# Chlamydia (genus)
**Chlamydia** is a genus of gram-negative, obligate intracellular bacteria in the family Chlamydiaceae (order Chlamydiales, phylum Chlamydiota) whose species cause a wide spectrum of disease in humans and other vertebrates, including trachoma, sexually transmitted infections, pneumonia, and zoonotic psittacosis[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf) • [2](https://pubchem.ncbi.nlm.nih.gov/taxonomy/810). Chlamydiae are non-motile and cannot be grown on ordinary artificial media; they replicate only inside eukaryotic cells, where they alternate between two morphologically distinct forms, the extracellular infectious elementary body (EB) and the intracellular replicative reticulate body (RB)[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf) • [3](https://www.sciencedirect.com/science/article/pii/S0168644505000239). The type species is *Chlamydia trachomatis*[2](https://pubchem.ncbi.nlm.nih.gov/taxonomy/810).
| Key fact | Detail |
|---|---|
| Lifestyle | Gram-negative, non-motile, obligate intracellular bacteria with a biphasic developmental cycle[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf) • [4](https://wwwn.cdc.gov/nchs/data/nhanes/public/2011/labmethods/chlmda_g_met_chlamydia.pdf) |
| Developmental forms | Elementary body (EB), 0.25 to 0.3 μm, infectious and extracellular; reticulate body (RB), 0.5 to 0.6 μm, replicative and intracellular[5](https://www.ncbi.nlm.nih.gov/books/NBK8091/) |
| Cycle timing | Eclipse phase of about 20 hours after EB entry; maximal yield of EB progeny 36 to 50 hours after infection[5](https://www.ncbi.nlm.nih.gov/books/NBK8091/) |
| Human pathogens | Three species infect humans: *C. trachomatis*, *C. pneumoniae*, and *C. psittaci*[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf) |
| Species count | 14 species in the genus, including three Candidatus species[6](https://journals.pdau.edu.ua/visnyk/article/view/1130) |
| Genome | *C. pneumoniae* genome of 1,230,230 nt; about 80% of predicted coding sequences are orthologs shared with *C. trachomatis*[7](https://pubmed.ncbi.nlm.nih.gov/10192388/) |
| Taxonomy | Genus *Chlamydia* Jones, Rake and Stearns, 1945; briefly split into *Chlamydia* and *Chlamydophila* (1999), later reunited into a single genus[8](https://www.sciencedirect.com/science/article/abs/pii/S0723202015000028) |
## The developmental cycle
The chlamydial life cycle is unique among bacteria in alternating between two forms. The elementary body is a small, non-replicating, infectious particle released from ruptured infected cells and transmitted to new hosts; it attaches to and invades susceptible cells[3](https://www.sciencedirect.com/science/article/pii/S0168644505000239) • [5](https://www.ncbi.nlm.nih.gov/books/NBK8091/). After entry, the EB is internalized within a membrane-bound vacuole, differentiates into the reticulate body, and the vacuole is modified by chlamydia-produced inclusion membrane proteins into a parasitophorous vacuole termed an inclusion[9](https://pubmed.ncbi.nlm.nih.gov/25580274/). During early infection, 1 to 10 hours post infection, the vesicle migrates to a perinuclear location near the [Golgi apparatus](https://www.edgepedia.org/golgi-apparatus)[9](https://pubmed.ncbi.nlm.nih.gov/25580274/).
**Replication inside the inclusion.** Reticulate bodies divide by binary fission, and accumulating RBs position themselves at the periphery of the inclusion membrane, where type III secretion system needle-like structures (injectisomes) penetrate the membrane to secrete effector proteins into the host cytosol[9](https://pubmed.ncbi.nlm.nih.gov/25580274/). Late in the cycle, RBs differentiate back into infectious EBs; the host cell then lyses and releases EBs that infect neighboring cells[3](https://www.sciencedirect.com/science/article/pii/S0168644505000239). Under stressful conditions such as immune responses, antibiotics, or nutrient deprivation, the cycle is disrupted and large, aberrant RBs appear, with continued expression of [DNA replication](https://www.edgepedia.org/dna-replication) genes but not of cell division genes[3](https://www.sciencedirect.com/science/article/pii/S0168644505000239).
## The elementary body
Once considered an inert particle, the EB is now understood as a sophisticated entity that encounters at least three different environments during each infectious cycle[10](https://pubmed.ncbi.nlm.nih.gov/27197644/). Its spore-like cell wall is stabilized by a network of proteins crosslinked by disulfide bonds, the outer membrane complex, which confers resistance to osmotic and physical stress in the harsh extracellular environment[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC4886739/). During differentiation into the RB, these crosslinked complexes are reduced, providing the membrane fluidity required for replication[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC4886739/).
