Properties of Streptococcus mutans grown in a synthetic medium: binding of glucosyltransferase and in vitro adherence, and binding of dextran/glucan and glycoprotein and agglutination

Anti-bacterial and anti-biofilm activities of arachidonic acid against the cariogenic bacterium Streptococcus mutans

Streptococcus mutans is a Gram-positive, facultative anaerobic bacterium, which causes dental caries after forming biofilms on the tooth surface while producing organic acids that demineralize enamel and dentin. We observed that the polyunsaturated arachidonic acid (AA) (ω-6; 20:4) had an anti-bacterial activity against S. mutans, which prompted us to investigate its mechanism of action. The minimum inhibitory concentration (MIC) of AA on S. mutans was 25 μg/ml in the presence of 5% CO2, while it was reduced to 6.25-12.5 μg/ml in the absence of CO2 supplementation. The anti-bacterial action was due to a combination of bactericidal and bacteriostatic effects. The minimum biofilm inhibitory concentration (MBIC) was the same as the MIC, suggesting that part of the anti-biofilm effect was due to the anti-bacterial activity. Gene expression studies showed decreased expression of biofilm-related genes, suggesting that AA also has a specific anti-biofilm effect. Flow cytometric analyses using potentiometric DiOC2(3) dye, fluorescent efflux pump substrates, and live/dead SYTO 9/propidium iodide staining showed that AA leads to immediate membrane hyperpolarization, altered membrane transport and efflux pump activities, and increased membrane permeability with subsequent membrane perforation. High-resolution scanning electron microscopy (HR-SEM) showed remnants of burst bacteria. Furthermore, flow cytometric analysis using the redox probe 2',7'-dichlorofluorescein diacetate (DCFHDA) showed that AA acts as an antioxidant in a dose-dependent manner. α-Tocopherol, an antioxidant that terminates the radical chain, counteracted the anti-bacterial activity of AA, suggesting that oxidation of AA in bacteria leads to the production of cytotoxic radicals that contribute to bacterial growth arrest and death. Importantly, AA was not toxic to normal Vero epithelial cells even at 100 μg/ml, and it did not cause hemolysis of erythrocytes. In conclusion, our study shows that AA is a potentially safe drug that can be used to reduce the bacterial burden of cariogenic S. mutans.

Hydroquinone, a control agent of agglutination and adherence of Streptococcus mutans induced by sucrose

Hydroquinone was found to alter agglutination of Streptococcus mutans induced by sucrose. The newly formed agglutination product produced by hydroquinone does not kill this cariogenic bacterium and the formation is reversible. The agglutination altering activity of hydroquinone seems to be specific for strains of S. mutans. As a result, hydroquinone inhibits sucrose-induced adherence of S. mutans.

Glucan-binding proteins of Streptococcus sobrinus B13 grown in high glucose media

To improve the yield of an 87-kDa glucan-binding protein (GBP) of Streptococcus sobrinus B13 (serogroup d), trypticase-yeast extract (TYE) medium supplemented with higher (1 and 2%) than the usual amount (0.2%) of glucose was used for growth. The production of this GBP extracellularly in 1.0 and 2.0% glucose-TYE media was examined and compared with the control (0.2% glucose). Upon analysis using SDS-PAGE, extracellular culture concentrates of 1.0 and 2.0% glucose-TYE cultures revealed similar protein profiles as the control. Higher glucose concentrations did not inhibit the synthesis of the 87-kDa GBP. Cells grown in 1.0 or 2.0% glucose-supplemented media aggregated rapidly compared to those observed in the control cells (0.2% glucose grown). Higher cell yield and higher extracellular protein content were obtainable in both 1.0 and 2% glucose-TYE cultures, thus improving the yield of the 87 kDa GBP.

An 87-kilodalton glucan-binding protein of Streptococcus sobrinus B13

An 87-kDa glucan-binding protein (GBP) of Streptococcus sobrinus B13 (serotype d) was isolated and purified from extracellular culture supernatant by using affinity chromatography on Sephadex G-50 and elution with a guanidine HCl gradient. Western blot (immunoblot) analysis showed it to be antigenically related, but not completely identical, to the 74-kDa GBP of Streptococcus mutans Ingbritt. The 87-kDa GBP has no glucosyltransferase activity. A possible role for this GBP in the cariogenicity of S. sobrinus B13 is suggested.

