Clinical and Molecular Characteristics of Streptococcus dysgalactiae subsp. equisimilis Bacteremia in Japan

Background Streptococcus dysgalactiae subsp. equisimilis (SDSE) is an emerging cause of invasive infection, particularly in aging populations. We investigated the clinical and genomic characteristics of SDSE bacteremia at a Japanese medical center serving a large older-adult population. Method We retrospectively analyzed 73 consecutive episodes of SDSE bacteremia, including 63 initial and 10 recurrent episodes, from April 2020 through September 2024. Clinical features were compared with those of Streptococcus pyogenes (GAS) and Streptococcus agalactiae (GBS) bacteremia. Whole-genome sequencing was performed to assess strain relatedness, emm types, sequence types (STs), and virulence gene profiles. Results The median age of patients with initial SDSE bacteremia was 86.0 years, the highest among 10 major bloodstream pathogens evaluated, predominantly affecting super-elderly patients compared with GAS and GBS. Lower-extremity skin and soft tissue infections were the most common portal of entry. Among 10 recurrent episodes, 5 were caused by strains with STs and emm types identical to those of the initial isolates, consistent with probable relapse; the remaining episodes involved different genotypes, suggesting reinfection. The all-cause 30-day mortality rate was 4.8%. Phylogenetic analysis demonstrated genetic diversity, including 15 emm types and 14 STs. No specific lineage or virulence gene was significantly associated with clinical outcomes. Conclusions In our cohort, SDSE bacteremia predominantly affected super-elderly adults and frequently arose from lower-extremity portals of entry. Recurrent episodes involved both probable relapse and potential reinfection. These findings suggest that localized interventions, such as skin care and edema management, may be important adjuncts to antimicrobial therapy for preventing recurrence.