Genome-based reclassification of <i>Parabacteroides chartae</i> as a later heterotypic synonym of <i>Macellibacteroides fermentans</i>

Strain PK034 was isolated from a municipal wastewater treatment plant in Stillwater, OK, USA. A comparison of 16S rRNA gene sequences revealed ≥99.4% sequence similarity to the type strains of <i>Macellibacteroides fermentans</i> (isolated from an up-flow anaerobic filter) and <i>Parabacteroides chartae</i> (isolated from wastewater from a paper mill), suggesting their close relatedness and the possibility of these strains belonging to the same species. The genus <i>Macellibacteroides</i> currently contains a single species, while the genus <i>Parabacteroides</i> currently contains 13 species. Phylogenetic analysis using the 16S rRNA gene, whole-genome phylogenomic analysis and overall genomic-based relatedness indices indicated that PK034 and the type strains of <i>M. fermentans</i> and <i>P. chartae</i> shared average nucleotide identity and digital DNA-DNA hybridization values >95% and >70%, which are the currently accepted thresholds for species-level delineation. Furthermore, such analysis also placed PK034 and the type strains of <i>M. fermentans</i> and <i>P. chartae</i> as a distinct branch within the <i>Tannerellaceae</i> family of the <i>Bacteroidales</i> order, clustering separately from all other species of the genus <i>Parabacteroides</i>. Comparison of phenotypic and physiological traits revealed a high level of shared characteristics and physiological optima between all isolates. Thus, it is proposed that <i>P. chartae</i> NS31-3^T has been erroneously classified as a member of the genus <i>Parabacteroides</i>, that strains PK034, <i>M. fermentans</i> and <i>P. chartae</i> constitute the same species and that <i>P. chartae</i> Tan <i>et al.</i> 2012 should be taken as a later heterotypic synonym of <i>M. fermentans</i> Jabari <i>et al.</i> 2012.