Induction of nitric oxide production by polyosides from the cell walls of Streptococcus mutans OMZ 175, a gram-positive bacterium, in the rat aorta

Oral streptococci: modulators of health and disease

Streptococci are primary colonizers of the oral cavity where they are ubiquitously present and an integral part of the commensal oral biofilm microflora. The role oral streptococci play in the interaction with the host is ambivalent. On the one hand, they function as gatekeepers of homeostasis and are a prerequisite for the maintenance of oral health - they shape the oral microbiota, modulate the immune system to enable bacterial survival, and antagonize pathogenic species. On the other hand, also recognized pathogens, such as oral Streptococcus mutans and Streptococcus sobrinus, which trigger the onset of dental caries belong to the genus Streptococcus. In the context of periodontitis, oral streptococci as excellent initial biofilm formers have an accessory function, enabling late biofilm colonizers to inhabit gingival pockets and cause disease. The pathogenic potential of oral streptococci fully unfolds when their dissemination into the bloodstream occurs; streptococcal infection can cause extra-oral diseases, such as infective endocarditis and hemorrhagic stroke. In this review, the taxonomic diversity of oral streptococci, their role and prevalence in the oral cavity and their contribution to oral health and disease will be discussed, focusing on the virulence factors these species employ for interactions at the host interface.

Virulence factors of Streptococcus anginosus - a molecular perspective

Streptococcus anginosus together with S. constellatus and S. intermedius constitute the Streptococcus anginosus group (SAG), until recently considered to be benign commensals of the human mucosa isolated predominantly from oral cavity, but also from upper respiratory, intestinal, and urogenital tracts. For years the virulence potential of SAG was underestimated, mainly due to complications in correct species identification and their assignment to the physiological microbiota. Still, SAG representatives have been associated with purulent infections at oral and non-oral sites resulting in abscesses formation and empyema. Also, life threatening blood infections caused by SAG have been reported. However, the understanding of SAG as potential pathogen is only fragmentary, albeit certain aspects of SAG infection seem sufficiently well described to deserve a systematic overview. In this review we summarize the current state of knowledge of the S. anginosus pathogenicity factors and their mechanisms of action.

StreptoBase: An Oral Streptococcus mitis Group Genomic Resource and Analysis Platform

The oral streptococci are spherical Gram-positive bacteria categorized under the phylum Firmicutes which are among the most common causative agents of bacterial infective endocarditis (IE) and are also important agents in septicaemia in neutropenic patients. The Streptococcus mitis group is comprised of 13 species including some of the most common human oral colonizers such as S. mitis, S. oralis, S. sanguinis and S. gordonii as well as species such as S. tigurinus, S. oligofermentans and S. australis that have only recently been classified and are poorly understood at present. We present StreptoBase, which provides a specialized free resource focusing on the genomic analyses of oral species from the mitis group. It currently hosts 104 S. mitis group genomes including 27 novel mitis group strains that we sequenced using the high throughput Illumina HiSeq technology platform, and provides a comprehensive set of genome sequences for analyses, particularly comparative analyses and visualization of both cross-species and cross-strain characteristics of S. mitis group bacteria. StreptoBase incorporates sophisticated in-house designed bioinformatics web tools such as Pairwise Genome Comparison (PGC) tool and Pathogenomic Profiling Tool (PathoProT), which facilitate comparative pathogenomics analysis of Streptococcus strains. Examples are provided to demonstrate how StreptoBase can be employed to compare genome structure of different S. mitis group bacteria and putative virulence genes profile across multiple streptococcal strains. In conclusion, StreptoBase offers access to a range of streptococci genomic resources as well as analysis tools and will be an invaluable platform to accelerate research in streptococci. Database URL: http://streptococcus.um.edu.my.

