β-Lactam Susceptibility of Streptococcus dysgalactiae subsp. equisimilis

All clinical isolates of Streptococcus dysgalactiae subsp. equisimilis (SDSE) are considered susceptible to β-lactams, the first-line drugs used for SDSE infections. However, penicillin-non-susceptible SDSE has been reported from Denmark. In this study, we attempted to detect β-lactam-non-susceptible clinical isolates of SDSE in Japan. One hundred and fifty clinical isolates of S. dysgalactiae were collected in 2018, and species identification was performed using Rapid ID Strep API. The minimum inhibitory concentrations (MIC) of six β-lactams (penicillin G, oxacillin, ceftizoxime, ceftibuten, cefoxitin, and cefaclor) were determined for 85 clinical isolates of SDSE using the agar dilution method standardized by the Clinical Laboratory Standards Institute. For the 85 isolates identified as SDSE, the MIC ranges of penicillin G, oxacillin, ceftizoxime, ceftibuten, cefoxitin, and cefaclor were 0.007–0.06, 0.03–0.12, 0.015–0.06, 0.25–2, 0.12–2, and 0.06–0.5 μg/mL, respectively. None of the clinical isolates were non-susceptible to penicillin G, indicating that all 85 clinical isolates of SDSE were susceptible to β-lactams. Our findings indicate that almost all clinical isolates of SDSE in several prefectures of Japan remain susceptible to β-lactams. Nevertheless, there remains a need for continuous and careful monitoring of drug susceptibility among clinical isolates of SDSE in Japan.