Prevalence of Csh-like fibrillar surface proteins among mitis group oral streptococci

E-Cigarette Aerosol Exposure Favors the Growth and Colonization of Oral Streptococcus mutans Compared to Commensal Streptococci

E-cigarettes (e-cigs) have drastically increased in popularity during the last decade, especially among teenagers. While recent studies have started to explore the effect of e-cigs in the oral cavity, little is known about their effects on the oral microbiota and how they could affect oral health and potentially lead to disease, including periodontitis and head and neck cancers. To explore the impact of e-cigs on oral bacteria, we selected members of the genus Streptococcus, which are abundant in the oral cavity. We exposed the commensals Streptococcus sanguinis and Streptococcus gordonii and the opportunistic pathogen Streptococcus mutans, best known for causing dental caries, to e-liquids and e-cig aerosols with and without nicotine and with and without menthol flavoring and measured changes in growth patterns and biofilm formation. Our results demonstrate that e-cig aerosols hindered the growth of S. sanguinis and S. gordonii, while they did not affect the growth of S. mutans. We also show that e-cig aerosols significantly increased biofilm formation by S. mutans but did not affect the biofilm formation of the two commensals. We found that S. mutans exhibits higher hydrophobicity and coaggregation abilities along with higher attachment to OKF6 cells than S. sanguinis and S. gordonii. Therefore, our data suggest that e-cig aerosols have the potential to dysregulate oral bacterial homeostasis by suppressing the growth of commensals while enhancing the biofilm formation of the opportunistic pathogen S. mutans. This study highlights the importance of understanding the consequences of e-cig aerosol exposure on selected commensals and pathogenic species. Future studies modeling more complex communities will provide more insight into how e-cig aerosols and vaping affect the oral microbiota. IMPORTANCE Our study shows that e-cigarette aerosol exposure of selected bacteria known to be residents of the oral cavity hinders the growth of two streptococcal commensals while enhancing biofilm formation, hydrophobicity, and attachment for the pathogen S. mutans. These results indicate that e-cigarette vaping could open a niche for opportunistic bacteria such as S. mutans to colonize the oral cavity and affect oral health.

Bacterial adhesion to collagens: implications for biofilm formation and disease progression in the oral cavity

Collagen is the most abundant structural protein in the body and the main component of the extracellular matrix of most tissues, including dentine and periodontal tissues. Despite the well-characterized role of collagen and specifically type-I collagen, as a ligand for host cells, its role as a substrate for bacterial adhesion and biofilm formation is less explored. Therefore, the purpose of this review is to discuss recent findings regarding the adhesion of oral bacteria to collagen surfaces and its role in the progression and severity of oral and systemic diseases. Initial oral colonizers such as streptococci have evolved collagen-binding proteins (cbp) that are important for the colonization of dentine and periodontal tissues. Also, periodontal pathogens such as Porphyromonas gingivalis and Tannerella forsythia utilise cbps for tissue sensing and subsequent invasion. The implications of bacteria-collagen coupling in the context of collagen biomaterials and regenerative dentistry approaches are also addressed. Furthermore, the importance of interdisciplinary techniques such as atomic force microscopy for the nanocharacterization of bacteria-collagen interactions is also considered. Overall, understanding the process of oral bacterial adhesion onto collagen is important for developing future therapeutic approaches against oral and systemic diseases, by modulating the early stages of biofilm formation.

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.

Biology of Oral Streptococci

Bacteria belonging to the genus Streptococcus are the first inhabitants of the oral cavity, which can be acquired right after birth and thus play an important role in the assembly of the oral microbiota. In this article, we discuss the different oral environments inhabited by streptococci and the species that occupy each niche. Special attention is given to the taxonomy of Streptococcus, because this genus is now divided into eight distinct groups, and oral species are found in six of them. Oral streptococci produce an arsenal of adhesive molecules that allow them to efficiently colonize different tissues in the mouth. Also, they have a remarkable ability to metabolize carbohydrates via fermentation, thereby generating acids as byproducts. Excessive acidification of the oral environment by aciduric species such as Streptococcus mutans is directly associated with the development of dental caries. However, less acid-tolerant species such as Streptococcus salivarius and Streptococcus gordonii produce large amounts of alkali, displaying an important role in the acid-base physiology of the oral cavity. Another important characteristic of certain oral streptococci is their ability to generate hydrogen peroxide that can inhibit the growth of S. mutans. Thus, oral streptococci can also be beneficial to the host by producing molecules that are inhibitory to pathogenic species. Lastly, commensal and pathogenic streptococci residing in the oral cavity can eventually gain access to the bloodstream and cause systemic infections such as infective endocarditis.

