Chlamydiae are bacterial species with a developmental cycle requiring replication in eukaryotic cells. They infect the cells of mammals, birds, marsupials, fish, reptiles, amphibians, insects and protozoa. The most famous species, Chlamydia trachomatis, causes sexually transmitted disease in humans. Analysis of available genome sequences for these bacterial species has shown that their common ancestor lived over a billion years ago. Because of their diversity, Chlamydiae are often recognised and grouped by using rDNA PCR methods. All Chlamydiae have 16S or 23S ribosomal ribonucleic acid (rRNA) sequences that are at least 80% identical to the rRNA of the Chlamydiaceae family. Chlamydiae are also characterised by four unique phenotypic traits: they have an electron dense infectious form (0.2 μm), they have a morphologically and physiologically distinct replicative form (approximately 1.0 μm), the outer membrane has lipopolysaccharide and is structurally dependent on disulfide‐crosslinked cysteine‐rich proteins, and in host cells Chlamydiae generally are located within membrane‐bound inclusions.

Key Concepts:

  • Chlamydia was long believed to be a simple and mysterious disease.

  • Today we know it is a tiny intracellular bacterium so unique that it is classified at the highest level: phylum Chlamydiae.

  • Chlamydiae probably includes thousands of species.

  • So far (since 1980) 25 chlamydial species have been described in some detail.

  • These species are found in animals, amoebae, insects and a worm living on the bottom of the ocean.

  • The complete genomes of 12 chlamydial species have been DNA sequenced.

  • Genome sequencing reveals that Chlamydiae existed a billion years ago.

  • The common ancestor of all Chlamydiae was directly related to the ancestors of plants and animals and other bacteria.

  • All genome‐sequenced Chlamydiae have 493 genes in common – approximately one quarter to one half of each genome.

  • Chlamydial phenotypes today display the observable effects of these genes.

Keywords: Chlamydia(e); disease; psittacosis; genome; intracellular; bacteria; evolution; amoeba; virus; veterinary

Figure 1.

Chlamydial taxonomy and phylogeny, 2011. Heavy bars=the year the family was named is indicated. Scale bar=‘estimated evolutionary change’; clustal alignment manually refined, Neighbour joining – no filter. Courtesy of Astrid Collingro, Department of Microbial Ecology, University of Vienna.

Figure 2.

The classic Chlamydiales developmental cycle of infection and replication. EB and RB morphological shape variations can include cocci, crescent, bow‐tie, elongated, pear‐shaped, vacuolated and head‐and‐tail. Some species are also found living directly in the cytoplasm. In amoebae some Chlamydiae can survive in cysts. The cycle typically takes 2–3 days, although some species have longer cycles (e.g. Simkaniaceae). EB, elementary body; RB, reticulate body.

Figure 3.

Electron micrograph of Chlamydia pneumoniae in the Alzheimer's brain. Arrows=EBs; arrowheads=RBs; Bar, 0.5 μm. Unpublished, courtesy of Brian J. Balin, Center for Chronic Disorders of Aging, Philadelphia College of Osteopathic Medicine, Philadelphia, PA; Alan P Hudson, Professor of Immunology and Microbiology, Wayne State University, Detroit, MI.

Figure 4.

Cell walls and inclusion wall of intracellular Chlamydiae. COMC, chlamydial outer membrane complex; T3SS, type‐3 secretion system.

Figure 5.

Eukaryotic evolution timescale indicating lineages infected with Chlamydiae (*). The phylogeny is based on SSU rRNA and calibrated using the continuous Phanerozoic microfossil record (Berney and Pawlowski, ). Rectangles delimit 95% confidence intervals. Lineages that host Chlamydiae in order of known frequency of infection: Metazoan>Amoebozoa>Excavates>Alveolates (Casson et al., ; Horn, ; Everett KDE and Meijer A, unpublished).



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Further Reading

Collingro A, Tischler P, Weinmaier T et al. (2011) Unity in variety – the pan‐genome of the Chlamydiae. Molecular Biology and Evolution June 20. (Epub ahead of print).

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How to Cite close
Everett, Karin DE(Jul 2011) Chlamydiae. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000451.pub2]