Association of a novel high molecular weight, serine‐rich protein (SrpA) with fibril‐mediated adhesion of the oral biofilm bacterium Streptococcus cristatus

The surface of the oral plaque bacterium Streptococcus cristatus is decorated with a lateral tuft of fibrils. The fibrillar tuft functions in the adhesion of S. cristatus to heterologous bacterial species in the plaque biofilm. The tuft typically consists of a densely packed fringe of shorter fibrils 238 ± 19 nm long with longer, less abundant fibrils 403 ± 66 nm long projecting through the fringe of short fibrils. The two types of fibrils in the tufts of S. cristatus have been refractory to biochemical separation, complicating their characterization. A hexadecane partition assay was used to enrich for subpopulations of S. cristatus CR311 (type strain NCTC 12479) having distinct fibrillar morphotypes. Negative staining in the TEM revealed that cells of a hydrophobic subpopulation of S. cristatus (CR311var1) carried only the long fibrils (395 ± 32 nm). A hydrophilic subpopulation of S. cristatus (CR311var3) consisted of mixed morphotypes having no fibrils or remnant short fibrils (223 ± 49 nm). No long fibrils were observed on any cells in the CR311var3 subpopulation. The CR311var3 morphotype, unlike the wild‐type strain and CR311var1, was not able to form corncobs with either Corynebacterium matruchotii or Fusobacterium nucleatum . Variant CR311var3 did not express the novel gene srpA , which encodes a high molecular weight (321,882 Da) serine‐rich protein, SrpA. The SrpA protein contains two extensive repeat motifs of 17 and 71 amino acids and a gram‐positive cell wall anchor consensus sequence (LPNTG). The unusual properties of SrpA most closely resemble those of Fap1, the fimbrial‐associated adhesin protein of Streptococcus parasanguis. The association of long fibrils, high surface hydrophobicity, ability to form corncob formations, and expression of the srpA gene suggest that SrpA is a long fibril protein in S. cristatus .