Novel Two-Component System of Streptococcus sanguinis Affecting Functions Associated with Viability in Saliva and Biofilm Formation

Streptococcus sanguinis is a pioneer species of teeth and a common opportunistic pathogen of infective endocarditis. In this study, we identified a two-component system, S. sanguinis SptRS (SptRS _Ss ), affecting S. sanguinis survival in saliva and biofilm formation. Isogenic mutants of sptR_Ss (SKsptR) and sptS_Ss (SKsptS) showed reduced cell counts in ex vivo assays of viability in saliva compared to those of parent strain SK36 and complemented mutants. Reduced counts of the mutants in saliva were associated with reduced growth rates in nutrient-poor medium (RPMI) and increased susceptibility to the deposition of C3b and the membrane attach complex (MAC) of the complement system, a defense component of saliva and serum. Conversely, sptR_Ss and sptS_Ss mutants showed increased biofilm formation associated with higher levels of production of H₂O₂ and extracellular DNA. Reverse transcription-quantitative PCR (RT-qPCR) comparisons of strains indicated a global role of SptRS _Ss in repressing genes for H₂O₂ production (2.5- to 15-fold upregulation of spxB, spxR, vicR, tpk, and ackA in sptR_Ss and sptS_Ss mutants), biofilm formation, and/or evasion of host immunity (2.1- to 11.4-fold upregulation of srtA, pcsB, cwdP, iga, and nt5e). Compatible with the homology of SptR _Ss with AraC-type regulators, duplicate to multiple conserved repeats were identified in 1,000-bp regulatory regions of downstream genes, suggesting that SptR _Ss regulates transcription by DNA looping. Significant transcriptional changes in the regulatory genes vicR, spxR, comE, comX, and mecA in the sptR_Ss and sptS_Ss mutants further indicated that SptRS _Ss is part of a regulatory network that coordinates cell wall homeostasis, H₂O₂ production, and competence. This study reveals that SptRS _Ss is involved in the regulation of crucial functions for S. sanguinis persistence in the oral cavity.

Surface proteins involved in the adhesion of Streptococcus salivarius to human intestinal epithelial cells

The adhesion properties of 14 Streptococcus salivarius strains to mucus (HT29-MTX) and non-mucus secreting (Caco-2/TC7) human intestinal epithelial cells were investigated. Ability to adhere to these two eukaryotic cell lines greatly differs between strains. The presence of mucus played a major factor in adhesion, likely due to high adhesiveness to mucins present in the native human mucus layer covering the whole cell surface. Only one S. salivarius strain (F6-1), isolated from the feces of a healthy baby, was found to strongly adhere to HT-29 MTX cells at a level comparable to that of Lactobacillus rhamnosus GG, a probiotic strain considered to be highly adherent. By sequencing the genome of F6-1, we were able to identify 36 genes encoding putative surface proteins. Deletion mutants were constructed for six of them and their adhesion abilities on HT-29 MTX cells were checked. Our study confirmed that four of these genes encode adhesins involved in the adhesion of S. salivarius to host cells. Such adhesins were also identified in other S. salivarius strains.

Innocent until proven guilty: mechanisms and roles of Streptococcus-Candida interactions in oral health and disease

Candida albicans and streptococci of the mitis group colonize the oral cavities of the majority of healthy humans. While C. albicans is considered an opportunistic pathogen, streptococci of this group are broadly considered avirulent or even beneficial organisms. However, recent evidence suggests that multi-species biofilms with these organisms may play detrimental roles in host homeostasis and may promote infection. In this review we summarize the literature on molecular interactions between members of this streptococcal group and C. albicans, with emphasis on their potential role in the pathogenesis of opportunistic oral mucosal infections.

