Emergence of a novel group B streptococcus CC61 clade associated with human infections in southern China

Emerging human pathogens of animal origin have become an increasing public health concern in recent years. The aim of this study was to investigate the transmission of group B streptococcus (GBS) clonal complex (CC) 61 strains in the southern Chinese population and analyze their genetic characteristics.

Whole-genome sequencing was performed on 693 clinical isolates of GBS collected from southern China between 2016 and 2021, and the prevalence of human CC61 isolates was investigated by genomic epidemiology. Phylogenetic analysis and Bayesian analysis of population structure were used to define genetic clades by combining CC61 genomes from global sources. Unique characteristics of human CC61 isolates were analyzed by comparison with the genomes of other isolates.

We identified 21 CC61 isolates from 19 patients (including four neonates), most of which belonged to sequence type (ST) 929 (n=17) and a few to ST931 (n=2) and ST61 (n=2). Phylogenetic analysis showed that the ST929 and ST931 isolates formed a novel clade associated with human infections (CC61H), which is a sister clade to the traditional bovine CC61 isolates. Population structure analysis indicated that CC61H has developed a unique population structure distinct from known lineages, representing an as-yet-unknown lineage. Comparative genomic analysis revealed that the epidemic success of CC61H in southern China was associated with the horizontal transfer of antibiotic resistance and virulence gene clusters. The insertion of a gene cluster encoding pilus island 1 may have contributed to the higher prevalence of ST929 relative to ST931. Furthermore, novel variants of the major pilin subunits BP-1 and BP-2b and the bacterial adhesin bibA were identified in CC61H, with bibA acquiring a pathogenic fragment of the homologous gene from the neonatal hypervirulent lineage CC17.

A novel clade of GBS CC61 associated with human infections was discovered in southern China. Given its multidrug resistance, high virulence and genomic characterization, the surveillance of CC61H strains should be more highly prioritized.