Streptococcus oralis Neuraminidase Modulates Adherence to Multiple Carbohydrates on Platelets

Abstract

Adherence to host surfaces is often mediated by bacterial binding to surface carbohydrates. Although it is widely appreciated that some bacterial species express glycosidases, previous studies have not considered whether bacteria bind to multiple carbohydrates within host glycans as they are modified by bacterial glycosidases. Streptococcus oralis is a leading cause of subacute infective endocarditis. Binding to platelets is a critical step in disease; however, the mechanisms utilized by S. oralis remain largely undefined. Studies revealed that S. oralis, like Streptococcus gordonii and Streptococcus sanguinis, binds platelets via terminal sialic acid. However, unlike those organisms, S. oralis produces a neuraminidase, NanA, which cleaves terminal sialic acid. Further studies revealed that following NanA-dependent removal of terminal sialic acid, S. oralis bound exposed ␤-1,4-linked galactose. Adherence to both these carbohydrates required Fap1, the S. oralis member of the serine-rich repeat protein (SRRP) family of adhesins. Mutation of a conserved residue required for sialic acid binding by other SRRPs significantly reduced platelet binding, supporting the hypothesis that Fap1 binds this carbohydrate. The mechanism by which Fap1 contributes to ␤-1,4-linked galactose binding remains to be defined; however, binding may occur via additional domains of unknown function within the nonrepeat region, one of which shares some similarity with a carbohydrate binding module. This study is the first demonstration that an SRRP is required to bind ␤-1,4-linked galactose and the first time that one of these adhesins has been shown to be required for binding of multiple glycan receptors.

Biology of Oral Streptococci

“…Among them, GspB and Hsa are serine-rich repeat glycoproteins (Srr) that bind to sialylated glycans present on the soluble human salivary mucin MG2/MUC7, a component of the mucosal pellicle (64), and the platelet glycoprotein GPIb (65, 66). Srr proteins are important for biofilm formation and have been identified in many other oral streptococci, including Fap1 of S. parasanguinis (67) and S.oralis (68), SrpA of S. sanguinis (69) and S. cristatus (70), and SrpA, B and C of S. salivarius (71). Interestingly, Srr homologs are not present in the caries pathogen S. mutans .…”

Streptococcus oralis subsp. dentisani Produces Monolateral Serine-Rich Repeat Protein Fibrils, One of Which Contributes to Saliva Binding via Sialic Acid

“…Given that other SRRPs are associated with fibrils on the bacterial surface, we named these open reading frames fapA (13,992 bp), fapB (13,692 bp), and fapC (15,144 bp), where fap represents fibril-associated protein. Each of these open reading frames is predicted to encode the previously described characteristics of SRRPs, including an atypical N-terminal signal sequence, two serinerich repeat domains that flank the nonrepeat region, and a cell wall anchor domain (2,6,20,21,(36)(37)(38)(39)(40)(41). The atypical signal sequence of SRRPs is approximately 90 amino acids long and is required for secretion via the accessory Sec system (39).…”

“…It should be noted that all three predicted SRRPs are distinct from Fap1, the previously characterized S. oralis subsp. oralis SRRP (2).…”

“…However, it is clear that adhesion is of critical importance in both environments. For several streptococcal species, binding to host surfaces, including platelets, saliva, and oral epithelial cells, can be mediated by members of the SRRP family (2,6,(33)(34)(35)(36). It has previously been reported that an S. oralis subsp.…”

“…It has previously been reported that an S. oralis subsp. oralis endocarditis isolate encodes an SRRP, Fap1, that is required for binding both terminal sialic acid and a cryptic ␤-1,4-linked galactose revealed by S. oralis neuraminidase (2). However, Fap1 is not present in the genome sequence of S. oralis subsp.…”