Molecular Characterization of a Streptococcus gallolyticus Genomic Island Encoding a Pilus Involved in Endocarditis

Background. Streptococcus gallolyticus is a causative agent of infective endocarditis associated with colon cancer. Genome sequence of strain UCN34 revealed the existence of 3 pilus loci (pill, pil2, and pil3). Pili are long filamentous structures playing a key role as adhesive organelles in many pathogens. The pill locus encodes 2 LPXTG proteins (Gallo2178 and Gallo2179) and 1 sortase C(Gallo2177). Gallo2179 displaying a functional collagen-binding domain was referred to as the adhesin, whereas Gallo2178 was designated as the major pilin. Methods. S. gallolyticus UCN34, Pill⁺ and Pill ⁻, expressing various levels of pill, and recombinant Lactococcus lactis strains, constitutively expressing pill, were studied. Polyclonal antibodies raised against the putative pilin subunits Gallo2178 and Gallo2179 were used in immunoblotting and immunogold electron microscopy. The role of pill was tested in a rat model of endocarditis. Results. We showed that the pill locus (gallo2179-78-77) forms an operon differentially expressed among S. gallolyticus strains. Short pilus appendages were identified both on the surface of S. gallolyticus UCN34 and recombinant L. lactis-exp ressing pill. We demonstrated that Pill pilus is involved in binding to collagen, biofilm formation, and virulence in experimental endocarditis. Conclusions. This study identifies Pill as the first virulence factor characterized in S. gallolyticus.