Clinical and molecular characteristics of Streptococcus agalactiae bacteremia in nonpregnant adults: a single-center analysis over 16 years in Hiroshima, Japan

PURPOSE: Molecular epidemiological data on Streptococcus agalactiae (Group B Streptococcus [GBS]) causing bacteremia in non-pregnant adults in Japan remain limited. This study elucidated the clinical characteristics and molecular epidemiology of GBS strains causing bacteremia in this population. METHODS: Data on non-pregnant adults with GBS treated at Hiroshima University Hospital between 2008 and 2023 were retrospectively reviewed. Clinical data were collected, and strains were characterized for antimicrobial susceptibility by susceptibility testing and whole-genome sequencing. RESULTS: A total of 73 adult patients with 74 episodes of GBS bacteremia were identified. The most common infection sites were skin and soft tissue (25.7%) and bacteremia with unknown focus (20.3%). Streptococcal toxic shock syndrome (STSS) was observed in 17 patients (23.0%), and the 30-day mortality rate was 25.7%. The predominant serotype/clonal complex (CC) lineages were Ib/CC12 (n = 14, 18.9%), V/CC1 (n = 12, 16.2%), and Ia/CC23 (n = 7, 9.5%). Notably, all seven Ia/CC23 strains and 10 (83.3%) V/CC1 strains were recovered after 2016. Single-nucleotide polymorphism-based analysis revealed the presence of strains within the ST17 and ST464 lineages circulating between pediatric and adult populations. CONCLUSIONS: This study demonstrates the emergence of the Ia/CC23 and V/CC1 lineages among invasive GBS strains causing bacteremia in non-pregnant adults in Japan over the past 10 years. Identifying the transmission pathways linking different age groups may help reduce the burden of invasive GBS infection in both pediatric and adult populations and develop effective public health and preventive strategies.