Comparative genomic analysis of hypervirulent group B streptococcus of ST12/serotype Ib suggests potential virulence...

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.

Sag37, an ST12/serotype Ib isolate, was recovered from a fatal neonatal case and exhibited features suggestive of hypervirulence. To define this lineage's prevalence and virulence, we analyzed 432 non-duplicate GBS isolates collected in Shanghai (2016 ~ 2022), and subtyped them by multilocus sequence typing (MLST) and capsular serotyping via a multiplex polymerase chain reaction. MLST revealed 50 sequence types, with ST10 and ST19 being most common. Nine clonal complexes were identified using the eBURST program, dominated by CC12 and CC19. Among seven serotypes identified, Ib was most prevalent, followed by III, V and Ia. Guided by the epidemiological data, additional clinical ST12/serotype Ib isolates, along with representative strains from other STs/serotypes, were selected for virulence assessment. For survival, 8-week-old CD1 mice were challenged subcutaneously with 1 × 10⁸ CFU and monitored for 7 days. ST12/serotype Ib isolates caused 100% mortality within 48 h, whereas other GBS subtypes resulted in delayed or no mortality. For the median lethal dose (LD₅₀), mice received intraperitoneal doses from 10³ to 10⁸ CFU; logistic regression estimated LD₅₀ values ranging from ≤ 10³ to ≥ 10⁸ CFU within ST12/serotype Ib, indicating marked intra-lineage heterogeneity. To further investigate the molecular mechanisms underlying the hypervirulence, whole-genome sequencing was conducted on 33 ST12/serotype Ib GBS strains identified herein. A pyseer-based genome-wide association study was performed using LD₅₀ as a quantitative trait. Multiple genetic variations potentially related to virulence were identified, providing insights into the molecular basis of hypervirulence of ST12/serotype Ib GBS.

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.

Not applicable.