Whole-genome characterization of neonatal group B Streptococcus infection isolates and contemporaneous maternal colonizing isolates in Southwest China

Background Regional genomic data on neonatal GBS infection in Southwest China remain limited, particularly for studies integrating neonatal infection isolates with contemporaneous maternal colonizing isolates. This study defined the antimicrobial susceptibility, molecular composition, resistance and virulence gene profiles, and phylogenetic context of neonatal GBS infection in this region. Methods We retrospectively analyzed 52 neonatal GBS infection isolates collected from March 2020 to March 2025 and compared them with 52 maternal vaginal colonizing isolates selected from the same period. Antimicrobial susceptibility testing, whole-genome sequencing, capsular serotyping, multilocus sequence typing, clonal complex assignment, gene screening, and core-genome phylogenetic analysis were performed. The 52 neonatal infection isolates were further contextualized with 1,060 publicly available blood- or cerebrospinal fluid-derived genomes from PubMLST. Results All neonatal infection isolates were susceptible to penicillin and ampicillin, whereas resistance to erythromycin, clindamycin, tetracycline, and moxifloxacin was common. Serotype III was the most frequent serotype, followed by Ib. Compared with maternal colonizing isolates, neonatal infection isolates showed significant enrichment of ST17/CC17 and the III-2/CC17 lineage. The predominant neonatal infection-associated lineages were Ib/CC12, III-2/CC17, and III-1/CC19. Resistance and virulence gene carriage was strongly structured by clonal background, with several aminoglycoside resistance genes and fbsB concentrated in CC17. Global phylogenetic analysis showed that local neonatal infection isolates were embedded within established GBS lineages. Conclusion Neonatal GBS infection in Southwest China was associated with selected phylogenetic backgrounds rather than directly mirroring the maternal colonizing population. These findings support integrated surveillance of antimicrobial susceptibility, molecular lineages, virulence markers, and vaccine-relevant serotypes.