Methods : Bacteria were incubated with unlabeled or fluorescence (Cy3)-labeled saliva at 37°C. After washing the bacteria, bound salivary components were eluted with SDS-containing buffer and resolved by gel electrophoresis. Bound salivary proteins were located by staining with Coomassie blue/periodic acid-Schiff or by fluorescence scanning. Protein bands were excised and analyzed by mass spectrometry. In parallel, limited on-bacteria trypsin digestion was performed. Proteins were identified by matching peptides to the Swiss-Prot database using the MASCOT algorithm and further confirmed by western blotting.
Results : Salivary components bound to S. aureus included salivary mucin-7, salivary agglutinin (gp340), secretory component, carbonic anhydrase VI, immunoglobulins, S100-A9, lysozyme, and parotid secretory protein (SPLUNC2). Binding reached maximum levels at 10 min and did not change when incubation times were extended to 2 h. Binding was not solely due to non-specific physico-chemical interactions, because it was not affected by NaCl or urea. Significantly, components bound to S. aureus were different from those bound to Streptococcus gordonii, in that salivary α-amylase bound strongly to S. gordonii, but did not bind to S. aureus. Biofilm-grown cells, known to express less S. aureus surface protein A (SpA), bound fewer salivary components, including immunoglobulins, which suggested that SpA is involved in binding. This was supported by S. epidermidis, known not to express SpA, and confirmed using a SpA-deficient S. aureus strain and SpA-coated sepharose beads.
Conclusion: A limited number of salivary proteins, some of which participate in innate immunity, selectively bind to S. aureus. These proteins may facilitate the clearance of S. aureus from the oral cavity.
Keywords: Adherence and colonization, Biofilm, Host-microbial interactions, Proteins and Saliva