Rapid evolution of virulence and drug resistance in the emerging zoonotic pathogen Streptococcus suis

BACKGROUND

Streptococcus suis is a zoonotic pathogen that infects pigs and can occasionally cause serious infections in humans. S. suis infections occur sporadically in human Europe and North America, but a recent major outbreak has been described in China with high levels of mortality. The mechanisms of S. suis pathogenesis in humans and pigs are poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

The sequencing of whole genomes of S. suis isolates provides opportunities to investigate the genetic basis of infection. Here we describe whole genome sequences of three S. suis strains from the same lineage: one from European pigs, and two from human cases from China and Vietnam. Comparative genomic analysis was used to investigate the variability of these strains. S. suis is phylogenetically distinct from other Streptococcus species for which genome sequences are currently available. Accordingly, approximately 40% of the approximately 2 Mb genome is unique in comparison to other Streptococcus species. Finer genomic comparisons within the species showed a high level of sequence conservation; virtually all of the genome is common to the S. suis strains. The only exceptions are three approximately 90 kb regions, present in the two isolates from humans, composed of integrative conjugative elements and transposons. Carried in these regions are coding sequences associated with drug resistance. In addition, small-scale sequence variation has generated pseudogenes in putative virulence and colonization factors.

CONCLUSIONS/SIGNIFICANCE

The genomic inventories of genetically related S. suis strains, isolated from distinct hosts and diseases, exhibit high levels of conservation. However, the genomes provide evidence that horizontal gene transfer has contributed to the evolution of drug resistance.

Serotype-specific polysaccharide of Streptococcus mutans contributes to infectivity in endocarditis

Streptococcus mutans and other viridans streptococci have been implicated as major etiological agents of infective endocarditis. The serotype-specific rhamnose-glucose polysaccharide (RGP) of S. mutans has several biological functions that appear to be essential for the induction of infective endocarditis. The aim of this study was to examine the contribution of RGP to the infectivity of S. mutans in infective endocarditis using a rat model. The RGP-defective mutant of S. mutans showed reduced ability to induce infective endocarditis compared to the parental strain. The ability of S. mutans to induce infective endocarditis was not consistent with the binding capacity of the organism to extracellular matrix proteins. The results suggest that S. mutans containing whole RGP is more virulent than the RGP-defective mutant, and the RGP has an important role for the induction of infective endocarditis by S. mutans.

Insertional inactivation of pac and rmlB genes reduces the release of tumor necrosis factor alpha, interleukin-6, and interleukin-8 induced by Streptococcus mutans in monocytic, dental pulp, and periodontal ligament cells

Streptococcus mutans possesses different cell wall molecules, such as protein of the I/II family, the serotype f polysaccharide rhamnose glucose polymer (RGP), and lipoteichoic acid (LTA), which act as adhesins and modulins, allowing S. mutans to colonize teeth and cause dental caries and pulpitis. We tested several isogenic mutants of S. mutans defective in protein I/II and/or RGP, as well as purified modulins such as protein I/II, RGP, and LTA, for their binding and activation abilities on monocytic, dental pulp (DP), and periodontal ligament (PDL) cells. Our results demonstrate that both protein I/II and RGP play important roles in streptococcal adherence to human monocytic and fibroblastic cells, whereas LTA is only a minor adhesin. In the activation process, the cytokine response elicited is polarized toward a Th1 response which seems principally due to protein I/II and RGP. Even if protein I/II seems to be more efficient in its purified form in triggering cells to release interleukin-8 (IL-8), RGP is the most efficient cytokine-stimulating component in intact bacteria, while LTA plays only a minor role. In cell activation, we showed, by using either cytochalasin D or coated ligands, that internalization of either S. mutans, S. mutans isogenic mutants, or purified ligands is not necessary to trigger cells to release IL-8. We also showed that, besides the implication of monocytes in pulpal inflammation, fibroblast-like cells such as DP and PDL cells are also actively implicated in local inflammation and in the generation of a Th1 response after stimulation with S. mutans cells or antigens.

Expression and characterization of streptococcal rgp genes required for rhamnan synthesis in Escherichia coli