Glucans synthesized in situ in experimental salivary pellicle function as specific binding sites for Streptococcus mutans

Many researchers have suggested that the role of glucan-mediated interactions in the adherence of Streptococcus mutans is restricted to accumulation of this cariogenic bacterium following its sucrose (i.e., glucan)-independent binding to saliva-coated tooth surfaces. However, the presence of enzymatically active glucosyltransferase in salivary pellicle suggests that glucans could also promote the initial adherence of S. mutans to the teeth. In the present study, the commonly used hydroxyapatite adherence assay was modified to include the incorporation of glucosyltransferase and the synthesis of glucans in situ on saliva-coated hydroxyapatite beads. Several laboratory strains and clinical isolates of S. mutans were examined for their ability to adhere to experimental pellicles, either with or without the prior formation of glucans in situ. Results showed that most strains of S. mutans bound stereospecifically to glucans synthesized in pellicle. Inhibition studies with various polysaccharides and fungal dextranase indicated that alpha 1,6-linked glucose residues were of primary importance in the glucan binding observed. Scanning electron microscopic analysis showed direct binding of S. mutans to hydroxyapatite surface-associated polysaccharide and revealed no evidence of trapping or cell-to-cell binding. S. mutans strains also attached to host-derived structures in experimental pellicles, and the data suggest that the bacterial adhesins which recognize salivary binding sites were distinct from glucan-binding adhesins. Furthermore, glucans formed in experimental pellicles appeared to mask the host-derived components. These results support the concept that glucans synthesized in salivary pellicle can promote the selective adherence of the cariogenic streptococci which colonize human teeth.

Gallotannins inhibit growth, water-insoluble glucan synthesis, and aggregation of mutans streptococci

During screening for anti-plaque agents of plant origin, ethanolic extracts from Melaphis chinensis (Bell), the Chinese Nutgall, exhibited strong inhibition of glucosyltransferase (GTF), in vitro adherence and glucan-induced agglutination of Streptococcus mutans 3209 and S. sobrinus B13. More than 91% inhibition of water-insoluble glucan synthesis from sucrose by GTF was noted at a concentration as low as 7.8 micrograms/mL. Bactericidal effects on other mutans streptococci, S. salivarius, and Actinomyces viscosus were also evident. Through chemical fractionation and analyses, along with bioassays, the active components were identified as gallotannins.

Attachment of Treponema pallidum to fibronectin, laminin, collagen IV, and collagen I, and blockage of attachment by immune rabbit IgG

As shown by scanning electron and phase contrast microscopy, Treponema pallidum attached in vitro to basement membranes purified from kidney cortex tissues or from retinal vessels. This organism also attached to the extracellular matrix remaining after cultured cells had been solubilised with Triton X. Fibronectin, laminin, collagen, IV, collagen I, and hyaluronic acid are structural components of basement membranes and extracellular matrices. Experiments were performed to investigate the in vitro attachment of T pallidum to each of these components. Viable or heat inactivated treponemes were added to glass coverslips precoated with different concentrations of each component. After various times of incubation, coverslips were washed and the attached organisms were counted. Large numbers of viable organisms attached to each of these five components. In contrast, heat inactivation sharply reduced numbers of attached organisms. The IgG fractions of immune and non-immune rabbit serum samples were affinity purified using protein A. T pallidum was preincubated with both fractions, then incubated with either intact cultured cells or with coverslips coated with the five tissue components. The IgG from immune serum blocked treponemal attachment to the cultured cells and to fibronectin, laminin, collagen IV, and collagen I, but not to hyaluronic acid. These results are discussed in terms of attachment mechanisms of T pallidum and potential applications to in vivo infection.

