CRISPR interference in a Streptococcus agalactiae Multi-locus Sequence Type 17 Strain

Group B Streptococcus (GBS), a common colonizer of the human genital and gastrointestinal tracts, is a leading cause of neonatal bacterial meningitis, which can lead to severe neurological complications. The hypervirulent serotype III, sequence type 17 (ST-17) strain COH1 is strongly associated with late-onset disease due to its unique set of virulence factors. However, genetic manipulation of ST-17 strains is notoriously challenging, limiting the ability to study key pathogenic genes. In this study, we developed a CRISPR interference (CRISPRi) system utilizing an endogenous catalytically inactivated Cas9 (dCas9) in the COH1 strain, enabling targeted and tunable gene expression knockdown. We confirmed the efficacy of this system through hemolysis assays, qPCR transcriptional analysis, and in vitro infection models using human brain endothelial cells. The CRISPRi system successfully produced phenotypic knockdowns of essential virulence genes, including pilA, srr2 , and iagA , reducing adhesion, invasion, and inflammatory responses at the blood-brain barrier. This platform enables rapid gene knockdowns for functional genomics in ST-17 GBS, enabling high-throughput screening and pathogenesis research.