ypmS promotes capsular polysaccharide shedding and in vivo fitness of Group B Streptococcus.

Group B Streptococcus (GBS) is a leading cause of neonatal sepsis and meningitis worldwide. Capsular polysaccharide (CPS) is a major virulence factor that aids GBS in colonizing the gastrointestinal (GI) and vaginal tracts, surviving in whole blood and evading the immune system. CPS is also the basis for a GBS CPS-protein conjugate vaccine candidate. In addition to its function on the surface of the bacteria, CPS can also be released and shed into the environment. In Streptococcus pneumoniae, shed CPS has been shown to absorb antimicrobial peptides and CPS-specific antibodies, allowing the bacteria to evade the innate immune system and potentially limiting vaccine protection. In this work, we show that CPS shedding occurs in ~75% of GBS isolates that we screened from a large library of clinical strains. We then identified ypmS as a genetic determinant of CPS shedding using an indexed transposon library. We used a clean in-frame deletion mutant of ypmS to assess the importance of shedding in GBS colonization and immune evasion. We explored the role of ypmS in colonization using murine models of GI and vaginal co-colonization and showed that the wild-type strain outcompeted the ΔypmS mutant in both scenarios. We also assessed the effect of shed capsule on opsonophagocytosis. Free CPS was protective against bacterial killing in an opsonophagocytic killing assay using CPS-specific antibodies. With this work, we have identified a novel genetic cause of CPS shedding and shown its contribution to GBS colonization fitness and immune evasion.