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

A conserved bacterial genetic basis for commensal-host specificity

Animals selectively acquire specific symbiotic gut bacteria from their environments that aid host fitness. To colonize, a symbiont must locate its niche and sustain growth within the gut. Adhesins are bacterial cell surface proteins that facilitate attachment to host tissues and are often virulence factors for opportunistic pathogens. However, the attachments are often transient and nonspecific, and additional mechanisms are required to sustain infection. In this work, we use live imaging of individual symbiotic bacterial cells colonizing the gut of living Drosophila melanogaster to show that Lactiplantibacillus plantarum specifically recognizes the fruit fly foregut as a distinct physical niche. L. plantarum establishes stably within its niche through host-specific adhesins encoded by genes carried on a colonization island. The adhesin binding domains are conserved throughout the Lactobacillales, and the island also encodes a secretion system widely conserved among commensal and pathogenic bacteria.

Insights into the role of Streptococcus oralis as an opportunistic pathogen in infectious diseases

Streptococcus oralis, belonging to the viridans group streptococci (VGS), has been considered a member of normal flora mainly inhabiting the oral cavity. However, more recently, there has been growing recognition of its role as a causative agent in various life-threatening infectious diseases such as infective endocarditis (IE) and meningitis. Additionally, the differences in the prevalence, clinical features, and prognosis of opportunistic infections between S. oralis and other VGS species have been addressed. Particularly the predominance of S. oralis in IE has drawn critical attention. In potentially fatal infections, clinical neglect of S. oralis as an instigating agent might significantly impede early diagnosis and treatment. Nevertheless, to date, the infectious diseases associated with S. oralis have not yet been comprehensively described. Therefore, this review will give an overview of infectious diseases caused by S. oralis to uncover its hidden role as an opportunistic pathogen.

Putative pathogenic factors underlying Streptococcus oralis opportunistic infections

Streptococcus oralis, belonging to the viridans group streptococci (VGS), has been considered a component of the normal flora predominantly inhabiting the oral cavity. In recent years, a growing body of literature has revealed that dental procedures or daily tooth brushing activities can cause the spread of S. oralis from the oral cavity into various body sites leading to life-threatening opportunistic infections such as infective endocarditis (IE) and meningitis. However, very little is currently known about the pathogenicity of S. oralis. Thus, the aim of this review is to update the current understanding of the pathogenic potential of S. oralis to pave the way for the prevention and treatment of S. oralis opportunistic infections.

Colorimetric detection of oral bacteria using functionalized gold nanoparticles as a plasmonic biosensor array

Early detection of specific oral bacterial species would enable timely treatment and prevention of certain oral diseases. In this work, we investigated the sensitivity and specificity of functionalized gold nanoparticles for plasmonic sensing of oral bacteria. This approach is based on the aggregation of positively charged gold nanoparticles on the negatively charged bacteria surface and the corresponding localized surface plasmon resonance (LSPR) shift. Gold nanoparticles were synthesized in different sizes, shapes and functionalization. A biosensor array was developed consisting of spherical- and anisotropic-shaped (1-hexadecyl) trimethylammonium bromide (CTAB) and spherical mercaptoethylamine (MEA) gold nanoparticles. It was used to detect four oral bacterial species (Aggregatibacter actinomycetemcomitans, Actinomyces naeslundii, Porphyromonas gingivalis and Streptococcus oralis). The plasmonic response was measured and analysed using RGB and UV-vis absorbance values. Both methods successfully detected the individual bacterial species based on their unique responses to the biosensor array. We present an in-depth study relating the bacteria zeta potential and AuNP aggregation to plasmonic response. The sensitivity depends on multiple parameters, such as bacterial species and concentration as well as gold nanoparticle shape, concentration and functionalization.

Serine-rich repeat proteins: well-known yet little-understood bacterial adhesins

Serine-rich-repeat proteins (SRRPs) are large mucin-like glycoprotein adhesins expressed by a plethora of pathogenic and symbiotic Gram-positive bacteria. SRRPs play major functional roles in bacterial-host interactions, like adhesion, aggregation, biofilm formation, virulence, and pathogenesis. Through their functional roles, SRRPs aid in the development of host microbiomes but also diseases like infective endocarditis, otitis media, meningitis, and pneumonia. SRRPs comprise shared domains across different species, including two or more heavily O-glycosylated long stretches of serine-rich repeat regions. With loci that can be as large as ~40 kb and can encode up to 10 distinct glycosyltransferases that specifically facilitate SRRP glycosylation, the SRRP loci makes up a significant portion of the bacterial genome. The significance of SRRPs and their glycans in host-microbe communications is becoming increasingly evident. Studies are beginning to reveal the glycosylation pathways and mature O-glycans presented by SRRPs. Here we review the glycosylation machinery of SRRPs across species and discuss the functional roles and clinical manifestations of SRRP glycosylation.

