Low but Increasing Prevalence of Reduced Beta-lactam Susceptibility Among Invasive Group B Streptococcal Isolates, US Population-Based Surveillance, 1998–2018

Background Invasive group B Streptococcus (iGBS) isolates with mutations in the pbp2x gene that encodes penicillin binding protein 2x can have reduced beta-lactam susceptibility (RBLS) when susceptible by Clinical and Laboratory Standards Institute (CLSI) criteria. We assessed the emergence and characteristics of RBLS strains in US iGBS isolates. Methods We analyzed iGBS isolates from 8 multistate population-based surveillance sites from 1998 to 2018. During 1998–2014, phenotypic antimicrobial susceptibility was determined by broth microdilution; criteria for 6 antibiotics were used to identify RBLS, followed by whole-genome sequencing (WGS). WGS for all isolates was added in 2015; we used phenotypic and genotypic results of >2000 isolates to validate phenotypic RBLS criteria and genotypic predictions. Since 2016, WGS has been used to screen for RBLS with broth microdilution confirmation of predicted RBLS isolates. Results Of 28 269 iGBS isolates, 28 (0.1%) were nonsusceptible by CLSI criteria; 137 (0.5%) met RBLS criteria. RBLS isolates were detected in all Active Bacterial Core surveillance sites. The RBLS proportion increased, especially since 2013 (odds ratio, 1.17; 95% CI, 1.03–1.32); the proportion that were nonsusceptible remained stable. Conclusions The RBSL proportion was low but increasing among US iGBS isolates. Ongoing monitoring is needed to detect emerging threats to prevention and treatment of GBS infections.