Phylogenetically Distinct CC128-Related Group A Streptococcus dysgalactiae subsp. equisimilis Lineages Identified in Bacteremia Cases in Japan.

Streptococcus dysgalactiae subsp. equisimilis (SDSE) expressing the Lancefield group A antigen is an uncommon variant that is thought to have acquired the group A carbohydrate synthesis genes through horizontal gene transfer from Streptococcus pyogenes. Its prevalence and genomic characteristics remain incompletely understood. We retrospectively investigated 48 SDSE isolates recovered from blood cultures at a tertiary care hospital in Japan, between 2010 and 2023. Lancefield grouping was performed for all isolates, and whole-genome sequencing and phylogenetic analyses were performed for the two group A SDSE isolates. Among the 48 isolates, 44 (91.7%) were group G, two (4.2%) were group A, and two (4.2%) were group C. The two group A isolates, HUSD042 and HUSD047, belonged to clonal complex 128 (CC128); HUSD042 belonged to sequence type 177 (ST177), a single-locus variant of ST128, and HUSD047 belonged to ST128. Both isolates carried the emm subtype stG485.0. Comparative genomic analysis revealed a hybrid gac/gcc carbohydrate synthesis locus, in which the central region corresponding to the group C SDSE gccF-gccK region was replaced by an S. pyogenes-derived gacF-gacL region, consistent with a previously described invasive ST128-related group A SDSE lineage. Bayesian phylogenetic analysis showed that HUSD042 clustered within a clade predominantly composed of Japanese isolates, whereas HUSD047 clustered within a clade composed mainly of isolates from the United States. These findings demonstrate the presence of phylogenetically distinct CC128-related group A SDSE lineages in Japan and highlight the genomic diversity of this rare variant. Continued genomic surveillance with broader geographic sampling and detailed epidemiological information is warranted to clarify its evolution and geographic distribution.