Molecular characterization of Group B strep in South Africa
Background: Group B Streptococcus (GBS) is a leading cause of neonatal meningitis and sepsis and an important cause of disease in adults. Capsular polysaccharide and protein-based GBS vaccines are currently under development.  Methods: Through national laboratory-based surveillance, invasive GBS isolates were collected from patients of all ages between 2019 and 2020. Phenotypic serotyping and antimicrobial susceptibility testing were conducted, followed by whole-genome sequencing for analysis of population structure and surface protein and resistance genes. Results: 1748 invasive GBS cases were reported. Of these, 661 isolates underwent characterization, with 658 yielding both phenotypic and genotypic results. Isolates (n=658) belonged to five clonal complexes (CC1, CC8/10, CC17, CC19, and CC23) and six serotypes were detected: III (42.8%), Ia (27.9%), V (11.9%), II (8.4%), Ib (6.7%), and IV (2.3%). Phenotypically, only one isolate exhibited reduced penicillin susceptibility (MIC 0.25ug/ml). Phenotypic resistance to erythromycin, clindamycin, and tetracycline was observed in 16.1%, 3.8%, and 91.5% of isolates, respectively. ermTR (34.9%) and mefA/E (29.2%) genes were most common among erythromycin-resistant isolates, while ermB predominated clindamycin-resistant isolates (32.0%). tetM accounted for 95.5% of tetracycline resistance. All isolates carried at least one of the three pilus gene clusters, one of the four homologous alpha/Rib family determinants, and 98% harbored one of the serine-rich repeat protein genes.  hvgA was found exclusively in  CC17 isolates. Conclusion: In our setting, ß-lactam antibiotics remain appropriate for GBS treatment and polysaccharide and protein-based vaccines under development are expected to provide good coverage.