Gene-centric analysis of Raskinella chloraquaticus reveals a functionally conserved taxonomic group in global drinking water distribution systems

A recent metagenomic survey of drinking water systems revealed that a highly prevalent and dominant uncultured bacterial genus (Raskinella) was represented globally by a single species (Raskinella chloraquaticus). R. chloraquaticus comprises of two sub-species groups, Lineages 1 and 2, the former representing a globally prevalent genomovar. The objective of this study was to perform comparative analysis of the gene content of R. chloraquaticus to characterize the gene-level diversity and determine factors shaping the diversity of this species. Pangenome analysis revealed that R. chloraquaticus possesses a core set of genes that constitute a major portion (87.74%) of the known gene content of the genome. Furthermore, most of the gene diversity of R. chloraquaticus is associated with Lineage 2 organisms, which consists of at least four distinct genomovars. Lineage 1 organisms consist of a higher proportion of identical genes than would have been expected if changes primarily occurred through random mutations and thus is potentially indicative of recombination. In contrast, Lineage 2 organisms appear to have emerged through random mutations and display stronger geographic preference. These results indicate that homologous recombination and geographic isolation likely shape the genetic repertoire of R. chloraquaticus. Further, the high level of gene conservation in R. chloraquaticus may be reflective of highly selective environment in drinking water systems. Thus, R. chloraquaticus may represent a model organism to probe selective pressures shaping the drinking water microbiome.