Streptococcus mutans major adhesion surface protein, P1 (I/II), does not contribute to attachment to valvular vegetations or to the development of endocarditis in a rat model

The Multifaceted Nature of Streptococcal Antigen I/II Proteins in Colonization and Disease Pathogenesis

Streptococci are Gram-positive bacteria that belong to the natural microbiota of humans and animals. Certain streptococcal species are known as opportunistic pathogens with the potential to cause severe invasive disease. Antigen I/II (AgI/II) family proteins are sortase anchored cell surface adhesins that are nearly ubiquitous across streptococci and contribute to many streptococcal diseases, including dental caries, respiratory tract infections, and meningitis. They appear to be multifunctional adhesins with affinities to various host substrata, acting to mediate attachment to host surfaces and stimulate immune responses from the colonized host. Here we will review the literature including recent work that has demonstrated the multifaceted nature of AgI/II family proteins, focusing on their overlapping and distinct functions and their important contribution to streptococcal colonization and disease.

Anti-caries vaccine based on clinical cold-adapted influenza vaccine: A promising alternative for scientific and public-health protection against dental caries

Dental caries remains one of the most pervasive infectious disease around the world. Protection against dental caries can be achieved experimentally by eliciting salivary IgA targeting surficial antigens of S. mutans, however, no such a vaccine has been launched for human use yet. Live vectored vaccines hold the greatest feasibility to induce potent and long-lasting immunity in the host. The FDA approved intranasal cold-adapted influenza vaccine has been used in clinical settings for many years. The vaccine can not only induce broad adaptive immune responses especially mucosal immunity, but the member strains can also circumvent existing immunity, presenting promising candidates for live vectored anti-caries vaccine. Moreover, the genetic techniques for modification of cold-adapted influenza viruses are well developed and widely applicable. Thus, we hypothesize that effective anti-caries vaccine can be developed with the backbone of cold-adapted influenza viruses by inserting specific antigenic identifier sequences of S. mutans into the viral genome, which is anticipated to protect against dental caries in humans with easy inoculation. The immune efficacies of recombinant cold-adapted influenza viruses expressing exogenous antigens have been verified by in vivo experiments for multiple infectious diseases, giving us great confidence to validate the safety properties and protection effect with this chimeric vaccine in animals or even humans. Existing data suggests that the live anti-caries vaccine may help improve public oral health by controlling the caries disease itself.

Role of Streptococcus mutans surface proteins for biofilm formation

Streptococcus mutans has been implicated as a primary causative agent of dental caries in humans. An important virulence property of the bacterium is its ability to form biofilm known as dental plaque on tooth surfaces. In addition, this organism also produces glucosyltransferases, multiple glucan-binding proteins, protein antigen c, and collagen-binding protein, surface proteins that coordinate to produce dental plaque, thus inducing dental caries. Bacteria utilize quorum-sensing systems to modulate environmental stress responses. A major mechanism of response to signals is represented by the so called two-component signal transduction system, which enables bacteria to regulate their gene expression and coordinate activities in response to environmental stress. As for S. mutans, a signal peptide-mediated quorum-sensing system encoded by comCDE has been found to be a regulatory system that responds to cell density and certain environmental stresses by excreting a peptide signal molecule termed CSP (competence-stimulating peptide). One of its principal virulence factors is production of bacteriocins (peptide antibiotics) referred to as mutacins. Two-component signal transduction systems are commonly utilized by bacteria to regulate bacteriocin gene expression and are also related to biofilm formation by S. mutans.

Contribution of cell surface protein antigen c of Streptococcus mutans to platelet aggregation

INTRODUCTION

Streptococcus mutans is considered to be one of the pathogens that cause infective endocarditis. The purpose of the present study was to examine the properties of S. mutans with regard to platelet aggregation by focusing on its high molecular protein antigen c (PAc).

METHODS

The platelet aggregation properties of six clinical strains and one isogenic mutant strain of S. mutans were analysed using an aggregometer and confocal microscopy, as well as with an inhibition assay of platelet aggregation using anti-PAc serum.

RESULTS

S. mutans strains with PAc expression induced platelet aggregation, while a PAc-deficient mutant and two clinical isolates with no PAc expression did not. When platelets were pretreated with higher amounts of anti-PAc serum, the platelet aggregation rate was reduced in a dose-dependent manner, indicating that PAc binds directly to platelets.

CONCLUSION

S. mutans PAc is involved in human platelet aggregation and may be one of the virulence factors in the pathogenesis of infective endocarditis.

Serotype classification of Streptococcus mutans and its detection outside the oral cavity

Streptococcus mutans, generally known as a major pathogen of dental caries, is also a possible causative agent of bacteremia and infective endocarditis. S. mutans is classified into serotypes c, e, f and k based on the chemical composition of serotype-specific polysaccharides, with approximately 70-80% of strains found in the oral cavity classified as serotype c, followed by e (approximately 20%), and f and k (less than 5% each). Serotype k was recently designated as a novel serotype and shown to possess unique features, the most prominent being a defect of the glucose side chain in serotype-specific rhamnose-glucose polymers, which is related to a higher incidence of detection in cardiovascular specimens, owing to phagocytosis resistance. Molecular analyses of cardiovascular specimens showed a high detection frequency for S. mutans DNA, among which the detection rate for serotype k was quite high. These findings suggest that serotype k S. mutans possibly has a high level of virulence for systemic diseases.

Molecular analyses of bacterial DNA in extirpated heart valves from patients with infective endocarditis

BACKGROUND/AIMS

Infective endocarditis (IE) is caused by a microbial infection of the endothelial surface of the heart. Although blood culture examinations are commonly used to determine the associated bacterial species, molecular techniques, which enable rapid identification of targeted bacterial species, have recently been applied in clinical cases.

