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

<i>Bifidobacterium faecale</i> CU 3-7^T (=JCM 19861^T=KACC 17904^T) was identified as a novel species primarily through phylogenetic analyses of the 16S rRNA gene, heat shock protein 60 (<i>hsp60</i>) and <i>dnaJ1</i> sequences, and <i>in vitro</i> DNA-DNA hybridization (DDH) compared to <i>Bifidobacterium adolescentis</i>. However, genome-based taxonomic approaches have not been studied yet. This study reassessed the taxonomic relationship between <i>B. faecale</i> and <i>B. adolescentis</i> 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 <i>B. adolescentis</i>-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 <i>B. faecale</i> should be reclassified as a later heterotypic synonym of <i>B. adolescentis</i>.