One of the key initial processes involved in bacterial infection is the attachment to host cell receptors, typically glycans (sugars) conjugated to proteins or lipids on epithelial cell surfaces. Accordingly, many pathogens have developed adhesins or lectins that attach to specific glycan epitopes characteristic of the host cells they target. In turn, we propose that the host provides an ingenious innate defence mechanism that uses the glycoproteins in their secreted fluids to competitively bind to the bacterial pathogens to prevent infection.

Our research has demonstrated that bacteria bind differentially to glycan moieties on secreted glycoproteins from human tears, milk, saliva and sweat, potentially providing decoys for selective adhesion and clearance of pathogens as the secretory fluids wash the epithelial cell surface. Here we demonstrate the differential involvement of sialic acid in the adhesion of pathogenic and commensal bacteria to secreted glycoproteins in these human secretions. Our techniques include a 96 well plate bacterial adhesion assay in which the glycoproteins bind to the PVDF membrane in the wells through hydrophobic interactions, leaving the glycan molecules exposed on the membrane surface. The presented glycans are then probed with fluorescently-labelled bacteria and bacterial binding quantified using a fluorescent plate reader. The importance of terminal sialic acid in the bacterial–secreted glycoprotein interaction was determined by detecting any change in the bacterial binding following neuraminidase treatment.

The work demonstrates a natural mechanism by which sialic acid contributes to the attachment of pathogenic bacteria to the glycoproteins secreted in a diverse range of body fluids and suggests there is a glycan-mediated protective process against infection in all human secretions.