Fellowship for Christian Ahearn: Sialic acid mediated interbacterial adhesion among oral bacteria

PROJECT SUMMARY/ABSTRACT Glycan-mediated interbacterial coadhesion is a critical part of multispecies biofilm formation in the human mouth. Certain strains of oral streptococci express serine-rich repeat protein (SRRP) adhesins that bind to sialic acids, common terminal sugars expressed on human oral mucosal epithelia and glycosylated salivary proteins. Sialic acids (Sias) are a group of nine-carbon sugars of which over fifty different subtypes exist. Streptococcal SRRPs are genetically diverse among strains, which likely evolved to bind to the multitude of Sia subtypes. To determine the full scope of diverse Sia-binding SRRP sequences encoded by streptococci, the applicant used hidden Markov models designed to identify Sia-binding SRRPs among over six thousand publicly available streptococcal genome sequences. The applicant identified hundreds of Sia-binding SRRP sequences, which likely reflect binding specificities for different Sia subtypes. For example, several strains of oral streptococci preferentially bind to either of the two Sia subtypes, N-acetylneuraminic acid (Neu5Ac) or N-glycolylneuraminic acid (Neu5Gc). Certain strains of bacteria evolved to either de novo synthesize or scavenge Sias from the environment and display them on their surface. Since Sia subtypes are diverse and since the applicant’s data show that streptococcal Sia-binding SRRP sequences are equally diverse, we believe that specificity of Sia-subtype- mediated streptococcal binding expands beyond the mere difference between Neu5Ac and Neu5Gc. Together, these data served as the basis for the applicant to formulate his independent hypothesis that oral streptococci bind to an assortment of Sia subtypes expressed by other oral bacteria. In this project, the applicant aims to identify Sia-expressing oral bacteria that serve as coadhesion partners of Sia-binding oral streptococci. In Aim 1, Sia-expressing oral bacteria will be identified by both an in-silico approach and isolated from human oral biofilms, then tested for coadhesion with Sia-binding oral streptococci. In Aim 2, bacterial strains known to express Sias as well as those newly identified strains in Aim 1 will be used to determine how different Sia subtypes expressed by bacteria affect coadhesion. The results of this proposed investigation will demonstrate how Sia subtypes influence glycan-mediated interbacterial coadhesion. Investigating the precise mechanism of Sia subtype-mediated interbacterial binding will help to understand how members of the oral microbiome interact to form potentially disease-associated microbial biofilms in the human oral cavity. The planned project will build on Dr. Ahearn’s skills acquired during his PhD dissertation research in bioinformatic and experimental analysis of bacterial surface-exposed components. In this project, he will expand his expertise into the emerging field of microbial glycobiology. Data derived from this project will allow Dr. Ahearn to advance to the next stage of his career by investigating the significance of Sia- and other glycan-mediated coadhesion during multispecies oral biofilm formation and its impact on both human oral health and systemic disease.