Corncob structures in dental plaque reveal microhabitat taxon specificity

BACKGROUND

The human mouth is a natural laboratory for studying how bacterial communities differ across habitats. Different bacteria colonize different surfaces in the mouth-teeth, tongue dorsum, and keratinized and non-keratinized epithelia-despite the short physical distance between these habitats and their connection through saliva. We sought to determine whether more tightly defined microhabitats might have more tightly defined sets of resident bacteria. A microhabitat may be characterized, for example, as the space adjacent to a particular species of bacterium. Corncob structures of dental plaque, consisting of coccoid bacteria bound to filaments of Corynebacterium cells, present an opportunity to analyze the community structure of one such well-defined microhabitat within a complex natural biofilm. Here, we investigate by fluorescence in situ hybridization and spectral imaging the composition of the cocci decorating the filaments.

RESULTS

The range of taxa observed in corncobs was limited to a small subset of the taxa present in dental plaque. Among four major groups of dental plaque streptococci, two were the major constituents of corncobs, including one that was the most abundant Streptococcus species in corncobs despite being relatively rare in dental plaque overall. Images showed both Streptococcus types in corncobs in all individual donors, suggesting that the taxa have different ecological roles or that mechanisms exist for stabilizing the persistence of functionally redundant taxa in the population. Direct taxon-taxon interactions were observed not only between the Streptococcus cells and the central corncob filament but also between Streptococcus cells and the limited subset of other plaque bacteria detected in the corncobs, indicating species ensembles involving these taxa as well.

CONCLUSIONS

The spatial organization we observed in corncobs suggests that each of the microbial participants can interact with multiple, albeit limited, potential partners, a feature that may encourage the long-term stability of the community. Additionally, our results suggest the general principle that a precisely defined microhabitat will be inhabited by a small and well-defined set of microbial taxa. Thus, our results are important for understanding the structure and organizing principles of natural biofilms and lay the groundwork for future work to modulate and control biofilms for human health. Video Abstract.

Streptococcus oralis Employs Multiple Mechanisms of Salivary Mucin Binding That Differ Between Strains

Streptococcus oralis is an oral commensal and opportunistic pathogen that can enter the bloodstream and cause bacteremia and infective endocarditis. Here, we investigated the mechanisms of S. oralis binding to oral mucins using clinical isolates, isogenic mutants and glycoconjugates. S. oralis bound to both MUC5B and MUC7, with a higher level of binding to MUC7. Mass spectrometry identified 128 glycans on MUC5B, MUC7 and the salivary agglutinin (SAG). MUC7/SAG contained a higher relative abundance of Lewis type structures, including Lewis b/y, sialyl-Lewis a/x and α2,3-linked sialic acid, compared to MUC5B. S. oralis subsp. oralis binding to MUC5B and MUC7/SAG was inhibited by Lewis b and Lacto-N-tetraose glycoconjugates. In addition, S. oralis binding to MUC7/SAG was inhibited by sialyl Lewis x. Binding was not inhibited by Lacto-N-fucopentaose, H type 2 and Lewis x conjugates. These data suggest that three distinct carbohydrate binding specificities are involved in S. oralis subsp. oralis binding to oral mucins and that the mechanisms of binding MUC5B and MUC7 differ. Efficient binding of S. oralis subsp. oralis to MUC5B and MUC7 required the gene encoding sortase A, suggesting that the adhesin(s) are LPXTG-containing surface protein(s). Further investigation demonstrated that one of these adhesins is the sialic acid binding protein AsaA.

Origins of glycan selectivity in streptococcal Siglec-like adhesins suggest mechanisms of receptor adaptation

Bacterial binding to host receptors underlies both commensalism and pathogenesis. Many streptococci adhere to protein-attached carbohydrates expressed on cell surfaces using Siglec-like binding regions (SLBRs). The precise glycan repertoire recognized may dictate whether the organism is a strict commensal versus a pathogen. However, it is currently not clear what drives receptor selectivity. Here, we use five representative SLBRs and identify regions of the receptor binding site that are hypervariable in sequence and structure. We show that these regions control the identity of the preferred carbohydrate ligand using chimeragenesis and single amino acid substitutions. We further evaluate how the identity of the preferred ligand affects the interaction with glycoprotein receptors in human saliva and plasma samples. As point mutations can change the preferred human receptor, these studies suggest how streptococci may adapt to changes in the environmental glycan repertoire.