Community interactions of oral streptococci

It is now clear that the most common oral diseases, dental caries and periodontitis, are caused by mixed-species communities rather than by individual pathogens working in isolation. Oral streptococci are central to these disease processes since they are frequently the first microorganisms to colonize oral surfaces and they are numerically the dominant microorganisms in the human mouth. Numerous interactions between oral streptococci and other bacteria have been documented. These are thought to be critical for the development of mixed-species oral microbial communities and for the transition from oral health to disease. Recent metagenomic studies are beginning to shed light on the co-occurrence patterns of streptococci with other oral bacteria. Refinements in microscopy techniques and biofilm models are providing detailed insights into the spatial distribution of streptococci in oral biofilms. Targeted genetic manipulation is increasingly being applied for the analysis of specific genes and networks that modulate interspecies interactions. From this work, it is clear that streptococci produce a range of extracellular factors that promote their integration into mixed-species communities and enable them to form social networks with neighboring taxa. These "community integration factors" include coaggregation-mediating adhesins and receptors, small signaling molecules such as peptides or autoinducer-2, bacteriocins, by-products of metabolism including hydrogen peroxide and lactic acid, and a range of extracellular enzymes. Here, we provide an overview of various types of community interactions between oral streptococci and other microorganisms, and we consider the possibilities for the development of new technologies to interfere with these interactions to help control oral biofilms.

Thermal Treatments Modulate Bacterial Adhesion to Dental Enamel

Numerous studies have demonstrated the effects of laser-induced heat on demineralization of enamel; however, no studies have investigated the link between heat/laser-induced changes in physicochemical properties and bacterial adhesion. In this study, we investigated the effects of thermal treatment on surface properties of enamel such as hydrophobicity and zeta potential. Bacterial adhesion to treated surfaces was characterized by confocal laser scanning microscopy, and adhesion force was quantified by atomic force microscopy. The hydrophobicity of enamel increased after heating (p < 0.05), and the zeta potential of heated enamel became more negative than that of the control (p < 0.01). Streptococcus oralis and S. mitis were more hydrophilic than S. sanguis, with more negative zeta potential (all p < 0.01). S. mitis and S. oralis occupied significantly less area on enamel after being heated (p < 0.05). Heating reduced the adhesion force of both S. mitis and S. oralis to enamel with or without saliva coating. Reduction of adhesion force was statistically significant for S. mitis (p < 0.01), whereas that of S. oralis was not statistically significant (p > 0.05). Heating did not affect the adhesion of S. sanguis with or without saliva coating. In conclusion, thermal treatment and photothermal/laser treatments may modulate the physicochemical properties of enamel, preventing the adhesion of some bacterial species.

Mitis group streptococci express variable pilus islet 2 pili

Background

Streptococcus oralis, Streptococcus mitis, and Streptococcus sanguinis are members of the Mitis group of streptococci and agents of oral biofilm, dental plaque and infective endocarditis, disease processes that involve bacteria-bacteria and bacteria-host interactions. Their close relative, the human pathogen S. pneumoniae uses pilus-islet 2 (PI-2)-encoded pili to facilitate adhesion to eukaryotic cells.

Methodology/Principal Findings

PI-2 pilus-encoding genetic islets were identified in S. oralis, S. mitis, and S. sanguinis, but were absent from other isolates of these species. The PI-2 islets resembled the genetic organization of the PI-2 islet of S. pneumoniae, but differed in the genes encoding the structural pilus proteins PitA and PitB. Two and three variants of pitA (a pseudogene in S. pneumoniae) and pitB, respectively, were identified that showed ≈20% difference in nucleotide as well as corresponding protein sequence. Species-independent combinations of pitA and pitB variants indicated prior intra- and interspecies horizontal gene transfer events. Polyclonal antisera developed against PitA and PitB of S. oralis type strain ATCC35037 revealed that PI-2 pili in oral streptococci were composed of PitA and PitB. Electronmicrographs showed pilus structures radiating >700 nm from the bacterial surface in the wild type strain, but not in an isogenic PI-2 deletion mutant. Anti-PitB-antiserum only reacted with pili containing the same PitB variant, whereas anti-PitA antiserum was cross-reactive with the other PitA variant. Electronic multilocus sequence analysis revealed that all PI-2-encoding oral streptococci were closely-related and cluster with non-PI-2-encoding S. oralis strains.

Conclusions/Significance

This is the first identification of PI-2 pili in Mitis group oral streptococci. The findings provide a striking example of intra- and interspecies horizontal gene transfer. The PI-2 pilus diversity provides a possible key to link strain-specific bacterial interactions and/or tissue tropisms with pathogenic traits in the Mitis group streptococci.

