Emergence of Group B streptococcus with reduced susceptibility to penicillin in mainland China

Objectives To characterize the first identified Group B Streptococcus (GBS) isolate with reduced susceptibility to penicillin in mainland China by analyzing its antimicrobial susceptibility profile and genetic determinants. Methods GBS identification was performed by latex agglutination, CAMP test and MALDI-TOF MS. Serotyping was carried out using a latex agglutination kit covering serotypes Ia-IX. Antimicrobial susceptibility testing was conducted using both microdilution (Sensititre™ ARIS™ 2X) and E-test strips. Whole-genome sequencing was performed to identify the sequence type (ST) and clonal complex (CC). The amino acid sequences of penicillin-binding proteins (PBP1A, PBP1B, PBP2A, PBP2B, PBP2X) were compared with those of the reference strain Streptococcus agalactiae 2603V/R (NC_004116). Results This GBS isolate was identified as serotype III and ST23 belonging to CC23. It exhibited a penicillin MIC of 0.25 mg/L, as determined by both automated and E-test methods. Three amino acid substitutions (I377V, V510I, Q557E) were determined in the PBP2X, in which Q557E was known as critical variation associated with educed susceptibility to penicillin. In PBP2B, an amino acid substitution (V80A), an insertion (ins382D), and a deletion (446delN) were identified. In PBP1B, only one amino acid substitution (A95D) was observed. Multiple amino acid substitutions were identified in PBP1A (M1V, A29T, G739S, V746A), accompanied by an insertion (ins182N) and a deletion (740_743delNGNN). No variations were detected in PBP2A. Conclusion This study reports the first penicillin-resistant GBS strain isolated in mainland China. Multiple PBPs variations were found except for the critical Q557E substitution in PBP2X. These findings highlight the importance of continued surveillance of penicillin susceptibility and resistance mechanisms in GBS .