Streptococcus pneumoniae Binding to Salivary Glycoproteins

Streptococcus pneumoniae colonizes mucous surfaces in the respiratory tract but genetically belongs to the Mitis group of streptococci, of which most other members colonize the oral cavity. S. pneumoniae TIGR4 carries a serine-rich repeat protein (SRRP), called PsrP, that resembles other SRRPs on oral Mitis group streptococci, including Hsa on S. gordonii DL1 and SrpA on S. sanguinis SK36. Both Hsa and SrpA are lectin-like adhesins that recognize terminal sialic acids on O-linked glycans and mediate streptococcal binding to surface-adsorbed salivary sialoglycoproteins lining the oral cavity.  Objective:  Because of these similarities to oral streptococci, it was investigated whether S. pneumoniae binds to salivary sialoglycoproteins. Methods:  S. pneumoniae TIGR4 (encapsulated), PsrP-deficient TIGR4 mutant STM199, capsule-deficient mutant HR1001.1, and PsrP-deficient HR1001.1 mutant, as well as S. gordonii DL1, S. sanguinis SK36, and respective SRRP-deficient mutants were labeled with fluorescein isothiocyanate (FITC). Salivary proteins were immobilized in form of dot blots, or transferred to nitrocellulose after separation by SDS-PAGE. Dot blots or transfers were overlaid with FITC-tagged bacteria and, after washing, protein dots or bands that served as counter-receptors for bacterial adhesion were detected by a fluorescence scanner. In addition, hemagglutination assays were performed to test for lectin-mediated binding activities on the bacteria.Results:  Only capsule-deficient S. pneumoniae HR1001.1 bound to salivary glycoproteins, but binding was not influenced by prior sialidase treatment of the proteins. The bacterial overlay revealed major bands at 55 kD and 150 kD as putative counter-receptors for S. pneumoniae binding. PsrP-deficiency did not decrease bacterial binding to salivary proteins, regardless of sialidase treatment, but diminished hemagglutination.  Conclusions:  Only capsule-deficient S. pneumoniae can bind to salivary proteins. At the present stage, neither an involvement of PsrP nor a dependency on the presence of sialic acid could be found for binding of S. pneumoniae to salivary glycoproteins.