How bacteria utilize sialic acid during interactions with the host: snip, snatch, dispatch, match and attach

N -glycolylneuraminic acid (Neu5Gc), and its precursor N-acetylneuraminic acid (Neu5Ac), commonly referred to as sialic acids, are two of the most common glycans found in mammals. Humans carry a mutation in the enzyme that converts Neu5Ac into Neu5Gc, and as such, expression of Neu5Ac can be thought of as a 'human specific' trait. Bacteria can utilize sialic acids as a carbon and energy source and have evolved multiple ways to take up sialic acids. In order to generate free sialic acid, many bacteria produce sialidases that cleave sialic acid residues from complex glycan structures. In addition, sialidases allow escape from innate immune mechanisms, and can synergize with other virulence factors such as toxins. Human-adapted pathogens have evolved a preference for Neu5Ac, with many bacterial adhesins, and major classes of toxin, specifically recognizing Neu5Ac containing glycans as receptors. The preference of human-adapted pathogens for Neu5Ac also occurs during biosynthesis of surface structures such as lipo-oligosaccharide (LOS), lipo-polysaccharide (LPS) and polysaccharide capsules, subverting the human host immune system by mimicking the host. This review aims to provide an update on the advances made in understanding the role of sialic acid in bacteria-host interactions made in the last 5-10 years, and put these findings into context by highlighting key historical discoveries. We provide a particular focus on 'molecular mimicry' and incorporation of sialic acid onto the bacterial outer-surface, and the role of sialic acid as a receptor for bacterial adhesins and toxins.

Expression and diversity of the sialic acid-binding adhesin and its homologs associated with oral streptococcal infection

Streptococcus gordonii, one of the early colonizers of oral biofilms, is involved in the development of dental caries, periodontal disease, and infective endocarditis. The Hsa adhesin of S. gordonii DL1 has the ability to bind strongly to the terminal sialic acid groups of host glycoproteins via the binding region, NR2, which is important for the pathogenicity of S. gordonii DL1. Low similarity with the NR2 of Hsa homologs among other streptococcal species has been reported. However, the reports have been limited to certain strains. This study attempted to assess frequency of the expression on the bacterial cell surface and to analyze the diversity of Hsa homologs among different wild strains of oral streptococci. We isolated 186 wild-type strains of oral streptococci from healthy volunteers and analyzed their hemagglutinating activity on human erythrocytes and their Hsa homologs and NR2 homologous regions by dot immunoblotting using anti-Hsa and anti-NR2 antisera, respectively. We found 30 strains reacted with anti-NR2 antiserum (NR2-positive) and determined the sequence of the NR2 regions. Many strains with high hemagglutinating activity were also NR2-positive, suggesting that the NR2 region may be associated with hemagglutinating activity. Among the NR2-positive strains, four different amino acid sequence patterns were observed, demonstrating diversity in the NR2 region. Notably, S. gordonii strains frequently possessed Hsa homologs and NR2-like antigens compared to other streptococci. It is speculated that the possessing frequency of Hsa homologs and the amino acid sequence of NR2 region may vary among streptococcal species. This article is protected by copyright. All rights reserved.

Proteoglycan 4 (lubricin) is a highly sialylated glycoprotein associated with cardiac valve damage in animal models of infective endocarditis

S. gordonii and S. sanguinis are primary colonizers of tooth surfaces, and are generally associated with oral health, but can also cause infective endocarditis (IE). These species express "Siglec-like" adhesins that bind sialylated glycans on host glycoproteins, which can aid the formation of infected platelet-fibrin thrombi (vegetations) on cardiac valve surfaces. We previously determined that the ability of S. gordonii to bind sialyl T-antigen (sTa) increased pathogenicity, relative to recognition of sialylated core 2 O-glycan structures, in an animal model of IE. However, it is unclear when and where the sTa structure is displayed, and which sTa-modified host factors promote valve colonization. In this study, we identified sialylated glycoproteins in the aortic valve vegetations and plasma of rat and rabbit models of this disease. Glycoproteins that display sTa versus core 2 O-glycan structures were identified by using recombinant forms of the streptococcal Siglec-like adhesins for lectin blotting and affinity capture, and the O-linked glycans were profiled by mass spectrometry. Proteoglycan 4 (PRG4), also known as lubricin, was a major carrier of sTa in the infected vegetations. Moreover, plasma PRG4 levels were significantly higher in animals with damaged or infected valves, as compared with healthy animals. The combined results demonstrate that, in addition to platelet GPIbα, PRG4 is a highly sialylated mucin-like glycoprotein found in aortic valve vegetations and may contribute to the persistence of oral streptococci in this protected endovascular niche. Moreover, plasma PRG4 could serve as a biomarker for endocardial injury and infection.