Studies using a host-free (axenic) system indicate that EBs have substantial metabolic and biosynthetic activity and depend on d-glucose-6-phosphate as an energy source; quantitative proteomics show an abundance of proteins for central metabolism and glucose catabolism, possibly supporting a burst of metabolic activity on host cell entry[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC4886739/).
## Species and diseases
Three species of *Chlamydia* are known to infect humans, and they vary in host spectrum, pathogenicity, and clinical disease[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf).
- ***C. trachomatis*** is historically considered an exclusively human pathogen; infection causes trachoma, an ocular infection that leads to blindness, and sexually transmitted diseases including pelvic inflammatory disease, chronic pelvic pain, ectopic pregnancy, and epididymitis[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf) • [7](https://pubmed.ncbi.nlm.nih.gov/10192388/).
- A human pathogen by nature, C. pneumoniae brings about pneumonia and bronchitis; within the United States it accounts for roughly 10% of pneumonia cases and 5% of bronchitis cases. Chronic outcomes following respiratory infection may include reactive airway disease, adult-onset asthma, and potentially lung cancer, and infection has been associated with atherosclerosis[7](https://pubmed.ncbi.nlm.nih.gov/10192388/). It is considered the least pathogenic human species, often producing subclinical or asymptomatic infections[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf).
- ***C. psittaci*** is a zoonotic agent that commonly infects psittacine (parrot family) birds and is highly pathogenic for humans; with appropriate treatment, mortality is about 1%[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf).
The genus also includes pathogens of other animals, such as *C. pecorum*, a pathogen of ruminants recognized as a new species through molecular analyses[12](https://wwwnc.cdc.gov/eid/article/2/4/96-0406_article). A 2021 review listed 14 species in the genus, including *C. abortus*, *C. avium*, *C. caviae*, *C. felis*, *C. gallinacea*, *C. muridarum*, *C. suis*, and three Candidatus species[6](https://journals.pdau.edu.ua/visnyk/article/view/1130).
## Genomes and evolution
Chlamydiae are phylogenetically separated from other bacterial divisions, and their genomes are highly conserved in gene content and order[7](https://pubmed.ncbi.nlm.nih.gov/10192388/) • [13](https://pmc.ncbi.nlm.nih.gov/articles/PMC111046/). About 80% of the predicted coding sequences of *C. pneumoniae* and *C. trachomatis* serovar D are orthologs, and roughly 80% conservation of genes and gene order also holds between *C. trachomatis*, *C. pneumoniae*, and *C. abortus*, reflecting the common biological requirements of their intracellular niche[7](https://pubmed.ncbi.nlm.nih.gov/10192388/) • [14](https://academic.oup.com/femspd/article/55/2/115/550166). The *C. pneumoniae* genome is about 0.15 Mb larger than that of *C. trachomatis* and contains about 200 genes not found in *C. trachomatis*, including an expanded family of 21 sequence-variant outer-membrane proteins, a conserved type III secretion virulence system, and additional purine and biotin biosynthetic capability[7](https://pubmed.ncbi.nlm.nih.gov/10192388/) • [13](https://pmc.ncbi.nlm.nih.gov/articles/PMC111046/).
**Reduced genomes, host dependence.** The reduced chlamydial genome makes the bacteria dependent on specific interactions with the host cell for nutrients to survive and replicate[15](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1281823/full). Their dependence on an intracellular growth niche and the peculiar developmental cycle long hampered genetic investigation, but the last decade has produced large collections of mutant strains and a plasmid-based transformation system enabling complementation, reporter gene expression, and targeted gene disruptions[16](https://www.annualreviews.org/content/journals/10.1146/annurev-micro-102215-095539).
## Taxonomy
The genus name *Chlamydia* dates to Jones, Rake and Stearns, 1945. Early taxonomists proposed several generic names for the psittacosis-lymphogranuloma venereum-trachoma group, including *Miyagawanella*, *Bedsonia*, and *Rakeia*; a 1966 revision concluded that shared morphology, developmental cycle, and group antigen outweighed the separating traits, and unified the group in the genus *Chlamydia*, replacing the earlier names[17](https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/00207713-16-2-223). Originally the order Chlamydiales contained one family, Chlamydiaceae, with one genus and just two species, *C. trachomatis* and *C. psittaci* (Page, 1968); chlamydiae were recognized as gram-negative bacteria in 1966[18](https://backoffice.biblio.ugent.be/download/6957831/6957832).