Six genes (rgpA through rgpF) that were involved in assembling the rhamnose-glucose polysaccharide (RGP) in Streptococcus mutans were previously identified (Y. Yamashita, Y. Tsukioka, K. Tomihisa, Y. Nakano, and T. Koga, J. Bacteriol. 180:5803-5807, 1998). The group-specific antigens of Lancefield group A, C, and E streptococci and the polysaccharide antigen of Streptococcus sobrinus have the same rhamnan backbone as the RGP of S. mutans. Escherichia coli harboring plasmid pRGP1 containing all six rgp genes did not synthesize complete RGP. However, E. coli carrying a plasmid with all of the rgp genes except for rgpE synthesized the rhamnan backbone of RGP without glucose side chains, suggesting that in addition to rgpE, another gene is required for glucose side-chain formation. Synthesis of the rhamnan backbone in E. coli required the initiation of transfer of N-acetylglucosamine to a lipid carrier and the expression of the rgpC and rgpD genes encoding the putative ABC transporter specific for RGP. The similarities in RGP synthesis between E. coli and S. mutans suggest common pathways for rhamnan synthesis. Therefore, we evaluated the rhamnosyl polymerization process in E. coli by high-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the lipooligosaccharide (LOS). An E. coli transformant harboring rgpA produced the LOS modified by the addition of a single rhamnose residue. Furthermore, the rgpA, rgpB, and rgpF genes of pRGP1 were independently mutated by an internal deletion, and the LOS chemotypes of their transformants were examined. The transformant with an rgpA deletion showed the same LOS profile as E. coli without a plasmid. The transformant with an rgpB deletion showed the same LOS profile as E. coli harboring rgpA alone. The transformant with an rgpF deletion showed the LOS band with the most retarded migration. On the basis of these results, we speculated that RgpA, RgpB, and RgpF, in that order, function in rhamnan polymerization.

Immunization against dental caries

Dental caries is one of the most common infectious diseases. Of the oral bacteria, mutans streptococci, such as Streptococcus mutans and S. sobrinus, are considered to be causative agents of dental caries in humans. There have been numerous studies of the immunology of mutans streptococci. To control dental caries, dental caries vaccines have been produced using various cell-surface antigens of these organisms. Progress in recombinant DNA technology and peptide synthesis has been applied to the development of recombinant and synthetic peptide vaccines to control dental caries. Significant protective effects against dental caries have been shown in experimental animals, such as mice, rats and monkeys, which have been subcutaneously, orally, or intranasally immunized with these antigens. Only a few studies, however, have examined the efficacy of dental caries vaccines in humans. Recently, local passive immunization using murine monoclonal antibodies, transgenic plant antibodies, egg-yolk antibodies, and bovine milk antibodies to antigens of mutans streptococci have been used to control the colonization of the organisms and the induction of dental caries in human. Such immunization procedures may be a safer approach for controlling human dental caries than active immunization.

Adventitia-derived nitric oxide in rat aortas exposed to endotoxin: cell origin and functional consequences

The role of adventitial cells in bacterial lipopolysaccharide (LPS)-induced vascular nitric oxide (NO) overproduction has been largely ignored. In rat aortas exposed to LPS in vitro or in vivo, it was found that adventitia contained the major part of NO synthase (NOS)-2 protein (Western blot and immunohistochemistry) and generated the largest amount of NO (electron paramagnetic resonance spin trapping). NOS-2 immunoreactive cells were mainly resident macrophages at an early stage (5 h, in vitro or in vivo) and fibroblasts at a later stage (20 h, in vitro). Adventitial NOS-2 activity largely accounted for 1) the relaxing effect of L-arginine in rings exposed to LPS in vivo, 2) generation of an "NO store" revealed by N-acetylcysteine-induced relaxation, and 3) formation of protein-bound dinitrosyl iron complexes in the medial layer of aortic rings exposed to LPS in vitro. In conclusion, the adventitia is a powerful source of NO triggered by LPS in the rat aorta. This novel source of NO has an important impact on smooth muscle function and might be implicated in various inflammatory diseases.

Muramyl dipeptide potentiates cytokine-induced activation of inducible nitric oxide synthase in rat astrocytes

We investigated the influence of muramyl dipeptide (MDP), a cell wall component of Gram-positive bacteria, on cytokine-induced nitric oxide (NO) production in rat primary astrocytes. MDP alone did not induce NO release in astrocyte cultures. However, MDP increased astrocyte NO production and subsequent nitrite accumulation triggered by IFN-gamma. IFN-gamma-activated expression of mRNA for inducible NO synthase (iNOS) and iNOS transcription factor interferon regulatory factor-1 (IRF-1) was markedly enhanced in astrocytes treated with MDP. The potentiating effect of MDP on IFN-gamma-induced NO production in astrocytes was completely blocked with protein tyrosine kinase (PTK) inhibitor genistein or mitogen activated protein kinase (MAPK) inhibitor PD98059. In contrast, protein kinase C (PKC) inhibitor calphostin C did not affect ability of MDP to augment IFN-gamma-triggered astrocyte NO synthesis. These results suggest that MDP synergizes with IFN-gamma in the induction of iNOS gene in astrocytes through mechanisms involving PTK and MAPK, but not PKC activation. Finally, MDP also augmented NO production and iNOS mRNA expression in astrocytes treated with IL-1beta.