A novel sialic acid-binding adhesin present in multiple species contributes to the pathogenesis of Infective endocarditis

Bacterial binding to platelets is a key step in the development of infective endocarditis (IE). Sialic acid, a common terminal carbohydrate on host glycans, is the major receptor for streptococci on platelets. So far, all defined interactions between streptococci and sialic acid on platelets are mediated by serine-rich repeat proteins (SRRPs). However, we identified Streptococcus oralis subsp. oralis IE-isolates that bind sialic acid but lack SRRPs. In addition to binding sialic acid, some SRRP^- isolates also bind the cryptic receptor β-1,4-linked galactose through a yet unknown mechanism. Using comparative genomics, we identified a novel sialic acid-binding adhesin, here named AsaA (associated with sialic acid adhesion A), present in IE-isolates lacking SRRPs. We demonstrated that S. oralis subsp. oralis AsaA is required for binding to platelets in a sialic acid-dependent manner. AsaA comprises a non-repeat region (NRR), consisting of a FIVAR/CBM and two Siglec-like and Unique domains, followed by 31 DUF1542 domains. When recombinantly expressed, Siglec-like and Unique domains competitively inhibited binding of S. oralis subsp. oralis and directly interacted with sialic acid on platelets. We further demonstrated that AsaA impacts the pathogenesis of S. oralis subsp. oralis in a rabbit model of IE. Additionally, we found AsaA orthologues in other IE-causing species and demonstrated that the NRR of AsaA from Gemella haemolysans blocked binding of S. oralis subsp. oralis, suggesting that AsaA contributes to the pathogenesis of multiple IE-causing species. Finally, our findings provide evidence that sialic acid is a key factor for bacterial-platelets interactions in a broader range of species than previously appreciated, highlighting its potential as a therapeutic target.

Infective endocarditis (IE) is typically a bacterial infection of the heart valves that causes high mortality. Infective endocarditis can affect people with preexisting lesions on their heart valves (Subacute IE). These lesions contain platelets and other host factors to which bacteria can bind. Growth of bacteria and accumulation of host factors results in heart failure. Therefore, the ability of bacteria to bind platelets is key to the development of IE. Here, we identified a novel bacterial protein, AsaA, which helps bacteria bind to platelets and contributes to the development of disease. Although this virulence factor was characterized in Streptococcus oralis, a leading cause of IE, we demonstrated that AsaA is also present in several other IE-causing bacterial species and is likely relevant to their ability to cause disease. We showed that AsaA binds to sialic acid, a terminal sugar present on platelets, thereby demonstrating that sialic acid serves as a receptor for a wider range of IE-causing bacteria than previously appreciated, highlighting its potential as a therapeutic target.

Streptococcal Serine-Rich Repeat Proteins in Colonization and Disease

Glycosylation of proteins, previously thought to be absent in prokaryotes, is increasingly recognized as important for both bacterial colonization and pathogenesis. For mucosal pathobionts, glycoproteins that function as cell wall-associated adhesins facilitate interactions with mucosal surfaces, permitting persistent adherence, invasion of deeper tissues and transition to disease. This is exemplified by Streptococcus pneumoniae and Streptococcus agalactiae, which can switch from being relatively harmless members of the mucosal tract microbiota to bona fide pathogens that cause life-threatening diseases. As part of their armamentarium of virulence factors, streptococci encode a family of large, glycosylated serine-rich repeat proteins (SRRPs) that facilitate binding to various tissue types and extracellular matrix proteins. This minireview focuses on the roles of S. pneumoniae and S. agalactiae SRRPs in persistent colonization and the transition to disease. The potential of utilizing SRRPs as vaccine targets will also be discussed.

Tandem sialoglycan-binding modules in a Streptococcus sanguinis serine-rich repeat adhesin create target dependent avidity effects

Sialic acid-binding immunoglobulin-like lectins (Siglec)-like domains of streptococcal serine-rich repeat (SRR) adhesins recognize sialylated glycans on human salivary, platelet, and plasma glycoproteins via a YTRY sequence motif. The SRR adhesin from Streptococcus sanguinis strain SK1 has tandem sialoglycan-binding domains and has previously been shown to bind sialoglycans with high affinity. However, both domains contain substitutions within the canonical YTRY motif, making it unclear how they interact with host receptors. To identify how the S. sanguinis strain SK1 SRR adhesin affects interactions with sialylated glycans and glycoproteins, we determined high-resolution crystal structures of the binding domains alone and with purified trisaccharides. These structural studies determined that the ligands still bind at the noncanonical binding motif, but with fewer hydrogen-bonding interactions to the protein than is observed in structures of other Siglec-like adhesins. Complementary biochemical studies identified that each of the two binding domains has a different selectivity profile. Interestingly, the binding of SK1 to platelets and plasma glycoproteins identified that the interaction to some host targets is dominated by the contribution of one binding domain, whereas the binding to other host receptors is mediated by both binding domains. These results provide insight into outstanding questions concerning the roles of tandem domains in targeting host receptors and suggest mechanisms for how pathogens can adapt to the availability of a range of related but nonidentical host receptors. They further suggest that the definition of the YTRY motif should be changed to ϕTRX, a more rigorous description of this sialic acid-recognition motif given recent findings.