Streptococcus adherence and colonization

Streptococci readily colonize mucosal tissues in the nasopharynx; the respiratory, gastrointestinal, and genitourinary tracts; and the skin. Each ecological niche presents a series of challenges to successful colonization with which streptococci have to contend. Some species exist in equilibrium with their host, neither stimulating nor submitting to immune defenses mounted against them. Most are either opportunistic or true pathogens responsible for diseases such as pharyngitis, tooth decay, necrotizing fasciitis, infective endocarditis, and meningitis. Part of the success of streptococci as colonizers is attributable to the spectrum of proteins expressed on their surfaces. Adhesins enable interactions with salivary, serum, and extracellular matrix components; host cells; and other microbes. This is the essential first step to colonization, the development of complex communities, and possible invasion of host tissues. The majority of streptococcal adhesins are anchored to the cell wall via a C-terminal LPxTz motif. Other proteins may be surface anchored through N-terminal lipid modifications, while the mechanism of cell wall associations for others remains unclear. Collectively, these surface-bound proteins provide Streptococcus species with a "coat of many colors," enabling multiple intimate contacts and interplays between the bacterial cell and the host. In vitro and in vivo studies have demonstrated direct roles for many streptococcal adhesins as colonization or virulence factors, making them attractive targets for therapeutic and preventive strategies against streptococcal infections. There is, therefore, much focus on applying increasingly advanced molecular techniques to determine the precise structures and functions of these proteins, and their regulatory pathways, so that more targeted approaches can be developed.

Adhesion determinants of the Streptococcus species

Streptococci are clinically important Gram-positive bacteria that are capable to cause a wide variety of diseases in humans and animals. Phylogenetic analyses based on 16S rRNA sequences of the streptococcal species reveal a clustering pattern, reflecting, with a few exceptions, their pathogenic potential and ecological preferences. Microbial adhesion to host tissues is the initial critical event in the pathogenesis of most infections. Streptococci use multiple adhesins to attach to the epithelium, and their expression is regulated in response to environmental and growth conditions. Bacterial adhesins recognize and bind cell surface molecules and extracellular matrix components through specific domains that for certain adhesin families have been well defined and found conserved across the streptococcal species. In this review, we present the different streptococcal adhesin families categorized on the basis of their adhesive properties and structural characteristics, and, when available, we focus the attention on conserved functional domains.

The role of Staphylococcus aureus surface protein SasG in adherence and biofilm formation

Staphylococcus aureus colonizes the moist squamous epithelium of the anterior nares. One of the adhesins likely to be responsible is the S. aureus surface protein G (SasG), which has sequence similarity with the proteins Pls (plasmin sensitive) of S. aureus and Aap (accumulation associated protein) of Staphylococcus epidermidis. Expression of SasG by a laboratory strain of S. aureus could not be detected by Western immunoblotting. To enable investigation of SasG, the gene was cloned into two expression vectors, the IPTG-inducible pMUTIN4 and the tetracycline-inducible pALC2073, and introduced into S. aureus. Expression of SasG masked the ability of exponentially grown S. aureus cells expressing protein A (Spa), clumping factor B (ClfB) and the fibronectin binding proteins A and B (FnBPA and FnBPB) to bind to IgG, cytokeratin 10 and fibronectin, respectively. SasG also masked binding to fibrinogen mediated by both ClfB and the FnBPs. Western immunoblotting showed no reduction in expression of the blocked adhesins following induction of SasG. SasG size variants with eight, six or five B repeats masked binding to the ligands, whereas variants with four, two or one repeats had no effect. SasG-expressing strains formed peritrichous fibrils (53.47+/-2.51 nm long) of varying density on the cell wall, which were labelled by immunogold negative staining with anti-SasG antibodies. SasG-expressing strains of S. aureus also formed biofilm independently of the polysaccharide intercellular adhesin (PIA). SasG variants with eight, six and five repeats formed biofilm, whereas variants with four, two or one repeats did not. It was concluded that the fibrillar nature of SasG explains its ability to mask binding of S. aureus microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) to their ligands and to promote formation of biofilm. In addition, the strong adhesion of SasG to desquamated nasal epithelial cells likely compensates for its blocking of the binding of S. aureus ClfB to cytokeratin 10, which is important in adhesion to squames by cells lacking SasG. Several clinical isolates expressed SasG at levels similar to those of SH1000 sasG : : pMUTIN4, indicating that the properties described in the laboratory strain SH1000 may be relevant in vivo.

