Multilocus sequence typing (MLST) for the infra-generic taxonomic classification of entomopathogenic Rickettsiella bacteria

The genus Rickettsiella comprises intracellular bacterial pathogens of a wide range of arthropods that are currently classified in four recognized species and numerous further pathotypes. However, both the delineation of and the synonymization of pathotypes with species are highly problematic. In the sequel of a previous phylogenomic study at the supra-generic level, nine selected genes - the 16S and 23S rRNA genes and the protein-encoding genes dnaG, ftsY, gidA, ksgA, rpoB, rpsA, and sucB - were evaluated for their potential as markers for the generic and infra-generic taxonomic classification of Rickettsiella-like bacteria. A methodological approach combining phylogenetic reconstruction with likelihood-based significance testing was employed on the basis of sequence data from the species Rickettsiella grylli and Rickettsiella popilliae, pathotypes 'Rickettsiella melolonthae' and 'Rickettsiella tipulae'. This study provides the first multilocus sequence typing (MLST) data for the genus Rickettsiella and identifies two new genetic markers, gidA and sucB, for the infra-generic classification within this taxon. In particular, aforesaid genes were found more reliable and informative markers than the corresponding 16S rRNA-encoding sequences that failed to produce strictly significant infra-generic taxonomic assignments. However, gidA- and sucB-based phylogenies were consistent with the currently accepted view of species delineation and species-pathotype synonymization within the genus Rickettsiella.