Clinical and Microbiological Characteristics of Invasive Group A Streptococcus Infection: Four Case Series of Re-Emerging Pathogens.

INTRODUCTION: Group A Streptococcus (GAS), particularly the M1UK lineage, has re-emerged as a major global public health concern following the COVID-19 pandemic, with a rise in invasive GAS (iGAS) and streptococcal toxic shock syndrome (STSS). Although STSS is under national surveillance in Japan, comprehensive molecular monitoring of iGAS infections remains limited, and the clinical characteristics of M1UK-associated iGAS have not been fully elucidated. METHODS: We retrospectively reviewed four consecutive iGAS cases requiring intensive care between March and May 2024. Detailed clinical, microbiological, and genomic investigations were performed to characterize the causative strains and their associated virulence profiles. RESULTS: All patients required respiratory and/or circulatory support with surgical debridement. Three cases involved necrotizing fasciitis, and one involved intra-abdominal infection secondary to ovarian tumor rupture. All four patients received penicillin G and clindamycin as definitive antimicrobial therapy, with two developing severe drug-related adverse events. Genotypic analysis identified three isolates as emm1 strains, including two M1UK lineage strains. The two M1UK isolates commonly harbored multiple superantigen genes. All isolates remained susceptible to β-lactam, clindamycin, and macrolide antibiotics. CONCLUSION: This case series documents the identification of the M1UK lineage among critically ill patients with iGAS infections in Japan. Our findings support the need for continued molecular surveillance while reinforcing the importance of prompt surgical source control and appropriate antimicrobial therapy in the management of severe iGAS.