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Bacteria; Chlamydiae/Verrucomicrobia group; Chlamydiae; Chlamydiae (class); Chlamydiales; Parachlamydiaceae; Parachlamydia
Chromosome: 2,414,465 bp
Chlamydiae have for long been known as obligate intracellular bacteria exclusively living in animals and humans and which cause a wide variety of disease. Chlamydiae were recognised as having a very wide host range while being closely related with each other, forming a common evolutionary lineage quite distinct from all other known bacteria.
Electron microscopic studies of these new obligate intracellular bacteria indicated that they possessed a life cycle very similar to the unique and characteristic chlamydial developmental cycle although they have an additional crescent body growth stage associated with prolonged incubation and which may be a characteristic of the Parachlamydiaceae. Elementary and crescent bodies are released into the extracellular medium within vesicles or after amoebal lysis [Greub and Raoult, 2002b]. Comparative ribosomal RNA sequence analysis identified them as being close but distinct living relatives of the previously described chlamydiae belonging to the Chlamydiaceae. However some of the difference between them suggested a hidden diversity of chlamydia-like bacteria which awaited discovery.
Free-living amoebae (FLA) are ubiquitous and have been isolated from soil, fresh water, tap water, hydrotherapy and dental equipment, humidifiers, sewage treatment works, and even from dust and the air. FLA feed on bacteria and other micro organisms including fungi, yeasts, algae and other protozoa. They thus have an important ecological role as predators controlling microbial communities.
FLA belonging to the genus Acanthamoeba are genetically diverse, comprising up to 14 18S rDNA-based lineages [see: Corsaro et al., 2003]. Acanthamoeba can be commonly isolated from human mucosal and epithelial surfaces, being found in 13 of 140 recruits (9.3%) [Michel et al., 1982]. In humans they have been associated with infection of the surface of the eye and contact lenses leading to a severe keratitis [Matias et al., 1991]. In addition, acanthamoebae can cause a granulomatous encephalitis and even systemic infections in immuno-compromised patients [Visvesvara, 1995].
Although acanthamoebae trophozoites feed on other micro organisms, several bacteria are able to survive phagocytosis to multiply within the amoebal cell. This intracellular replication is particularly well studied in the case of the respiratory pathogen Legionella pneumophila. It is suggested that only the prior multiplication in FLA and the subsequent aspiration of vesicles filled with L. pneumophila leads to the occurrence of Legionnaire’s disease in susceptible humans [Harb et al., 2000].
In addition to transient associations of acanthamoebae and bacteria, around 25% of all investigated Acanthamoeba isolates were found to be colonised with obligate intracellular bacterial symbionts (Fritsche et al., 1993). Several of these endosymbionts which had a rod-shaped morphology were identified as novel Alphaproteobacteria most closely related to the rickettsiae [Fritsche et al., 1999; Horn et al., 1999; Birtles et al., 2000]. Others were identified as novel Betaproteobacteria [Horn et al., 2002] or as novel members of the Bacteroidetes [Horn et al., 2001]. However, some endosymbionts of FLA were coccoid-shaped and turned out to represent previously unrecognized chlamydia-related bacteria.
Among Michel's nasal Acanthamoeba isolates were two strains Bn9 and Berg17, which were found to be colonised with bacteria showing a chlamydia-like morphology [Michel et al., 1994]. Similar Gram positive chlamydia-like particles were also identified in Acanthamoeba trophozoites from humans in an outbreak of humidifier fever in Vermont, in the USA [Lewis et al., 1990]. In a key paper, Amann et al., 1997 demonstrated, by sequencing of the 16S rRNA gene, that the bacterial endosymbiont from Bn9 was more closely related to the chlamydiae than other bacteria, proposing the organism as "Candidatus Parachlamydia acanthamoebae", with the type strain Bn9T and Berg17 a further isolate. Subsequently in the reclassification of the Chlamydiales [Everett et al., 1999] P. acanthamoebae was placed in a new family, the Parachlamydiaceae.
P. acanthamoebae have variable Gram staining characteristics and it was noted that they can be grown in Vero cells (although no data was provided for this observation), growing best at moderate temperatures (> 25°C). P. acanthamoebae can be specifically detected with the rRNA-targeted oligonucleotide probe Bn9658 (5'-TCCGTTTTCTCCGCCTAC-3'. Parachlamydiae are antigenically and morphologically distinct from the Chlamydiaceae and several phylotypes are known to exist [Corsaro et al., 2003].
Department of Microbial Ecology, Institute of Ecology and Conservation Biology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria. firstname.lastname@example.org
Horn M, Collingro A, Schmitz-Esser S, Beier CL, Purkhold U, Fartmann B, Brandt P, Nyakatura GJ, Droege M, Frishman D, Rattei T, Mewes HW, Wagner M., Science 304(5671):728-30 (2004 Apr 30).