PROJECT SUMMARY Sialic acids (Sias), the outermost termini of glycan chains decorating glycoproteins and glycolipids, comprise a diverse family of nine-carbon sugar acids, where structural variants are defined by substitutions at different carbon positions. Sias are involved in cell-cell recognition processes and modulate a wide variety of physiological and pathological processes, including recognition by viruses and other pathogenic microorganisms. On salivary glycoproteins, terminal Sias constitute important cognate glycan motifs recognized by Sia-binding adhesins of oral commensal streptococci. This recognition enables initial adhesion and colonization of saliva-coated oral surfaces by streptococci. However, if these normally harmless commensal Sia-binding streptococci happen to transgress the tissue barrier at the gingival margin and evade neutrophil defense, they can become dispersed within the bloodstream and act as agents of systemic diseases, including infectious endocarditis. Because Sias play a role in all of these events, the finer specificities of streptococcal binding to the various subtypes of Sias become an important question to explore. Our group has recently performed a comprehensive screening of clinical isolates of dental plaque streptococcal strains for their ability to bind to different Sia subtypes. Through analyzing their finer Sia-subtype binding specificities using a novel sialoglycan array, we found streptococci in the oral cavity of human individuals that bind to a non-human sialic acid, N-glycolylneuraminic acid (Neu5Gc). This is significant because humans, unlike most other mammals, are genetically unable to synthesize Neu5Gc. In this project, we aim to determine the structural basis for differential recognition of Neu5Gc and Neu5Ac by these Sia-binding streptococci, and to identify their natural receptors in the human oral cavity. We will investigate the molecular basis of Sia-mediated streptococcal binding to salivary sialoglycoproteins. We will identify other bacterial species in oral biofilms that express Sias on their surface and are targets for interbacterial adhesion by Neu5Gc-binding streptococci. Lastly, we will determine the molecular basis of Sia-mediated binding of streptococci to phagocytes and investigate the functional consequences of streptococcal binding to different Sia-subtypes on bacterial uptake and phagocyte activation. Overall, these studies are expected to break new ground by demonstrating how expression of Sia-subtypes influences bacteria-host interactions in the human oral cavity. The knowledge gained will likely have a positive impact on early diagnosis and prevention of dental, oral, and systemic diseases caused by oral microbes.