Sialic acid is involved in the differential binding of streptococcal species to milk and salivary glycoproteins.

The flow of human saliva constantly bathes the mouth and is thought to provide several mechanisms of innate immune protection against the huge number of micro-organisms to which the oral cavity is exposed; a similar mechanism is postulated for the oligosaccharides in milk as a protection against infant infection. Streptococcus gordonii is an early coloniser of human tooth enamel, whereas S. mutans colonises at a later stage in the development of the biofilm involved in the formation of dental caries. The glycoprotein component of saliva and milk is known to have antimicrobial properties and the attached sugar residues have been implicated in the interactions of salivary glycoproteins with bacteria. The binding of oral streptococci to salivary and milk glycoproteins and the involvement of terminal sialic acid residues in this pathogen-sugar interaction was investigated.

Binding of fluorescently-labelled Streptococcus gordonii F552 and S. mutans LT11 to saliva-coated hydroxyapatite crystals visually showed a stronger S. gordonii adherence to salivary glycoproteins. This difference in interaction of the two streptococci was confirmed and extended by quantitative measurement of the binding of these bacteria to both salivary and milk glycoproteins. Sialic acid residues on these proteins were demonstrated to be integral to streptococcal adherence to both human secreted fluids by i) altered binding of the two strains with pre-incubation of the bacteria with sialic acid, ii) washing of the bound bacteria with sialic acid and iii) enzymatic removal of sialic acid before bacterial binding. The two streptococcal strains demonstrated differential responses to the changes in glycoprotein sialylation in both saliva and milk. A rapid, facile assay of bacterial adherence to glycoproteins in the secreted fluids of saliva and milk allowed the contribution of terminal sialylation to be quantitatively determined. Our findings indicate that human saliva and milk bind similarly to common oral pathogens. S. gordonii binding to both milk and salivary glycoproteins was shown to rely on the presence of sialic acid residues on the glycoproteins, whereas S. mutans did not appear to use sialic acid epitopes for binding and bound more strongly with a decrease in sialylation.

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