PbsP, a major virulence factor of Streptococcus agalactiae, is regulated by the SaeRS two-component system

Two-component regulatory systems (TCSs) are signal transduction machineries commonly used by prokaryotes to sense and respond to environmental conditions by modifying gene expression. Pathogenic and opportunistic bacteria exploit TCSs to reprogram gene expression during different steps of host infection. In this study, we investigated the role of a two-components regulatory system in Streptococcus agalactiae, a leading cause of severe neonatal infections such as sepsis and meningitis, designated SaeRS which was recently discovered to be activated upon contact with vaginal fluids, suggesting its role in host colonization. In this context, SaeRS once activated, is able to up-regulate two important adhesins called PbsP and BvaP, enhancing vaginal mucosal colonization. PbsP (Plasminogen binding surface Protein) is a cell wall-anchored protein, required for binding of surface-associated Plasminogen and Vitronectin. This protein is also a virulence factor involved in bacterial dissemination from the blood to the brain, and a valuable vaccine candidate since immunization with PbsP is protective against infection in murine models. Moreover, pbsP gene is present and highly conserved in all sequenced human GBS strains. Here we employed SaeRS loss of function S. agalactiae strains to investigate its role in vivo, using different murine models of infection. The SaeR-impaired strains showed decreased ability to lead lethality and to disseminate in blood and distant organs. We performed a transcriptomic analysis using a genetic approach to activate signaling pathways of SaeRS. Transcriptome of this constitutionally activated mutant revealed a marked up-regulation of four genes, comprising pbsP, bvaP, saeR, and saeS, which represent the SaeR regulon. Moreover, the genes encoding for the two adhesins PbsP and BvaP was showed to be up-regulated in vivo during peritoneal infection. The strong up-regulation of PbsP in constitutionally activated mutant was found to be linked to an hyper-adhesive and hyper-invasive phenotype, using in vitro cellular assays. Also the in vitro ability to across the microvascular endothelial cells was enhanced in a PbsP-dependent way. These data suggest that SaeRS system is an important regulator of virulence, which is activated during infection and drive host-interaction changes trough the expression of PbsP. Due to its important role, this pathway could be the target of new therapeutic strategies.