Phylum Chlamydiae (Chlamydia and relatives)

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  • Phylum Chlamydiae
    • Class Chlamydiae
      • Order Chlamydiales
        • Family Chlamydiaceae
          • Genus Chlamydia
          • Genus Chlamydiophila
          • Genus Clavochlamyia
        • Family Parachlamydiaceae

          • Genus Parachlamydia
          • Genus Neochlamydia
          • Genus Protochlamydia
        • Family Simkaniaceae
          • Genus Simkania
          • Genus Fritschea
        • Family Criblamydiaceae
          • Genus Criblamydia
          • Genus Rhabdochlamydia
          • Genus Piscichlamydia
        • Family Waddliaceae
          • Genus Waddlia

About this phylum


Historically, there were only 3 species in this Phylum, all of the genus Chlamydia: the human pathogens C. trachomatis, C. psittaci and C. pneumoniae. (The latter two have been moved to the genus Chlamydiophila.) Although many new species are now known, this Phylum remains a collection of very closely-related, phenotypically similar organisms. However, environmental surveys using the rRNA PCR approach suggest the diversity both within and without known Families is very much broader than is represented by known species. Although they are sometimes discussed with viruses, because they are obligate intracellular parasites transmitted by small, metabolically inert particles, they are Bacteria phylogenetically and in every other meaningful way.

The Chlamdia are distantly related to Verrucomicrobium, and probably the Planctomycetes as well, all of which have little or no peptidoglycan in their cell walls.

Life cycle

The Chlamydiae are obligate intracellular parasites of eukaryotes with a biphasic life cycle. The ‘elementary body’ (EB) is the infectious phase found in interstitial fluids, secretions, and in the environment. EB’s are small, only 0.2-0.3μm in diameter, and metabolically inert. Although Chlamydiae lack detectable amounts of peptidoglycan, the envelop of EBs is rigid due to heavy disulfide crosslinking of the major outer-membrane protein (MOMP). EBs attach to the host cell surface, probably non-specifically rather than via any specific receptors or adhesins, and endocytosed. The EBs then develop into vegetative ‘reticulate bodies’ (RBs), which metabolize, grow, and divide within the endocytic vesicle. RBs are larger, ca. 1μm cells, and are non-infectious and osmotically fragile, apparently lacking MOMP crosslinking. When the resources of the host cell become limited, most of the RBs differentiate into EBs, which are then released into the surroundings either by host cell lysis or exocytosis.

Life clcyle
Diagram of an idealised chlamydial developmental cycle, courtesy of Dr Karin D Everett. The small, infectious elementary bodies are in red; the larger, replicating reticulate bodies are in green. (from


Although the genomes of Chlamydiae are reduced in animal pathogens to about 1000 genes, they retain the genes required for information processing (transcription, translation, replication), the cell envelop (including peptidoglycan synthesis, even though this is undetectable in practice), and much of central metabolism. Glycolytic enzymes genes are present, as are the genes for an incomplete TCA cycle. The genes for purine and pyrimidine biosynthesis are present in some species, but they seem to rely on the host for most amino acids and cofactors. ATPase, run in ‘reverse’ at the expense of ATP, is probably used to generate the proton gradient required to drive active transport of nutrients from the host cell; no electron transport chain is present.

The Chlamydiae are largely energy parasites. An ATP/ADP antiport is used to acquire ATP from the host an recycle ADP. The ability to synthesize ATP may supplement energy parasitism, or may be required only to generate ADP to supply the ATP/ADP antiport as the cells grow.


The Chlamydiae are all obligate intracellular parasites, predominately of animals (as far as we know). The "environmental" Chlamydiae, despite the name, are also obligate intracellular parasites, but they infect protists - especially amoebas. In fact, it may be that amoebas can act as intermediate hosts for the more traditional Chlamydia as well. These environmental species have the same life cycle as the other Chlamydiae, but the elementary bodies float around in the environment instead of the body fluids of the host.

Example : Chlamydia trachomatis

Trachoma : souce unknown

C. trachomatis is a human pathogen that is the most common venereal disease in the United States - 4 million cases/year. It is easily spread since most infections in females are asymptomatic and untreated. Infection can lead to PID in women, and eventually sterility, and urethritis in men. Repeated ocular infection ("trachoma"), usually in children, leads to blindness, primarily in the third world, and is the leading cause of childhood blindness in the world. Blindness is an indirect result of infection on the inner eyelid; scarring causes the eyelids to curl inwards such that the eyelashes rub painfully across the surface of the eye with every blink. This constant irritation clouds the cornea, obscuring vision. This species also infects the koala, resulting in infertility that, along with habitat loss, is a serious threat to the survival of the species. In fact, the four major infectious diseases of koalas are all chlamydial!

Example : Protochlamydia amoebophila

P. amoebophila UWE25 (pink) in its host Acanthamoeba.
Poppert S, Essig A, Marre R, Wagner M, Horn M.. Appl Environ Microbiol. 2002 68:4081-9

P. amoebophila grows symbiotically in amoeba of the genus Acanthamoeba. These amoebas are common environmental organisms, although some are opportunistically pathogenic to humans; A. castelliani is commonly found in the tear covering of your eyes, and can cause infections in the eyes, especially those of contact lens wearers. P. amoebophila is a model system for the investigation of the evolution of the human pathogen Chlamydiae. The genome of P. amoebophila less reduced than that of the pathogenic Chlamydiae. At about 2.4MBP in length, with over 2000 protein-encoding genes, it is twice the size as those of the pathogenic Chlamydiae, and as large as those of many free-living Bacteria. It has a complete TCA cycle, from which is can synthesize glycine, serine, glutamine, and proline. Unlike pathogenic Chlamydiae, it cannot synthesize tryptophan; this ability is a virulence factor in the pathogens. P. amoebophila has an abbreviated electron transport chain, which is probably used to generate a proton gradient for active transport, but may also be used to generate ATP by oxidation phosphorylation to supplement that acquired from the host.