB and GBIF taxonomies.<ref>{{cite web |title=g__Chlamydophila |department=GTDB Tree |website=gtdb.ecogenomic.org |url=https://gtdb.ecogenomic.org/tree?r=g__Chlamydophila }}</ref><ref>{{cite web |title=Chlamydophila {{small|(Everett & al., 1999)}} |website=gbif.org |url=https://www.gbif.org/species/3228240 |lang=en }}</ref> Joseph et al. 2015, which proposed new species from strains formerly known as ''C. psittaci'', also recovered a coherent ''Chlamydophila'' clade in their whole-genome tree, but with an unusual topology showing ''Chlamydophila'' to be sister to ''C. muridarum''.<ref name=Joseph2015/> Gupta et al. (2015) finds 1 CSI + 19 CSPs specific for ''Chlamydophilia'' and 2 CSIs + 19 CSPs specific for the three-species version of ''Chlamydia''.<ref>{{cite journal |last1=Gupta |first1=Radhey S. |last2=Naushad |first2=Sohail |last3=Chokshi |first3=Chirayu |last4=Griffiths |first4=Emma |last5=Adeolu |first5=Mobolaji |date=September 2015 |title=A phylogenomic and molecular markers based analysis of the phylum Chlamydiae: Proposal to divide the class Chlamydiia into two orders, Chlamydiales and Parachlamydiales ''ord. nov.'', and emended description of the class Chlamydiia |journal=Antonie van Leeuwenhoek (journal) |volume=108 |issue=3 |pages=765–781 |doi=10.1007/s10482-015-0532-1 |pmid=26179278 }}</ref>
=== Species additions === Many probable species were subsequently isolated, but no one bothered to name them. Many new species fall into the ''Chlamydophilia'' clade and were originally classified as aberrant strains of ''C. psittaci''. Complicating the picture is the fact that this clade shows signs of interspecies recombination.<ref name="Joseph2015"/>
* In 2013 a 10th species was added, ''C. ibidis'', known only from feral sacred ibis in France.<ref name=ibis>{{cite journal |last1=Vorimore |first1=Fabien |last2=Hsia |first2=Ru-ching |last3=Huot-Creasy |first3=Heather |last4=Bastian |first4=Suzanne |last5=Deruyter |first5=Lucie |last6=Passet |first6=Anne |last7=Sachse |first7=Konrad |last8=Bavoil |first8=Patrik |last9=Myers |first9=Garry |last10=Laroucau |first10=Karine |date=20 September 2013 |title=Isolation of a New ''Chlamydia'' species from the Feral Sacred Ibis (''Threskiornis aethiopicus'')- ''Chlamydia ibidis'' |journal=PLOS ONE |volume=8 |issue=9 |at=e74823 |doi=10.1371/journal.pone.0074823 |pmid=24073223 |pmc=3779242 |bibcode=2013PLoSO...874823V |doi-access=free }}</ref> * Two more species were added in 2014 (but validated 2015): ''C. avium'' which infects pigeons and parrots, and ''C. gallinacea'' infecting chickens, guinea fowl and turkeys.<ref name="Joseph2015"/> * Two of the species proposed for ''Chlamydophila'' in 1999 (''C. abortus'', ''C. felis'') were formally merged in 2015.<ref name=lpsn1/> ''C. caviae'' was covered by the same publication, but was only validated in 2024.<ref>{{cite web |title=Species: Chlamydia caviae |url=https://lpsn.dsmz.de/species/chlamydia-caviae |website=lpsn.dsmz.de |language=en}}</ref> * ''C. poikilotherma'' was validated in 2022, as a correction of the 2019 "Chlamydia poikilothermis".<ref name=lpsn1/> * ''C. buteonis'' was validated in 2023.<ref name=lpsn1/> * ''C. crocodili'' was validated in 2023.<ref name=lpsn1/>
There is one invalidly published ''Chlamydophilia'' species that has not been transferred back to ''Chlamydia'' as of 2025: "''Chlamydophila parapsittaci''",<ref>{{cite web |title=Species: Chlamydophila parapsittaci |url=https://lpsn.dsmz.de/species/chlamydophila-parapsittaci |website=lpsn.dsmz.de |language=en}}</ref> representative of an intermediate stage between ''C. abortus'' and ''C. psittaci''.<ref>{{cite journal |last1=Vafin |first1=R. R. |last2=Ravilov |first2=R. Kh. |last3=Gaffarov |first3=Kh. Z. |last4=Ravilov |first4=A. Z. |last5=Iskhakov |first5=G. M. |last6=Bakirov |first6=I. Kh. |last7=Kashov |first7=V. N. |title=On the nomenclature and classification of chlamydiae |journal=Molecular Genetics, Microbiology and Virology |date=December 2007 |volume=22 |issue=4 |pages=155–164 |doi=10.3103/S0891416807040040}}</ref> See {{section link|Chlamydia psittaci|Psittaci-abortus intermediate}} for a discussion of it.
