The Fish Pathogen "Candidatus Clavichlamydia salmonicola"-A Missing Link in the Evolution of Chlamydial Pathogens of Humans.

Chlamydiae like Chlamydia trachomatis and Chlamydia psittaci are well-known human and animal pathogens. Yet, the chlamydiae are a much larger group of evolutionary ancient obligate intracellular bacteria that includes predominantly symbionts of protists and diverse animals. This makes them ideal model organisms to study evolutionary transitions from symbionts in microbial eukaryotes to pathogens of humans. To this end, comparative genome analysis has served as an important tool. Genome sequence data for many chlamydial lineages are, however, still lacking, hampering our understanding of their evolutionary history. Here, we determined the first high-quality draft genome sequence of the fish pathogen "Candidatus Clavichlamydia salmonicola", representing a separate genus within the human and animal pathogenic Chlamydiaceae. The "Ca. Clavichlamydia salmonicola" genome harbors genes that so far have been exclusively found in Chlamydia species suggesting that basic mechanisms important for the interaction with chordate hosts have evolved stepwise in the history of chlamydiae. Thus, the genome sequence of "Ca. Clavichlamydia salmonicola" allows to constrain candidate genes to further understand the evolution of chlamydial virulence mechanisms required to infect mammals.

'Candidatus Branchiomonas cysticola' is a common agent of epitheliocysts in seawater-farmed Atlantic salmon Salmo salar in Norway and Ireland.

The prevalence and geographical distribution of the recently described endosymbiont 'Candidatus Branchiomonas cysticola' in Atlantic salmon Salmo salar gill epithelial cell cysts was investigated in seawater-farmed fish suffering proliferative gill inflammation (PGI). To this end, we developed a specific and sensitive real-time PCR assay for detection of the bacterium. 'Ca. B. cysticola' was found to be highly prevalent in Atlantic salmon gills sampled over 7 yr and from 17 geographically distant seawater locations in Norway and Ireland. 'Ca. B. cysticola' was found in significantly greater quantities in fish with large numbers of epitheliocysts, and fluorescence in situ hybridization confirmed its localisation within cysts. 'Ca. Piscichlamydia salmonis', a bacterium previously linked to epitheliocysts, was identified at relatively low levels of infection, apparently independent of epitheliocyst prevalence. These results suggest that 'Ca. B. cysticola' is the main cyst-forming bacterium in seawater-farmed Atlantic salmon in the studied countries. Our results also suggest a relationship between load of 'Ca. B. cysticola' and extent of pathological changes. Taken together with a previously described association between epitheliocyst load and severity of PGI in Norwegian salmon, the results could indicate a role for 'Ca. B. cysticola' in gill diseases such as PGI.

A novel betaproteobacterial agent of gill epitheliocystis in seawater farmed Atlantic salmon (Salmo salar).

Epitheliocystis, a disease characterised by cytoplasmic bacterial inclusions (cysts) in the gill and less commonly skin epithelial cells, has been reported in many marine and freshwater fish species and may be associated with mortality. Previously, molecular and ultrastructural analyses have exclusively associated members of the Chlamydiae with such inclusions. Here we investigated a population of farmed Atlantic salmon from the west coast of Norway displaying gill epitheliocystis. Although 'Candidatus Piscichlamydia salmonis', previously reported to be present in such cysts, was detected by PCR in most of the gill samples analysed, this bacterium was found to be a rare member of the gill microbiota, and not associated with the observed cysts as demonstrated by fluorescence in situ hybridization assays. The application of a broad range 16 S rRNA targeted PCR assay instead identified a novel betaproteobacterium as an abundant member of the gill microbiota. Fluorescence in situ hybridization demonstrated that this bacterium, tentatively classified as 'Candidatus Branchiomonas cysticola', was the cyst-forming agent in these samples. While histology and ultrastructure of 'Ca. B. cysticola' cysts revealed forms similar to the reticulate and intermediate bodies described in earlier reports from salmon in seawater, no elementary bodies typical of the chlamydial developmental cycle were observed. In conclusion, this study identified a novel agent of epitheliocystis in sea-farmed Atlantic salmon and demonstrated that these cysts can be caused by bacteria phylogenetically distinct from the Chlamydiae.