[Molecular characteristics and antimicrobial susceptibility of 31 clinical invasive isolates of Streptococcus dysgalactiae subspecies equisimilis in 5 provinces of China].

Objective: To analyze the molecular characteristics and antimicrobial susceptibility of clinically invasive Streptococcus dysgalactiae subsp. equisimilis (SDSE) strains isolated in China. Methods: A total of 31 non-duplicate SDSE strains isolated from blood, cerebrospinal fluid, and other sterile body fluids were collected from five provinces in China between 2019 and 2024. Antimicrobial susceptibility testing was performed using the agar dilution method to determine the minimum inhibitory concentrations of six antimicrobial agents: penicillin, cefotaxime, levofloxacin, clindamycin, erythromycin, and tetracycline. Whole-genome sequencing and bioinformatics analysis were conducted for species confirmation, multilocus sequence typing (MLST), emm typing, and analysis of virulence genes and antimicrobial resistance genes. Results: All strains were 100.0% susceptible to penicillin, cefotaxime, and levofloxacin, while the resistance rates to clindamycin, erythromycin, and tetracycline were 74.19%, 87.10%, and 61.29%, respectively. MLST identified 12 sequence types (ST), belonging to 6 clonal complexes (CC), with CC127 (41.9%) and CC269 (32.3%) being the predominant clones. The predominant emm types were stG245.0 (45.2%) and stG840.0 (35.5%), which were strongly associated with specific sequence types or clonal complexes (ST127-stG245.0, CC269-stG840.0). The primary antimicrobial resistance genes were tetM (71.0%) and ermB (61.9%), and distinct resistance gene profiles were observed across different clonal complexes. The key streptococcal virulence factor, the superantigen gene speG, was not carried by CC127 strains but was widely present in other clonal groups. Conclusions: Invasive SDSE strains in China remain highly susceptible to β-lactam antibiotics but exhibit severe resistance to clindamycin, erythromycin, and tetracycline. CC127 and CC269 are the major predominant clones, and the carrying of virulence factors varies among different clones.