| Chlamydia | |
|---|---|
 | |
| Chlamydia trachomatisinclusion bodies (brown) in a McCoy cell culture. | |
Scientific classification | |
| Domain: | Bacteria |
| Kingdom: | Pseudomonadati |
| Phylum: | Chlamydiota |
| Class: | Chlamydiia |
| Order: | Chlamydiales |
| Family: | Chlamydiaceae |
| Genus: | Chlamydia Jones, Rake & Stearns 1945 |
| Type species | |
| Chlamydia trachomatis (Busacca 1935) Rake 1957 | |
| Species[1] | |
| Synonyms | |
homotypic
heterotypic
| |
Chlamydia is agenus ofpathogenicGram-negative bacteria that areobligate intracellular parasites.Chlamydia infections are the most common bacterialsexually transmitted diseases in humans and are the leading cause of infectious blindness worldwide.[4]
Humans mainly contractC. trachomatis,C. pneumoniae,C. abortus, andC. psittaci.[5]
Because ofChlamydia's unique developmental cycle, it was taxonomically classified in a separate order.[6]Chlamydia is part of the order Chlamydiales, family Chlamydiaceae.[1]
Earlier criteria for differentiation of chlamydial species did not always work well. For example, at that timeC. psittaci was distinguished fromC. trachomatis bysulfadiazine resistance, although not all strains identified asC. psittaci at the time were resistant, andC. pneumoniae was classified by its appearance underelectron 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 genusChlamydia and reclassified them in the then newly created genusChlamydophila (Cp. thereafter). Five new species were added by splitting from existing species:[7]
| Old name | Host | New name |
|---|---|---|
| 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 |
| C. trachomatis | Humans | C. trachomatis |
| Swine | C. suis | |
| Mice and hamsters | C. muridarum |
According to the authors of the 1999 study, themeanDNA–DNA reassociation difference distinguishingChlamydophila fromChlamydia is 10.1%, an accepted value for genus separation. Although the16S 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 distinguishChlamydophila fromChlamydia. Also, the full-length23S ribosomal RNA genes of the species of the two genera were less than 95% identical.[7] Supporting criteria such asantigen detection,[9]glycogen staining, host association, and EM morphology were also employed, depending on applicability and availability.
| Genus | Approximate genome size (Mega base pairs) | Detectable glycogen | Number ofrDNA operons |
|---|---|---|---|
| Chlamydophila | 1.2 | No | 1 |
| Chlamydia | 1.0 | Yes | 2 |
In 2001 many bacteriologists strongly objected to the reclassification.[1] Comparative genomic analyses in 2006 identified a number of signature proteins that were uniquely present in species from the generaChlamydia andChlamydophila, which supported the distinctness ofChlamydophila, but did not support an "early separation" scenario as suggested by rRNA.[10]
In 2009 the validity ofChlamydophila was challenged by newer DNA analysis techniques (using 100 concatenated proteins instead of 16S rRNA), leading to a proposal to "reunite theChlamydiaceae into a single genus,Chlamydia". The authors pointed to the poorbootstrap support of the 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.[11] This reversion appears to have been accepted by the community[12] and was formally validated in 2015,[13][14] bringing the number of (valid)Chlamydia species up to 9 as of 2017.[15] The merger of the genusChlamydophila back into the genusChlamydia is, by 2018, generally accepted.[16][17][18][19]
However, the much newer analyses ofGenome Taxonomy Database using 120 concatenated proteins again show a split of those two genera to be valid (see§ Phylogeny below), and has lead to the resurrection of the genus in the GTDB andGBIF taxonomies.[20][21] Joseph et al. 2015, which proposed new species from strains formerly known asC. psittaci, also recovered a coherentChlamydophila clade in their whole-genome tree, but with an unusual topology showingChlamydophila to be sister toC. muridarum.[5] Gupta et al. (2015) finds 1 CSI + 19 CSPs specific forChlamydophilia and 2 CSIs + 19 CSPs specific for the three-species version ofChlamydia.[22]
Many probable species were subsequently isolated, but no one bothered to name them. Many new species fall into theChlamydophilia clade and were originally classified as aberrant strains ofC. psittaci. Complicating the picture is the fact that this clade shows signs of interspecies recombination.[5]
There is one invalidly publishedChlamydophilia species that has not been transferred back toChlamydia as of 2025: "Chlamydophila parapsittaci",[25] representative of an intermediate stage betweenC. abortus andC. psittaci.[26] SeeChlamydia psittaci § Psittaci-abortus intermediate for a discussion of it.
Chlamydia species have genomes around 1.0 to 1.3megabases in length.[27] Most encode ~900 to 1050 proteins.[28] Some species also contain a DNAplasmids orphage genomes (see Table). 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.[29]
| C. muridarum MoPn | C. trachomatis D | C. pneumoniae AR39 | C. pneumoniae 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) | 1 ssDNA phage | none |
Table 1. Genome features of selectedChlamydia species and strains. MoPn is a mouse pathogen while strain "D" is a human pathogen. About 80% of the genes inC. trachomatis andC. pneumoniae are orthologs. Adapted after Read et al. 2000,[28] nomenclature of MoPn following Carlson et al. 2008[30]
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 aspore. The elementary body may be 0.25 to 0.30 μm in diameter. This form is covered by a rigidcell wall (hence thecombining formchlamyd- in the genus name). The elementary body induces its ownendocytosis upon exposure to target cells. Onephagolysosome usually produces an estimated 100–1000 elementary bodies.[citation needed]
Chlamydia may also take the form of a reticulate body, which is in fact anintracytoplasmic 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 withiodine, 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 byexocytosis.[6]
Studies on the growth cycle ofC. trachomatis andC. psittaci incell culturesin 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.[29]
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.[31] The IhtA RNA is conserved acrossChlamydia species.[32]
Most chlamydial infections do not cause symptoms.[33] Symptomatic infections often include a burning sensation when urinating and abdominal or genital pain and discomfort.[34] All people who have engaged in sexual activity with potentially infected individuals may be offered one of several tests to diagnose the condition.[citation needed]Nucleic acid amplification tests (NAAT), which includepolymerase chain reaction (PCR),transcription-mediated amplification (TMA),ligase chain reaction (LCR), andstrand displacement amplification (SDA), are the most widely used diagnostic test forChlamydia.[35]
| 16S rRNA basedLTP_10_2024[36][37][38] | 120 marker proteins basedGTDB 09-RS220[39][40][41] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Data related toChlamydia at Wikispecies Data related toChlamydophila at Wikispecies| National | |
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Chlamydophila | |