METHODS

Nine heart valve specimens from IE patients (six subacute cases and three acute cases) were extirpated and collected, then bacterial DNA was extracted. Bacterial species in the specimens were determined by two different molecular methods and the results were compared with those from a conventional blood culture technique. In addition, a comparison between the two molecular methods was carried out using known numbers of six streptococcal species.

RESULTS

The conventional blood culture method revealed the bacterial species in eight cases, while one was found to be negative. Multiple species were identified in most of the cases by both molecular methods; however, those specified by one method were not always consistent with those specified by the other. Furthermore, the species determined by the blood culture technique were not always identified by the molecular methods. We also found that the two molecular methods used in the present study were extremely sensitive to detect from 1 to 100 cells of individual oral streptococcal species.

CONCLUSION

Our results suggest that species specified by molecular methods may have disseminated incidentally into the bloodstream, so interpretation of such results should be carefully undertaken in clinical situations.

Protein antigen in serotype k Streptococcus mutans clinical isolates

Streptococcus mutans, a major pathogen of dental caries and infective endocarditis, is classified into serotypes c, e, f, and k, with serotype k strains recently reported to be frequently detected in persons with infective endocarditis. Thus, we hypothesized that common properties associated with infective endocarditis are present in those strains. Fifty-six oral S. mutans strains, including 11 serotype k strains, were analyzed. Western blotting analysis revealed expression of the 3 types of glucosyltransferases in all strains, while expression of the approximately 190-kDa cell-surface protein (PA) was absent in 12 strains, among which the prevalence of serotype k (7/12) was significantly high. Furthermore, cellular hydrophobicity and phagocytosis susceptibility were lower in the group of serotype k strains. These results indicate that the absence of PA expression, low cellular hydrophobicity, and phagocytosis susceptibility are common bacterial properties associated with serotype k strains, which may be associated with virulence for infective endocarditis.

Contribution of cell surface protein antigen PAc of Streptococcus mutans to bacteremia

Streptococcus mutans, a major cariogenic bacterium, is occasionally isolated from the blood of patients with bacteremia and infective endocarditis. Mutant strains of S. mutans MT8148, defective in the major surface proteins glucosyltransferase (GTF) B-, C-, and D-, and protein antigen c (PAc), were constructed by insertional inactivation of each respective gene with an antibiotic resistant cassette. Susceptibility to phagocytosis was determined by analyses of interactions of the bacteria with human polymorphonuclear leukocytes, and the PAc-defective mutant strain (PD) showed the lowest rate of phagocytosis. Further, when PD and MT8148 were separately injected into the jugular veins of Sprague-Dawley rats, PD was recovered in significantly larger numbers and for a longer duration, and caused more severe systemic inflammation than MT8148, indicating that S. mutans PAc is associated with its systemic virulence in blood. Next, 100 S. mutans clinical isolates from 100 Japanese children and adolescents were analyzed by Western blotting using antisera raised against recombinant PAc, generated based on the pac sequence of MT8148. Four of the 100 strains showed no positive band and each exhibited a significantly lower phagocytosis rate than that of 25 randomly selected clinical strains (P < 0.01). In addition, three of the 100 strains possessed a lower molecular weight PAc and a significantly lower rate of phagocytosis than the 25 reference strains (P < 0.05). These results suggest that S. mutans PAc may be associated with phagocytosis susceptibility to human polymorphonuclear leukocytes, with approximately 7% of S. mutans clinical isolates possible high-risk strains for the development of bacteremia.

Contribution of glucan-binding protein C of Streptococcus mutans to bacteremia occurrence

Our previous analysis of major cell surface proteins of Streptococcus mutans isolated from the blood of a patient with bacteremia showed variations of glucan-binding protein C (GbpC) expression. In the present study, we analyzed the contribution of GbpC of S. mutans to bacteremia occurrence. A GbpC-defective mutant strain (C1) was significantly less susceptible to phagocytosis by human polymorphonuclear leukocytes than its parent strain (MT8148) (P < 0.001). When 21 rats were injected with C1 or streptomycin-resistant MT8148R into the jugular vein, strain C1 was recovered from blood in larger numbers and for a longer duration than MT8148R. Further, infection with C1 resulted in significant increases in serum sialic acid (SSA) concentrations, and splenomegaly, as well as body weight reduction. We also evaluated GbpC expression in 20 clinical oral isolates by immunoblotting with anti-GbpC serum, and found that expression intensity was positively correlated to phagocytosis rate (P < 0.05). These results suggest that S. mutans GbpC may be associated with systemic virulence, since a weak expression of GbpC causes the organisms to be refractory to phagocytosis, resulting in a longer survival of the bacterium in the bloodstream.

Streptococcus mutans strains harboring collagen-binding adhesin

A previously unidentified 120-kDa protein was detected in Streptococcus mutans strain Z1 and was involved in the cold-agglutination of the strain. We have identified the gene, designated cnm, as being involved in the agglutination of strain Z1 following random mutagenesis. The amino acid sequence of the deduced Cnm protein exhibited high similarity to those of collagen-binding adhesins from staphylococci and other organisms. To confirm whether the protein is involved in collagen-binding, we cloned a cnm gene fragment, overexpressed it in E.coli, and prepared crude extracts. The extracts containing recombinant protein exhibited binding to immobilized collagen and laminin but not to fibronectin. Compared with the parental strain Z1, the cold-agglutination-negative mutant 05A02 exhibited reduced binding to collagen and laminin but retained that to fibronectin. This gene was detected in some strains of S. mutans. Therefore, the cnm gene encoded a new strain-specific member of the collagen-binding adhesin family.