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.