Reclassification of Streptococcus ilei as a later heterotypic synonym of Streptococcus koreensis based on whole-genome sequence analysis

The genus Streptococcus, a member of family Streptococcaceae, is known for its wide range of industrial, clinical and human relevance. Among the species of genus Streptococcus two members, namely Streptococcus koreensis and Streptococcus ilei, were isolated from subgingival dental plaque and human small intestinal fluid, respectively. The 16S rRNA gene sequence similarity of the type strains of these members shows a similarity of 99.87%. In this study, we performed a systematic study to clarify the taxonomic assignment of these two species. Genome similarity assessment based on whole-genome sequence information such as average nucleotide identity using orthoANI and fastANI, digital DNA-DNA hybridization value between S. koreensis and S. ilei were 96.31, 96.60, 86.4 and 97.63, respectively. All these genome similarity values clearly exceeded the species delineation cutoffs. Phylogenetic assessment using 16S rRNA gene and whole-genome information using PhyloPhlAn, which uses around 400 conserved genes across bacterial phyla, provides additional evidence for these members forming a monophyletic clade in the phylogenetic tree. Pan genome analysis suggests a very large core genome (n = 1374) and the presence of no unique gene between the genomes of S. koreensis and S. ilei. Additionally, we found highly syntenic genomes of type strains of these two species. Based on these evidences, we propose S. ilei should be reclassified as a later heterotypic synonym of S. koreensis.