Identification of PilX, pilus component of Streptococcus sanguinis

Streptococcus sanguinis is an oral commensal bacterium that promotes dental biofilm formation and causes infective endocarditis. S. sanguinis strain SK36 produces pili comprising PilA, PilB, and PilC. This study determined whether the ssa1635 gene adjacent to the pilus-related gene locus encodes a pilus component and its roles in biofilm formation and eukaryotic cell adhesion.

Using a series of mutant strains and antisera against PilA, PilB, PilC, and SSA1635, immunoblot analyses and immunoprecipitation assays were performed for SSA1635 characterization. Both involvement of the deduced pilus-specific transpeptidase SrtC in pilus assembly and SSA1635 localization were examined by immunoblot analysis of various mutant strains. Furthermore, biofilm formation assays on saliva-coated surfaces and adhesion to HeLa cells were conducted to assess functions.

SSA1635, designated as PilX, formed complexes with PilA, PilB, and PilC. PilX was identified as a tip pilin incorporated into the pilus structure by SrtC. Notably, deletion of pilX impaired polymerization of other pilins. Furthermore, a pilX deletion mutant exhibited decreased biofilm formation compared with the wild-type and revertant strains and comparable rates of adherence to HeLa cells.

PilX is a potential pilin tip that may aid in facilitating the polymerization of other pilins. PilX contributes to biofilm formation, although it appears to be dispensable for adhesion to HeLa cells. Further characterization of PilX-binding specificities will provide valuable insights into the colonization mechanism of S. sanguinis.