== Evolution == {{see|Chlamydiota#Evolution}}
==Phylogeny== {| class="wikitable" |- ! colspan=1 | 16S rRNA based LTP_10_2024<ref>{{cite web|title=The LTP |url=https://imedea.uib-csic.es/mmg/ltp/#LTP| access-date=10 December 2024}}</ref><ref>{{cite web|title=LTP_all tree in newick format| url=https://imedea.uib-csic.es/mmg/ltp/wp-content/uploads/ltp/LTP_all_10_2024.ntree |access-date=10 December 2024}}</ref><ref>{{cite web|title=LTP_10_2024 Release Notes| url=https://imedea.uib-csic.es/mmg/ltp/wp-content/uploads/ltp/LTP_10_2024_release_notes.pdf |access-date=10 December 2024}}</ref> ! colspan=1 | 120 marker proteins based GTDB 10-RS226<ref name="about">{{cite web |title=GTDB release 10-RS226 |url=https://gtdb.ecogenomic.org/about#4%7C |website=Genome Taxonomy Database|access-date=1 May 2025}}</ref><ref name="tree_bac">{{cite web |title=bac120_r226.sp_label |url=https://data.gtdb.ecogenomic.org/releases/release226/226.0/auxillary_files/bac120_r226.sp_labels.tree |website=Genome Taxonomy Database|access-date=1 May 2025}}</ref><ref name="taxon_history">{{cite web |title=Taxon History |url=https://gtdb.ecogenomic.org/taxon_history/ |website=Genome Taxonomy Database|access-date=1 May 2025}}</ref> |- | style="vertical-align:top| {{Clade | style=font-size:90%;line-height:80% |1={{clade |1={{clade |1=''Chlamydia pecorum'' |2=''Chlamydia pneumoniae'' }} |2={{clade |label1=''Chlamydia'' s.s. |1={{clade |1=''C. muridarum'' |2={{clade |1=''C. suis'' |2=''C. trachomatis'' }} }} |label2=''Chlamydophila'' |2={{clade |1={{clade |1=''Chlamydia avium'' |2=''Chlamydia gallinacea'' }} |2={{clade |1=''C. felis'' |2={{clade |1=''C. psittaci'' |2={{clade |1={{clade |1=''C. abortus'' |2=''Chlamydia buteonis'' }} |2={{clade |1=''C. caviae'' |2={{clade |1=''Chlamydia crocodili'' |2=''Chlamydia poikilotherma'' }} }} }} }} }} }} }} }} }} | {{Clade | style=font-size:90%;line-height:80% |1={{clade |label1=''Chlamydia'' s.s. |1={{clade |1=''C. trachomatis'' <small>(Busacca 1935) Rake 1957</small> |2={{clade |1=''C. muridarum'' <small>Everett, Bush & Andersen 1999</small> |2=''C. suis'' <small>Everett, Bush & Andersen 1999</small> }} }} |label2=''Chlamydophila'' |2={{clade |1="''Chlamydia ibidis''" <small>Vorimore et al. 2013</small> |2={{clade |1={{clade |1=''C. pecorum'' <small>(Fukushi and Hirai 1992) Everett, Bush & Andersen 1999</small> |2={{clade |1="''Ca.'' Chlamydia sanziniae" <small>corrig. Taylor-Brown et al. 2016</small> |2={{clade |1="''Chlamydia serpentis''" <small>Staub et al. 2018</small> |2={{clade |1="''Ca.'' Chlamydia coralli" <small>corrig. Taylor-Brown et al. 2017</small> |2=''C. pneumoniae'' <small>(Grayston et al. 1989) Everett, Bush & Andersen 1999</small> }} }} }} }} |2={{clade |1={{clade |1=''Chlamydia avium'' <small>Sachse et al. 2015</small> |2=''Chlamydia gallinacea'' <small>Sachse et al. 2015</small> }} |2={{clade |1=''C. felis'' <small>Everett, Bush & Andersen 1999</small> |2={{clade |1={{clade |1=''C. caviae'' <small>Everett, Bush & Andersen 1999</small> |2={{clade |1=''Chlamydia crocodili'' <small>Chaiwattanarungruengpaisan et al. 2023</small> |2=''Chlamydia poikilotherma'' <small>Staub et al. 2022</small> }} }} |2={{clade |1=''C. psittaci'' <small>(Lillie 1930) Everett, Bush & Andersen 1999</small> |2={{clade |1=''C. abortus'' <small>Everett, Bush & Andersen 1999</small> |2=''Chlamydia buteonis'' <small>Laroucau et al. 2023</small> }} }} }} }} }} }} }} }} }} |} Unassigned species: * "''Chlamydia major''" <small>Shaw, Christiansen & Birkelund 1999</small> * "''Ca.'' Chlamydia testudinis" <small>Laroucau et al. 2020</small> * ''Chlamydia vaughanii'' <small>Marquis et al. 2025</small> * "''Chlamydophila parapsittaci''" <small>Vafin et al. 2007</small>
==Genomes== ''Chlamydia'' species have genomes around 1.0–1.3 megabases in length.<ref>{{cite web|url=http://www.ebi.ac.uk/genomes/bacteria.html| title=EMBL bacterial genomes|access-date= January 19, 2012}}</ref> Most encode ≈900~1050 proteins.<ref name=Read2000>{{cite journal |last1=Read |first1=T.D. |last2=Brunham |first2=R.C. |last3=Shen |first3=C. |last4=Gill |first4=S.R. |last5=Heidelberg |first5=J.F. |last6=White |first6=O. |last7=Hickey |first7=E.K. |last8=Peterson |first8=J. |last9=Utterback |first9=T. |display-authors=6 |date=2000-03-15 |df=dmy-all |title=Genome sequences of Chlamydia trachomatis MoPn and Chlamydia pneumoniae AR39 |journal=Nucleic Acids Research |volume=28 |issue=6 |pages=1397–1406 |issn=1362-4962 |pmc=111046 |pmid=10684935 |doi=10.1093/nar/28.6.1397 }}</ref> Some species also contain a DNA plasmids or phage genomes (see Table 1, below). The elementary body contains an RNA polymerase responsible for the transcription of the DNA genome after entry into the host cell cytoplasm and the initiation of the growth cycle. Ribosomes and ribosomal subunits are found in these bodies.<ref name=":0">{{cite book |last=Becker |first=Yechiel |year=1996 |title=Medical Microbiology |edition=4th |editor-last=Baron |editor-first=S. |publisher=University of Texas Medical Branch |place=Galveston, TX |isbn=0-9631172-1-1 |chapter=Chlamydia |pmid=21413294 |url=https://www.ncbi.nlm.nih.gov/books/NBK7627/ |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK8091/ |via=ncbi.nlm.nih.gov }}</ref>
:{| class="wikitable" style="text-align:center;" |+ '''Table 1.''' Genome features of selected ''Chlamydia'' species and strains<ref name=Read2000/> |- style="vertical-align:bottom;" ! ! ''C. muridarum''<br/>MoPn<ref name=Read2000/><ref name=Carlson-etal-2008/> ! ''C. trachomatis''<br/>D[[#strain_D_anchor|{{sup| [†]}}]] ! ''C. pneumoniae''<br/>AR39<ref name=Read2000/> ! ''C. pneumoniae''<br/>CWL029 |- | Size (nt) | 1,069,412 | 1,042,519 | 1,229,853 | 1,230,230 |- | ORFs | 924 | 894 | 1052 | 1052 |- | tRNAs | 37 | 37 | 38 | 38 |- | plasmids | 1 (7,501 nt) | 1 (7,493 nt) | {{nobr|1 ssDNA phage}} | none |} MoPn is a mouse pathogen while {{nobr|{{anchor|strain_D_anchor}}[†] strain "D"}} is a human pathogen. About 80% of the genes in ''C. trachomatis'' and ''C. pneumoniae'' are orthologs. Adapted after Read et al. 2000,<ref name=Read2000/> nomenclature of MoPn following Carlson et al. 2008.