Role of serotype-specific polysaccharide in the resistance of Streptococcus mutans to phagocytosis by human polymorphonuclear leukocytes

To clarify the role of cell surface components of Streptococcus mutans in resistance to phagocytosis by human polymorphonuclear leukocytes (PMNs), several isogenic mutants of S. mutans defective in cell surface components were studied with a luminol-enhanced chemiluminescence (CL) assay, a killing assay, and a transmission electron microscope. The CL responses of human PMNs to mutant Xc11 defective in a major cell surface antigen, PAc, and mutant Xc16 defective in two surface glucosyltransferases (GTF-I and GTF-SI) were the same as the response to the wild-type strain, Xc. In contrast, mutant Xc24R, which was defective in serotype c-specific polysaccharide, induced a markedly higher CL response than the other strains. The killing assay showed that human PMNs killed more Xc24R than the parent strain and the other mutants. The transmission electron microscopic observation indicated that Xc24R cells were more internalized by human PMNs than the parental strain Xc. These results may be reflected by the fact that strain Xc24R was more phagocytosed than strain Xc. The CL response of human PMNs to a mutant defective in polysaccharide serotype e or f was similar to the response to Xc24R. Furthermore, mutants defective in serotype-specific polysaccharide were markedly more hydrophobic than the wild-type strains and the other mutants, suggesting that the hydrophilic nature of polysaccharides may protect the bacterium from phagocytosis. We conclude that the serotype-specific polysaccharide, but not the cell surface proteins on the cell surface of S. mutans, may play an important role in the resistance to phagocytosis.

A novel gene required for rhamnose-glucose polysaccharide synthesis in Streptococcus mutans

Gene rgpG is required for biosynthesis of rhamnose-glucose polysaccharide (RGP) in Streptococcus mutans. Its deduced amino acid sequence had similarity to WecA, which initiates syntheses of enterobacterial common antigen and some O antigens in Escherichia coli. Gene rgpG complemented a wecA mutation of E. coli, suggesting that rgpG may function similarly in RGP synthesis.

Reactive oxygen species as a risk factor in verotoxin-1-exposed rats

It has been suggested the the interaction of Escherichia coli O157-derived verotoxins (VTs) with the vascular endothelium plays a central role in the pathogenesis of the thrombotic microangiopathy and ischemic lesions characteristic of hemolytic uremic syndrome (HUS) and E. coli O157-associated hemorrhagic colitis. Intravenous administration of both E. coli O157-derived VT1 and lipopolysaccharide (LPS) in the rat induced a synergistic increase in thiobarbituric acid (TBA) values in those animal's plasma, as compared with that injected with VT1 or LPS alone. We then hypothesized that an increase in lipid peroxidation in the rat plasma was due to an enhanced production of endothelial cell-derived reactive oxidant. Based on determination of rat sera and cultured human aortic endothelial cells (HAECs), VT1 had little if any effect on LPS-stimulated increase of nitric oxide and the resultant peroxynitrite generations. Both RT-PCR and Western blot studies of reactive oxygen species-related enzymes showed that VT1 markedly decreased the expression of catalase mRNA and protein in HAECs, but caused less alteration in the levels of Cu, Zn-superoxide dismutase, and NADPH oxidase mRNA. Further studies by spin trapping analysis using 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO) revealed a time-dependent increase in hydroxyl radicals by VT1 in HAECs. The accumulated data thus suggest that bacterial VT1 reduces mainly catalase levels in endothelial cells, which is synergistically potentiated by LPS, and that the resulting hydroxyl radical participates in endothelium injury through a marked enhancement of lipid peroxidation, leading to HUS.