Differential binding specificities of oral streptococcal antigen I/II family adhesins for human or bacterial ligands

The antigen I/II (AgI/II) family polypeptides, ranging from 1310 to 1653 amino acid (aa) residues, are cell wall anchored adhesins expressed by most indigenous species of oral streptococci. The polypeptides interact with a wide range of host molecules, in particular salivary agglutinin glycoprotein (SAG or gp340), and with ligands on other oral bacteria. To determine the receptor recognition properties of six different AgI/II family polypeptides from strains of Streptococcus gordonii, Streptococcus intermedius and Streptococcus mutans, the genes were cloned and expressed on the surface of the surrogate host Lactococcus lactis. The S. gordonii SspA and SspB polypeptides mediated higher binding levels of L. lactis cells to surface immobilized gp340 than did S. intermedius Pas protein, or S. mutans SpaP or PAc proteins. However, the AgI/II proteins were all similar in their abilities to mediate aggregation of lactococci by fluid phase gp340. The SpaP(I) polypeptide from S. mutans Ingbritt, which was C-terminally truncated by approximately 400 aa residues, did not bind gp340. Lactococci expressing AgI/II proteins, including SpaP(I), were aggregated by a synthetic 16 aa residue peptide SRCRP2 derived from the aa repeat block sequences within gp340. In coaggregation assays, SspB from S. gordonii was unique in mediating coaggregation with only group A and group E strains of Actinomyces naeslundii. All the other AgI/II polypeptides mediated coaggregation with group C and group D strains of A. naeslundii. Analysis of chimeric protein constructs revealed that coaggregation specificity was determined by sequences within the N-terminal half of AgI/II protein. A synthetic peptide (20 aa residues), which defines a putative adhesion epitope within the C-terminal region of polypeptide, inhibited AgI/II-mediated aggregation by gp340 but did not affect coaggregation with A. naeslundii. These results suggest that different mechanisms operate in interactions of AgI/II family polypeptides with native gp340, gp340 SRCR domain peptide, and A. naeslundii. Specificity of these interactions appears to be determined by discontinuous but interacting regions of the polypeptides, thus providing flexibility in receptor recognition for streptococcal colonization of the human host.

Association of a novel high molecular weight, serine-rich protein (SrpA) with fibril-mediated adhesion of the oral biofilm bacterium Streptococcus cristatus

The surface of the oral plaque bacterium Streptococcus cristatus is decorated with a lateral tuft of fibrils. The fibrillar tuft functions in the adhesion of S. cristatus to heterologous bacterial species in the plaque biofilm. The tuft typically consists of a densely packed fringe of shorter fibrils 238 +/- 19 nm long with longer, less abundant fibrils 403 +/- 66 nm long projecting through the fringe of short fibrils. The two types of fibrils in the tufts of S. cristatus have been refractory to biochemical separation, complicating their characterization. A hexadecane partition assay was used to enrich for subpopulations of S. cristatus CR311 (type strain NCTC 12479) having distinct fibrillar morphotypes. Negative staining in the TEM revealed that cells of a hydrophobic subpopulation of S. cristatus (CR311var1) carried only the long fibrils (395 +/- 32 nm). A hydrophilic subpopulation of S. cristatus (CR311var3) consisted of mixed morphotypes having no fibrils or remnant short fibrils (223 +/- 49 nm). No long fibrils were observed on any cells in the CR311var3 subpopulation. The CR311var3 morphotype, unlike the wild-type strain and CR311var1, was not able to form corncobs with either Corynebacterium matruchotii or Fusobacterium nucleatum. Variant CR311var3 did not express the novel gene srpA, which encodes a high molecular weight (321,882 Da) serine-rich protein, SrpA. The SrpA protein contains two extensive repeat motifs of 17 and 71 amino acids and a gram-positive cell wall anchor consensus sequence (LPNTG). The unusual properties of SrpA most closely resemble those of Fap1, the fimbrial-associated adhesin protein of Streptococcus parasanguis. The association of long fibrils, high surface hydrophobicity, ability to form corncob formations, and expression of the srpA gene suggest that SrpA is a long fibril protein in S. cristatus.