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.

Automated summary

Chlamydia, including human and animal pathogens like Chlamydia trachomatis and Chlamydia psittaci, are a diverse group of ancient obligate intracellular bacteria. Comparative genome analysis has been used to study the evolutionary transition of chlamydiae from symbionts in eukaryotes to pathogens in humans. However, genome sequence data for many chlamydial lineages are still missing, limiting our understanding of their evolutionary history. Thus, the genome sequence of the fish pathogen Candidatus Clavichlamydia salmonicola provides valuable information for studying the evolution of chlamydial virulence mechanisms.