Reclassification of Bifidobacterium faecale Choi et al. 2014 as a later heterotypic synonym of Bifidobacterium adolescentis Reuter 1963

Bifidobacterium faecale CU 3-7T (=JCM 19861T=KACC 17904T) was identified as a novel species primarily through phylogenetic analyses of the 16S rRNA gene, heat shock protein 60 (hsp60) and dnaJ1 sequences, and in vitro DNA-DNA hybridization (DDH) compared to Bifidobacterium adolescentis. However, genome-based taxonomic approaches have not been studied yet. This study reassessed the taxonomic relationship between B. faecale and B. adolescentis using overall genome-associated indices. The type strains of the two species possessed heterogeneous copies of the 16S rRNA gene within their genomes, with similarities below the species threshold of 98.7-99.0%. Therefore, we were unable to assess the 16S rRNA gene similarity-based taxonomic relationships. This indicated that the 16S rRNA gene is not an appropriate marker for studying the taxonomy of B. adolescentis-associated taxa. The type strains exhibited an ortho average nucleotide identity uncorrected value of 97.7% and a digital DDH value of 80.7%, indicating that they should be assigned to a single taxon. A genome-based re-assessment indicated that B. faecale should be reclassified as a later heterotypic synonym of B. adolescentis.