Binding between Human Salivary Components and Staphylococcus aureus

Objective : Studies of hospitalized pneumonia patients have suggested that the oral cavity serves as a reservoir for respiratory pathogens such as Staphylococcus aureus. Since saliva is known to modulate bacterial colonization, we attempted to identify the salivary components that interact with S. aureus.

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.