Comparative Genomic Analysis of Hypervirulent Group B streptococcus of ST12/Serotype Ib Suggests Potential Virulence Factors

Background Group B streptococcus is a leading cause of neonatal invasive diseases. ST12/serotype Ib GBS isolate, the second most prevalent lineage in East Asia, has aroused increasing attention, as a growing body of reports suggests they may be more virulent and fatal than the ST17/serotype III strain. Results Sag37, an ST12/serotype Ib strain, was initially identified from a fatal neonatal case and exhibited features suggestive of hypervirulence. To better understand the prevalence and virulence of this lineage, 432 non-duplicate GBS isolates collected in Shanghai between 2016 and 2022 were analyzed. Multilocus sequence typing revealed 50 STs, with ST10 and ST19 being most common. Nine clonal complexes were identified, dominated by CC12 and CC19. Among seven serotypes identified, Ib was most prevalent, followed by III, V and Ia. Based on the epidemiological data, additional clinical ST12/serotype Ib isolates, along with representative strains from other STs and serotypes, were selected for virulence assessment. In a mouse infection model, ST12/serotype Ib isolates caused 100% mortality within 48 hours, whereas other GBS subtypes resulted in delayed or no mortality. LD ₅₀ assessment further revealed intra-lineage virulence diversity in ST12/serotype Ib strains, with some causing 50% mortality at 10³ CFU, while others were non-lethal even at 10⁸ CFU. To further investigate the molecular mechanisms underlying the hypervirulence, whole-genome sequencing and genome-wide association study was conducted on 33 ST12/serotype Ib GBS strains identified herein. Multiple genetic variations potentially related to virulence were identified, providing insights into the molecular basis of hypervirulence of ST12/serotype Ib GBS. Conclusions This study underscored the correlation between ST12/serotype Ib GBS isolates and hypervirulence, and further and revealed multiple candidate loci potentially associated with this phenotype. These findings highlighted the need for ongoing surveillance and functional investigation of emerging GBS lineages. Clinical trial number: Not applicable.