<ref name=Carlson-etal-2008>{{cite journal |last1=Carlson |first1=John H. |last2=Whitmire |first2=William M. |last3=Crane |first3=Deborah D. |last4=Wicke |first4=Luke |last5=Virtaneva |first5=Kimmo |last6=Sturdevant |first6=Daniel E. |last7=Kupko |first7=John J. |last8=Porcella |first8=Stephen F. |last9=Martinez-Orengo |first9=Neysha |last10=Heinzen |first10=Robert A. |last11=Kari |first11=Laszlo |last12=Caldwell |first12=Harlan D. |display-authors=6 |date=June 2008 |title=The Chlamydia trachomatis Plasmid Is a Transcriptional Regulator of Chromosomal Genes and a Virulence Factor |journal=Infection and Immunity |volume=76 |issue=6 |pages=2273–2283 |doi=10.1128/iai.00102-08|pmid=18347045 |pmc=2423098 }}</ref>
== Developmental cycle == ''Chlamydia'' may be found in the form of an '''elementary body''' and a reticulate body. The elementary body is the nonreplicating infectious particle that is released when infected cells rupture. It is responsible for the bacteria's ability to spread from person to person and is analogous to a spore. The elementary body may be 0.25 to 0.30 μm in diameter. This form is covered by a rigid cell wall (hence the combining form ''chlamyd-'' in the genus name). The elementary body induces its own endocytosis upon exposure to target cells. One phagolysosome usually produces an estimated 100–1000 elementary bodies.{{cn|date=February 2023}}
''Chlamydia'' may also take the form of a reticulate body, which is in fact an intracytoplasmic form, highly involved in the process of replication and growth of these bacteria. The reticulate body is slightly larger than the elementary body and may reach up to 0.6 μm in diameter with a minimum of 0.5 μm. It does not have a cell wall. When stained with iodine, reticulate bodies appear as inclusions in the cell. The DNA genome, proteins, and ribosomes are retained in the reticulate body. This occurs as a result of the development cycle of the bacteria. The reticular body is basically the structure in which the chlamydial genome is transcribed into RNA, proteins are synthesized, and the DNA is replicated. The reticulate body divides by binary fission to form particles which, after synthesis of the outer cell wall, develop into new infectious elementary body progeny. The fusion lasts about three hours and the incubation period may be up to 21 days. After division, the reticulate body transforms back to the elementary form and is released by the cell by exocytosis.<ref name="Chlamydia trachomatis"/>
Studies on the growth cycle of ''C. trachomatis'' and ''C. psittaci'' in cell cultures ''in vitro'' reveal that the infectious elementary body (EB) develops into a noninfectious reticulate body (RB) within a cytoplasmic vacuole in the infected cell. After the elementary body enters the infected cell, an eclipse phase of 20 hours occurs while the infectious particle develops into a reticulate body. The yield of chlamydial elementary bodies is maximal 36 to 50 hours after infection.<ref name=":0" />
