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Family Chlamydiaceae

All species are Gram-negative and express the family-specific lipopolysaccharide epitope  (previously called the genus-specific epitope). The extracellular osmotic stability of the Chlamydiaceae EBs is maintained by a complex of disulfide-crosslinked envelope proteins that include the 40-kDa major outer membrane protein [MOMP, a translation product of ompA], a hydrophilic cysteine-rich 60-kDa protein, and a low-molecular mass cysteine-rich lipoprotein. When chlamydiae infect cells, the disulphide crosslinks within and among these envelope proteins become chemically reduced, allowing transformation of EBs into RBs. Using visible light microscopy, the inclusion may have a bright, particulate flickering that varies in intensity and duration and is suggestive of chlamydial Brownian motion. Chlamydiaceae have little or no detectable peptidoglycan although they have the necessary biosynthetic genes and no transfer RNA (tRNA) in the 16S–23S-rRNA intergenic spacer. The G+C content is approximately 40 mol%.

Chlamydia and Chlamydophila

In the 1999 chlamydial reclassification [Everett et al., 1999], five new species were validated and the former C. pneumoniae, C. pecorum and C. psittaci were moved to the new genus Chlamydophila [see classification]. In addition to genetic and protein sequence differences with the genus Chlamydia, Chlamydophila spp. do not produce detectable glycogen and have one ribosomal operon (Chlamydia spp. have two). Chlamydial strains and species have varying inclusion morphology and variable probabilities of having an extrachromosomal plasmid or sulphadiazine resistance.

Clavochlamydia

Karlsen et al., 2007 identified a new Candidatus species from freshwater salmonid fish in Norway. The organism was associated with epitheliocystis and inflammatory gill disease but was quite different to Piscichlamydia, another chlamydial group associated with these conditions.  The new organism was called 'Candidatus Clavochlamydia salmonica' reflecting its provisional taxonomic status and the fact that it has not been grown in cell culture [see taxonomy 2008]. Phylogenetic analysis of the 16SrRNA sequence suggested the new organism is loosely affiliated to the family Chlamydiaceae within which it may eventually form a new genus. The authors thought Clavochlamydia salmonicola may have branched from the rest of the Chlamydiaceae prior to the Chlamydia - Chlamydophila split.

Formal description of ‘Candidatus Clavochlamydia salmonicola’

‘Candidatus Clavochlamydia salmonicola’ [Cla.vo.chlamy'di.a L. adj. clavate club-like shape; L. F. n. Chlamydia name of bacterial taxon; L. F. n. Clavochlamydia Chlamydiae with club-like shape; sal.mo.ni.co'la. L. M. n. Salmo name of host genus; L. v. colere to inhabit; L. M. n. salmonicola infecting fish of the genus Salmo.]

The provisional taxon ‘Candidatus Clavochlamydia salmonicola’ contains intracellular bacteria that may infect gill cells of salmonid fish in freshwater environments. Members of this taxon have a developmental cycle of replication similar to the Chlamydiae. This includes pleomorphic RBs that may reach at least 2 μm in length. Reticulate bodies develop into IBs that are between 350 and 650 nm in length and contain a centrally located electron-dense nuclear area that is approximately 180 nm in diameter and separated from the ribosomes by an electron-lucent zone (about 30 nm). The IBs develop into EBs which are about 1 μm long and consist of a head region (about 350–400 nm in diameter) and a 550- to 600-nm-long tail region. The tail (diameter of about 80 nm) is expanded in the terminal part (diameter of about 150 nm). All three bodies are surrounded by a cell wall and a cytoplasmic membrane. Both the cell wall and the cytoplasmic membrane consist of a trilaminar membrane (unit membrane). Members of this taxon may be causative agents of the condition ‘epitheliocystis’. Affected cells may develop into large cysts of at least 56 μm, and the infection may spread to nearby cells. The 16S rRNA gene of ‘Candidatus Clavochlamydia salmonicola’ has been deposited in the GenBank with Accession No. DQ011662 and EF577392. The 16S rRNA gene shows phylogenetic affinity towards the family Chlamydiaceae [Karlsen et al., 2007]. 

[KDE]  2002 updated by [MEW] 2008

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References

Karlsen, M., Nylund, A., Watanabe, K., Helvik, J. V., Nylund, S. & Plarre, H. (2007). Characterization of 'Candidatus Clavochlamydia salmonicola': an intracellular bacterium infecting salmonid fish. Environmental Microbiology 10(1), 208 - 218. Full paper