Recombination between gtfB and gtfC is required for survival of a dTDP-rhamnose synthesis-deficient mutant of Streptococcus mutans in the presence of sucrose

The rml genes are involved in dTDP-rhamnose synthesis in Streptococcus mutans. A gene fusion between gtfB and gtfC, which both encode extracellular water-insoluble glucan-synthesizing enzymes, accompanied by inactivation of the rml genes was observed for cells grown in the presence of sucrose. The survival rates of rml mutants isolated in the absence of sucrose were drastically reduced in the presence of sucrose. The rates were consistent with the frequency of spontaneous gene fusions between gtfB and gtfC, suggesting that the spontaneous recombinant organisms were selected in the presence of sucrose. The rml mutants with a gtfB-gtfC fusion gene had markedly reduced water-insoluble glucan synthetic activity and lost the ability to colonize glass surfaces in the presence of sucrose. These results suggest that the rml mutants of S. mutans, which are defective in dTDP-rhamnose synthesis, can survive only in the absence of water-insoluble glucan synthesis.

The inducible nitric oxide synthase in vascular and cardiac tissue

Expression of the inducible form of nitric oxide synthase (iNOS) has been reported in a variety of cardiovascular diseases. The resulting high output nitric oxide (NO) formation, besides the level of iNOS expression, depends also on the expression of the metabolic pathways providing the enzyme with substrate and cofactor. NO may trigger short and long term effects which are either beneficial or deleterious, depending on the molecular targets with which it interacts. These interactions are governed by local factors (like the redox state). In the cardiovascular system, the major targets involve not only guanylyl cyclase, but also other haem proteins, protein thiols, iron-non-haem complexes, and superoxide anion (forming peroxynitrite). The latter has several intracellular targets and may be cytotoxic, despite the existence of endogenous defence mechanisms. These interactions may either trigger NO effects or represent releasable NO stores, able to buffer NO and prolong its effects in blood vessels and in the heart. Besides selectively inhibiting iNOS, a number of other therapeutic strategies are conceivable to alleviate deleterious effects of excessive NO formation, including peroxynitrite (ONOO-) scavenging and inhibition of metabolic pathways triggered by ONOO-. When available, these approaches might have the advantage to preserve beneficial effects of iNOS induction. Counteracting vascular hyper-responsiveness to endogenous vasoconstrictor agonists in septic shock, or inducing cardiac protection against ischaemia-reperfusion injury are examples of such beneficial effects of iNOS induction.

Streptococcus sanguis-induced platelet clotting in rabbits and hemodynamic and cardiopulmonary consequences

By mimicking hemostatic structural domains of collagen, Streptococcus sanguis (aggregation-positive phenotype; Agg+) induces platelets to aggregate in vitro. To test the hypothesis that aggregation occurs in vivo, S. sanguis (Agg+ or Agg- suspension) was infused intravenously into rabbits. The extent of hemodynamic and cardiopulmonary changes and the fate of circulating platelets were Agg+ strain dose dependent. Within 45 to 50 s of the start of infusion, 40 x 10(8) CFU of the Agg+ strain caused increased blood pressure. Thirty seconds after infusion, other changes occurred. Intermittent electrocardiographic abnormalities (13 of 15 rabbits), ST-segment depression (10 of 15 rabbits), and preventricular contractions (7 of 15 rabbits) manifested at 3 to 7 min, with frequencies dose dependent. Respiratory rate and cardiac contractility increased during this phase. Blood catecholamine concentration, thrombocytopenia, accumulation of 111Indium-labeled platelets in the lungs, and ventricular axis deviation also showed dose dependency. Rabbits were unaffected by inoculation of an Agg- strain. Therefore, Agg+ S. sanguis induced platelet aggregation in vitro. Platelet clots caused hemodynamic changes, acute pulmonary hypertension, and cardiac abnormalities, including ischemia.

Genes involved in cell wall localization and side chain formation of rhamnose-glucose polysaccharide in Streptococcus mutans

We identified in Streptococcus mutans six new genes (rgpA through rgpF), whose disruption results in a loss of serotype-specific antigenicity, specified by the glucose side chains of rhamnose-glucose polysaccharide from the cell wall. Rhamnose and glucose content of the cell wall decreased drastically in all these disruption mutants, except that in the rgpE mutant only the glucose content decreased. RgpC and RgpD are homologous to ATP-binding cassette transporter components and may be involved in polysaccharide export, whereas RgpE may be a transferase of side chain glucose.