Systematics of Prokaryotes Rickettsia Taxonomy to Harmonize With Current Standards for Prokaryotic Nomenclature

Rickettsial taxonomy poses significant problems for rickettsiologists. This is partly historical, related to limitations of working with obligately intracellular bacteria, and inapplicable methods used to study most other prokaryotes. Before molecular phylogenetic-based taxonomy, spotted fever group Rickettsia species were classified by mouse serotyping. Serotyping required antiserum preparation; thus, absent rickettsial isolation, a method based on the sequences of 5 genes was proposed for classification using statistics that concurred with mouse serotyping species designations in species including very closely related “new” species. This rapidly led to descriptions of many other new species, but not to adjustments to statistics that governed divisions based on nucleotide identities, while interrogating less than 1.5% of rickettsial genomes. The result was an unwieldy and confusing taxonomy and nomenclature poorly supported by other phenotypic characters, and not useful for clinical purposes. Whole genome analyses later confirmed the gene-targeted approach, but these were also calibrated using mouse serotype species, that are now known to reflect antigenic differences primarily in one protein encoded in sca0. At least two major prokaryote whole genome taxonomic analyses concluded that the organization, classification and ultimate Rickettsia taxonomy and nomenclature significantly diverged from prokaryotic standards, and recommended re-analysis. Thus, we propose convening a panel of interested experts to discuss these issues in Rickettsia taxonomy and nomenclature and to discern a reorganization that better conforms with prokaryotic standards and provides greater value to understand biologic similarities and distinctions, as well as rickettsial virulence and pathogenicity.