Genome-Based Reclassification of <i>Streptococcus taoyuanensis</i> ST2<sup>T</sup> as a Later Heterotypic Synonym of <i>Streptococcus caecimuris</i> CLA-AV-18<sup>T</sup>

This study systematically evaluated the taxonomic relationship between <i>Streptococcus taoyuanensis</i> ST2^T and <i>Streptococcus caecimuris</i> CLAAV18^T. Comparative genomic analysis revealed a high 16S rRNA gene sequence similarity of 99.6%, with the two strains clustering closely in the 16S rRNA-based phylogenetic tree. The genetic relatedness was further validated by Multi-Locus Sequence Typing (MLST) analysis: assessments of seven conserved housekeeping genes (<i>atpD</i>, <i>gapA</i>, <i>gyrB</i>, <i>GdhA</i>, <i>recA</i>, <i>dnaK</i>, and <i>sdhA</i>) demonstrated complete concordance in target fragment lengths (ranging from 33 bp to 121 bp). No size polymorphisms, insertions, or deletions were detected, indicating a highly conserved core genome. At the whole-genome level, the Average Amino Acid Identity (AAI), Average Nucleotide Identity (ANI), and digital DNA-DNA hybridization (dDDH) values between the two strains were 96.8%, 95.7%, and 84.6%, respectively. These values significantly exceed the established thresholds for species delineation (AAI: 95.5%; ANI: 95%; dDDH: 70%), providing robust genomic evidence that both strains belong to the same species. Furthermore, phenotypic testing confirmed nearly identical physiological characteristics, with only minor biochemical variations. Based on the integration of phylogenetic, genomic, and phenotypic evidence, we formally propose <i>Streptococcus taoyuanensis</i> as a later heterotypic synonym of <i>Streptococcus caecimuris</i>.