In 1999, based on ribosomal RNA gene sequence analysis, Everett and colleagues proposed subdividing the Chlamydiaceae into two genera, *Chlamydia* and *Chlamydophila*, and splitting *C. trachomatis* and *C. psittaci* into additional species[8](https://www.sciencedirect.com/science/article/abs/pii/S0723202015000028). Comparative genome analysis later showed that host-divergent strains are closely related biologically and ecologically, and that none of the species relationships met the less-than-95% 16S rRNA similarity threshold proposed for the family split; a 2009 paper proposed reuniting the Chlamydiaceae into a single genus, *Chlamydia*, a classification adopted in a 2015 emendation covering all 11 then-recognized species[14](https://academic.oup.com/femspd/article/55/2/115/550166) • [8](https://www.sciencedirect.com/science/article/abs/pii/S0723202015000028). Alternative nomenclature including *Chlamydophila pneumoniae* and *Chlamydophila psittaci* still appears in some sources[1](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf).
## References
1. [Chlamydia psittaci, C. trachomatis, C. pneumoniae, in Biosafety in Microbiological and Biomedical Laboratories, 6th Edition, CDC/NIH](https://www.cdc.gov/labs/media/pdfs/2025/08/SF__19a_308133-A_BMBL6_00-BOOK-WEB-final-3.pdf)
2. [Chlamydia, Taxonomy, PubChem (NCBI Taxonomy ID 810)](https://pubchem.ncbi.nlm.nih.gov/taxonomy/810)
3. [Abdelrahman & Belland. The chlamydial developmental cycle. FEMS Microbiology Reviews](https://www.sciencedirect.com/science/article/pii/S0168644505000239)
4. [Chlamydia trachomatis laboratory method, NHANES, CDC](https://wwwn.cdc.gov/nchs/data/nhanes/public/2011/labmethods/chlmda_g_met_chlamydia.pdf)
5. [Chlamydia, Medical Microbiology, NCBI Bookshelf](https://www.ncbi.nlm.nih.gov/books/NBK8091/)
6. [The renovation of the taxonomic classification of the Order Chlamydiales microorganisms, Scientific Progress & Innovations](https://journals.pdau.edu.ua/visnyk/article/view/1130)
7. [Kalman et al. Comparative genomes of Chlamydia pneumoniae and C. trachomatis. Nature Genetics (1999)](https://pubmed.ncbi.nlm.nih.gov/10192388/)
8. [Sachse et al. Emendation of the family Chlamydiaceae: proposal of a single genus, Chlamydia. Systematic and Applied Microbiology](https://www.sciencedirect.com/science/article/abs/pii/S0723202015000028)
9. [Recent advances in understanding and managing Chlamydia trachomatis infections, PubMed](https://pubmed.ncbi.nlm.nih.gov/25580274/)
10. [One Face of Chlamydia trachomatis: The Infectious Elementary Body, PubMed](https://pubmed.ncbi.nlm.nih.gov/27197644/)
11. [Chlamydia cell biology and pathogenesis, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4886739/)
12. [Chlamydiae as Pathogens: New Species and New Issues, Emerging Infectious Diseases (1996)](https://wwwnc.cdc.gov/eid/article/2/4/96-0406_article)
13. [Genome sequences of Chlamydia trachomatis MoPn and Chlamydia pneumoniae AR39, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC111046/)
14. [Divergence without difference: phylogenetics and taxonomy of Chlamydia resolved, Pathogens and Disease (2009)](https://academic.oup.com/femspd/article/55/2/115/550166)
15. [Molecular pathogenesis of Chlamydia trachomatis, Frontiers in Cellular and Infection Microbiology (2023)](https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2023.1281823/full)
16. [Molecular Genetic Analysis of Chlamydia Species, Annual Review of Microbiology](https://www.annualreviews.org/content/journals/10.1146/annurev-micro-102215-095539)
17. [Revision of the family Chlamydiaceae Rake (Rickettsiales), International Journal of Systematic Bacteriology](https://www.microbiologyresearch.org/content/journal/ijsem/10.1099/00207713-16-2-223)
18. [Taxonomy thesis, Ghent University repository](https://backoffice.biblio.ugent.be/download/6957831/6957832)
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*Topic: Encyclopedia › Life and health › Human health and medicine › Diseases and injuries › Urinary, reproductive, and developmental conditions › Sexually transmitted infections › STI pathogens › Chlamydia and chlamydial infections*
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