Pangenome and Phylogenomic Analysis of the Pathogenic Actinobacterium Rhodococcus equi

We report a comparative study of 29 representative genomes of the animal pathogen Rhodococcus equi The analyses showed that R. equi is genetically homogeneous and clonal, with a large core genome accounting for ≈80% of an isolates' gene content. An open pangenome, even distribution of accessory genes among the isolates, and absence of significant core-genome recombination, indicated that gene gain/loss is a main driver of R. equi genome evolution. Traits previously predicted to be important in R. equi physiology, virulence and niche adaptation were part of the core genome. This included the lack of a phosphoenolpyruvate:carbohydrate transport system (PTS), unique among the rhodococci except for the closely related Rhodococcus defluvii, reflecting selective PTS gene loss in the R. equi-R. defluvii sublineage. Thought to be asaccharolytic, rbsCB and glcP non-PTS sugar permease homologues were identified in the core genome and, albeit inefficiently, R. equi utilized their putative substrates, ribose and (irregularly) glucose. There was no correlation between R. equi whole-genome phylogeny and host or geographical source, with evidence of global spread of genomovars. The distribution of host-associated virulence plasmid types was consistent with the exchange of the plasmids (and corresponding host shifts) across the R. equi population, and human infection being zoonotically acquired. Phylogenomic analyses demonstrated that R. equi occupies a central position in the Rhodococcus phylogeny, not supporting the recently proposed transfer of the species to a new genus.