Gene cloning and characterization of Streptococcus intermedius fimbriae involved in saliva-mediated aggregation

Streptococcus intermedius, an oral commensal and a cause of systemic pyogenic disease, expresses fimbriae. To identify the gene(s) encoding these fimbriae, we used a serum raised against purified fimbriae to screen libaries of recombinant λ phages. The cloned gene cluster encoding S. intermedius fimbriae, (saliva-mediated aggregation and adherence-associated fimbriae), contained 4 ORFs, i.e. a putative ribonulease (Saf1), a putative adhesin (Saf2), the main pilus subunit (Saf3) and a sortase C (SrtC). Escherichia coli strains harboring recombinant phages and plasmids carrying the saf3 gene produced a 55 kDa protein recognized by the antifimbriae serum. Saf3 contains an N-terminal signal sequence and a C-terminal cell-wall-anchoring motif LPXTG. Among strains of the Streptococcus anginosus group, only serotype g and untypable strains were found to contain the saf3 gene, to possess the fimbrial antigen and to exhibit saliva-mediated aggregation. Knockout mutants made by insertion of an erythromycin resistance gene into saf3 did not produce fimbrial structures or fimbrial antigens and did not participate in saliva-mediated aggregation. The adherent activity of mutants toward plastic wells coated with salivary agglutinin was about 65% that of the parental strain, and the reaction depended on calcium. There was no significant difference in adherence to hydroxyapatite beads pretreated with salivary agglutinin between the parental and mutant strains. These results demonstrated that Saf3 is associated with aggregation, and suggested that other molecule(s) are associated with adherence of S. intermedius.