A histone like protein HctA and HctB play role in controlling the differentiation between the two cell types. The expression of HctA is tightly regulated and repressed by small non-coding RNA, IhtA until the late RB to EB re-differentiation.<ref>{{cite journal|last1=Grieshaber|first1=NA|last2=Grieshaber|first2=SS|last3=Fisher|first3=ER|last4=Hackstadt|first4=T|title=A small RNA inhibits translation of the histone-like protein Hc1 in Chlamydia trachomatis.|journal=Mol. Microbiol.|date=2006|issue=2|pages=541–50|doi=10.1111/j.1365-2958.2005.04949.x|pmid=16390448|volume=59|s2cid=11872982}}</ref> The IhtA RNA is conserved across ''Chlamydia'' species.<ref>{{cite journal|last1=Tattersall|first1=J|last2=Rao|first2=GV|last3=Runac|first3=J|last4=Hackstadt|first4=T|last5=Grieshaber|first5=SS|last6=Grieshaber|first6=NA|title=Translation inhibition of the developmental cycle protein HctA by the small RNA IhtA is conserved across Chlamydia.|journal=PLOS ONE|date=2012|volume=7|issue=10|doi=10.1371/journal.pone.0047439|pmid=23071807|pmc=3469542|article-number=e47439|bibcode=2012PLoSO...747439T|doi-access=free}}</ref>
==Pathology== Most chlamydial infections do not cause symptoms.<ref>{{cite web|url=http://www.nhs.uk/Livewell/focusonchlamydia/Pages/Focus-on-chlamydia-hub.aspx|title=Chlamydia protection|access-date=August 1, 2010|archive-date=March 24, 2016|archive-url=https://web.archive.org/web/20160324210149/http://www.nhs.uk/Livewell/focusonchlamydia/Pages/Focus-on-chlamydia-hub.aspx|url-status=dead}}</ref> Symptomatic infections often include a burning sensation when urinating and abdominal or genital pain and discomfort.<ref>{{Cite web |date=17 July 2023 |title=Chlamydia |url=https://www.who.int/news-room/fact-sheets/detail/chlamydia |access-date=2023-12-25 |website=World Health Organization |language=en}}</ref> All people who have engaged in sexual activity with potentially infected individuals may be offered one of several tests to diagnose the condition.{{cn|date=February 2023}} Nucleic acid amplification tests (NAAT), which include polymerase chain reaction (PCR), transcription-mediated amplification (TMA), ligase chain reaction (LCR), and strand displacement amplification (SDA), are the most widely used diagnostic test for ''Chlamydia''.<ref>{{Cite web |date= 27 July 2010|title=Facts about chlamydia |url=https://www.ecdc.europa.eu/en/chlamydia/facts |access-date=2023-12-25 |website=European Centre for Disease Prevention and Control |language=en}}</ref>
==See also== * List of bacterial orders * List of bacteria genera
==References== {{Reflist}}
==Further reading== * {{Wikispecies-inline|Chlamydia}} * {{Wikispecies-inline|Chlamydophila}}
== External links == *[https://web.archive.org/web/20130510031955/http://patricbrc.org/portal/portal/patric/Taxon?cType=taxon&cId=83553 Chlamydophila] genomes and related information at [http://patricbrc.org/ PATRIC], a Bioinformatics Resource Center funded by [https://www.niaid.nih.gov/ NIAID] *Taxonomic Outline of the Procaryotes, Bergey's Manual of Systematic Bacteriology, Second Edition Release 1.0, April c. [https://web.archive.org/web/20130720103419/http://www.cme.msu.edu/Bergeys/april2001-genus.pdf] * [https://web.archive.org/web/20131209074033/http://www.chlamydiae.com/ Chlamydiae.com]
{{Bacteria classification|state=collapsed}} {{Gram-negative non-proteobacterial bacterial diseases}} {{Taxonbar|from=Q846309|from2=Q2442594|from3=Q26385444}} {{Authority control|additional=Q2442594, Q26385444}}
Category:Chlamydiota Category:Pathogenic bacteria Category:Bacteria genera