Streptococcal recognition of sialic acid diversity in the salivary proteome
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Saliva facilitates in the preprocessing of food, and contributes to the protection of teeth and oral mucosa. Saliva also helps to maintain an ecological balance in the diverse oral microbiota through the clearance of undesirable pathogens, and fostering colonization of oral cavity by a physiological commensal microflora. Among the earliest residents that are frequently found colonizing the human teeth are members of the oral Mitis group of streptococci. These oral streptococci can bind to adsorbed salivary sialoglycoproteins of the acquired salivary pellicle through the recognition of host sialic acids by sialic acid-binding serine-rich repeat (SRR) bacterial adhesins. Among the best characterized are streptococcal adhesins Hsa and GspB of Streptococcus gordonii and SrpA of S. sanguinis , which are a group of commensal species that normally co-exist in the oral cavity. Besides S. gordonii and S. sanguinis , other closely related members of the Mitis group of streptococci also express homologous SRR adhesins. However, it still remains unclear if genetically related SRR adhesins such as Fap1 of S. parasanguinis FW213 and PsrP of S. pneumoniae TIGR4 can bind to sialic acids as well. The objective of our first study ( Chapter 2 ) was to determine if Fap1 or PsrP also bind to sialic acids on salivary sialoglycoproteins in a similar way as Hsa and SrpA. Pairs of wild-type and SRR adhesin-deficient mutant strains were tested for their binding to immobilized salivary secretions, as well as for sialic acid-dependent hemagglutination. Results obtained provided evidence that, though structurally and genetically similar, Fap1 and PsrP are different from Hsa and SrpA, in that the former two adhesins do not bind to sialic acids. Throughout the animal kingdom, the two most common forms of sialic acid in nature are N -acetylneuraminic acid (Neu5Ac) and N -glycolylneuraminic acid (Neu5Gc), and they only differ by a single oxygen atom. We aimed to ask in our second study ( Chapter 3 ), if the single oxygen atom difference between Neu5Ac and Neu5Gc is relevant to the binding specificities of oral streptococci that express sialic acid-binding SRR adhesins. Pairs of wild-type and SRR adhesin-deficient mutant strains were tested for their binding to sialoglycan microarrays containing various pairs of Neu5Ac- and Neu5Gc-carrying glycan structures, as well as for sialic acid-dependent agglutination of human (Neu5Ac-rich) and chimpanzee (Neu5Gc-rich) red blood cells. We found that sialic acid-binding SRR adhesins Hsa, GspB and SrpA exhibit different binding specificities to sialic acid subtypes, Neu5Ac and non-human mammalian Neu5Gc. It is one of the project's long-term goals to find out if the single oxygen atom difference in Neu5Ac and Neu5Gc can influence the colonization of the mouth by different types of sialic acid-binding oral streptococci. In this regard, chimpanzee and gorilla individuals are the most suitable animal models for this study, since they are the closest extant living relatives to modern humans, and share 95% genomic similarity with humans. Moreover, unlike the great apes, humans do not synthesize the sialic acid subtype Neu5Gc due to a gene mutation that occurred within the hominin lineage ∼2-3 million years ago. As a result, this mutation predisposes modern humans with an excess of Neu5Ac expression, as compared to the great apes, who demonstrate a mix of both Neu5Ac and Neu5Gc expression. To understand the role of sialic acid-binding in oral bacterial colonization, a first step is to find out more about the composition of saliva from the great apes, and to find out if their saliva is similar to that of humans. Hence, our objective for our final study ( Chapter 4 ) was to achieve a global comparison of the salivary proteomes of humans, chimpanzees and gorillas. Because of the high genomic similarity shared among humans and the great apes, it is therefore hypothesized that the salivary proteomes of humans and the great apes would be highly similar. Saliva obtained from humans, chimpanzees and gorillas were analyzed by various biochemical and proteomic approaches. Our findings showed that the composition of salivary proteins among humans, chimpanzees and gorillas is highly similar, with few distinct differences in expression levels and electrophoretic mobility of selected proteins.