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'Chlamydia-like' organisms

Rhabdochlamydia

Prior to 1999, chlamydiae were categorized as either Chlamydiaceae (in the order Chlamydiales) or 'Chlamydia-like.'  All of these bacteria shared 3 characteristics:  they had apparent biphasic intracellular development in eukaryotic cells, small size, and 16S rDNA sequences that were >80% identical.  Beginning in 1999, 'chlamydia-like' came to describe bacteria for which only one or two of these of these characteristics were known.  The next level up, ‘Candidatus,’ applies when there is evidence for a developmental cycle, small size, and >80% rDNA identity, but for which type strains have not been cultured for submission to two culture collections. Interestingly, most of the 'Candidatus' organisms are obligate intracellular bacteria that have not been cultured. The highest level, 'validated species' are those recognized by the International Journal of Systematic and Evolutionary Microbiology or published in a validation list of the International Committee on Systematics of Prokaryotes [see: Fuzeby, J. P., 2003].

Possession of a biphasic intracellular growth cycle is insufficient to characterize an organism as being a member of the Chlamydiales, as superficially similar growth cycles exist among alpha-proteobacteria Rickettsia and gamma-proteobacteria Coxiella.  Sequence data based on 16S rDNA provide useful taxonomic information, with Everett et al., 1999 defining sequence similarity threshold values of 95% for genera, 90% for families and 80% for the order Chlamydiales. Lineages discovered since 1999 belong to as many as eight more groups [Corsaro et al., 2003].  Reactivity with the so-called 'genus specific' epitope on chlamydial lipopolysaccharide is often used as a  taxonomic descriptor [see epitheliocystis] but it has only been characterized for members of the family Chlamydiaceae.

Today, 'chlamydia-like' organisms have been reported from a huge range of hosts including hydras, bivalves, isopod crustacea, spiders, scorpions, reptiles and fish. They have also been reported in an extraordinary variety of aquatic environmental habitats including waste water plants, Arctic and Antarctic sediment, the plant rhizosphere etc. [For a detailed review see Corsaro et al., 2003; 2004; Corsaro & Greub, 2006].  The first to emerge from this level of partially described chlamydiae are 'Candidatus Rhabdochlamydia porcellionis',  'Candidatus Fritschea bemesia' and 'Candidatus Fritschea eriococci'.

'Candidatus Rhabdochlamydia porcellionis'

Fig 1.  Rhabdochlamydia porcellionis in the wood louse, Porcellio scaber. Note the rod shaped elementary body (top). Figures kindly provided and © Drs Rok Kostanjsek & Gorazd Avgustin, 2004.	Fig 2. Elementary body of R. porcellionis showing the 5 layered cell wall (top).	Fig 3. Intermediate body of R. porcellionis showing DNA condensation into nucleoid.
Fig 4. Large reticulate body of R. porcellionis.	Fig 5. Low power view of R. porcellionis inclusions in the hepato-pancreas of the wood louse, Porcellio scaber.	Fig 6. Higher power details of an R. porcellionis inclusion in P. scaber showing the rod shaped, electron-dense elementary bodies and the larger, electron-lucent reticulate bodies, which replicate by binary fission.

Infections with intracellular 'chlamydia-like' or 'rickettsia-like' organisms have been reported in both aquatic and terrestrial crustacean species [for review see Drobne et al., 1999] as well as insects.  These bacteria are sometimes mistaken for one another, due to their similar morphology;  sequencing the 16S rDNA of ‘Rickettsiella porcellionis,’ an intracellular bacterium from the wood louse Porcellio scaber, showed that the gene was in fact closest (90.5 - 91.7% similar) to environmental chlamydiae lineage (ECL) VI group previously described by Horn and Wagner, 2001. Similarity with other ribosomal sequences from other Chlamydiales was 83.1 - 87.2%  [Kostanjsek et al., 2004]. A reclassification of these bacteria to 'Candidatus Rhabdochlamydia porcellionis' was approved by publication in the International Journal of Systematic and Evolutionary Microbiology, an ICSB-approved taxonomy journal. Phylogenetic analysis suggests that Rhabdochlamydia form a distinctive lineage within the Chlamydiales outside the known families and genera but most closely related to ECL VI and, next, at 86.6% similarity, to S. negevensis [Kostanjsek et al., 2004]. A specific flurorescent hybridisation probe was prepared [sequence 3'-CTTTACGTTTCCTGTCGTAG-5' ; Kostanjsek et al., 2004] which confirmed the localisation of the new organism to intracellular vacuoles in P. scaber [See: IJSEM additional figures].

Benign infection of  P. scaber with R. porcellionis was found in up to 10% of the animals and was associated with white spots of 50 to 150 microns diameter on the hepatopancreas . These spots were aggregations of vacuoles densely filled with bacteria in different phases of a developmental cycle that included: a) small, dense, rod-shaped "elementary bodies" approximately 250-700 nm long with a diameter of 100 - 150 nm with a 5-layered cell wall structure without discernable peptidoglycan layer and approximately 16nm thick; b) much larger spherical cells 1 - 4 microns in diameter were also present which divided by binary fission and resembled reticulate bodies; and c) spherical "intermediate bodies", approximately 350 - 650 nm in diameter, with a characteristic electron dense central area  [Kostanjsek et al., 2004]. The vacuoles, filled mainly with elementary bodies, were released into the lumen of the hepatopancreas.  Morphologically these organisms resembled the polyphyletic Rickettsiella chironomi group.  [Drobne et al., 1999]. Previously Shay et al., 1985 had described an organism they termed 'Chlamydia isopodii'  from the hepatopancreas of the same wood louse species. However this naming was not validly published in journals recommended by the International Committee for Systematic Bacteriology. It seems likely that the former  'C. isopodii' and 'Candidatus Rhabdochlamydia porcellionis' are the same species.

[Comment: The term Candidatus is used in taxonomy to describe obligate (ie never cultured) symbionts of eukaryotic cells. The name Rhabdochlamydia is derived as follows: "Rha.bdo.chla.my’di.a; Gr. fem. n. rhabdos the stick, rod; M. L. fem. n. Chlamydia taxonomic name of a bacterial genus; M. L. fem. n. Rhabdochlamydia rod-shaped Chlamydia, referring to the rod-like shape of the elementary bodies; por.ce.lli.o’.nis; N. L. gen. n. of Porcellio, taxonomic name of a terrestrial isopod genus; pertaining to the genus name of the host organism" [Kostanjsek et al., 2004]. 

Kostanjsek, R., Strus, J., Drobne, D. and Avgustin, G. (2004). 'Candidatus Rhabdochlamydia porcellionis', gen. nov., sp. nov., an intracellular bacterium from hepatopancreatic cells of the terrestrial isopod crustacean Porcellio scaber (Crustacea: Isopoda). International Journal of Systematic and Evolutionary Microbiology 54, 543 - 549 .  Supplementary pictures 

Shay, M. T., Bettica, A., Vernon, G. M. & Witkus, E. R. (1985). Chlamydia isopodii sp. n., an obligate intracellular parasite of Porcellio scaber. Experimental Cellular Biology 53, 115 - 120.

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