Specific and non-specific affinities of the extracellular glucosyltransferase complex of Streptococcus mutans 6715

Glucosyltransferases (GTF) from different strains of streptococci exhibited different elution profiles when fractionated on insoluble-dextran affinity columns. The proportions of unadsorbed and adsorbed GTF were not related to their extent of stimulation by exogenous dextran, and GTF preparations exposed to, and freed from, clinical dextran prior to fractionation lost their ability to bind to the dextran columns. Different proportions of bound GTF were released by irrigation of columns with different concentrations of salt and clinical dextran, and the "specific" binding and release of GTF exhibited by a column possessing covalently linked, clinical dextran ligands was duplicated on a control column that did not possess the dextran ligands. These results, and the high affinity of GTF for hydrophobic alkyl (Shaltiel) ligands, demonstrate that ionic and hydrophobic properties of impure GTF aggregates may lead to erroneous characterization of the dextran affinity of some protein fractions. Fractionations on DEAE-Sepharose and on hydroxylapatite showed that the two dextran-dependent GTF activities (GTF-S and GTF-I) were present in the major enzyme fraction (Streptococcus mutans 6715) recovered from a Sephacryl S-200 affinity column. A minor, dextran-independent GTF was not adsorbed onto the Sephacryl column. The presence of SDS (0.005%) and Triton X100 (0.01%) stabilized GTF activity during gel filtration and improved the separation of GTF-S and GTF-I in hydroxylapatite fractionation of the highly aggregated enzyme. A comparable separation of the two enzyme forms on DEAE-Sepharose was achieved only if T10 dextran (10 mg/mL) was included with the detergent mixture in the column irrigant.

An improved method for measuring aggregation of certain streptococcal bacteria found in dental plaque

Various cells were incubated with either dextran or sucrose and the reaction was terminated by the addition of a 20 per cent solution of formaldehyde. A reaction mixture consisting of cells and aggregated cells was applied to a step-gradient glycerol column (0-60 per cent) and 0.5 ml fractions were collected from the bottom of the column. Non-aggregated cells remained in the top layer of the column. Aggregated cells settled in the 30 per cent glycerol layer as determined by either spectrophotometric or radioactive methods. The amount of aggregated cells demonstrated in this layer increased with either incubation time or concentration of sucrose, dextran or cells. Cells of Strep, mutans strains 6715, AHT, 10449, OMZ-176, OMZ-175 and two mutants of Strep. mutans strain 6715 (designated C4 and C307) aggregated strongly. Cells of Strep. mutans strain LM7 aggregated weakly, whereas cells of Strep. mutans 6715 mutant UAB165, a mutant defective in aggregation. Strep. mutans strain BHT, Strep. salivarius and Strep. sanguis did not aggregate in the presence of either sucrose or dextran. Experiments testing co-aggregation between two different types of cells were done by using 3H-labelled and 14C-labelled cells. Strep. mutans strain 6715 co-aggregated with Strep. sanguis in the presence of sucrose or dextran, whereas Strep. mutans strain 6715 did not aggregate with Strep. salivarius. The method separates aggregated from non-aggregated cells and enables quantitation of sucrose- or dextran-induced aggregation and co-aggregation.

Effects of local immunization with glucosyltransferase on colonization of hamsters by Streptococcus mutans

Experiments were performed to study the effect of antibody to Streptococcus mutans glucosyltransferase (GTF) on the implantation of these organisms in hamsters. Salivary (immunoglobulin A) and serum (immunoglobulin G) antibodies to GTF and GTF-inhibiting activity were elicited by injection of GTF in Freund complete adjuvant in the salivary gland region. Sham-immunized and GTF-immunized groups were then orally challenged with approximately 10(7), 10(8), or 10(9) colony-forming units of cariogenic S. mutans 6715. The results were evaluated by systematically swabbing molars 4 days and approximately 4 weeks after challenge. In general, fewer GTF-immunized hamsters became infected with S. mutans after challenge with 10(7) or 10(8) organisms than did identically challenged sham-immunized hamsters. Of the animals that did become infected, fewer S. mutans colony-forming units were recovered from GTF-immunized hamsters. These results indicate that the presence of antibody to GTF can diminish the ability of S. mutans to implant in the oral cavity of immunized hamsters.

Laser light scattering measurement of dextran-induced Streptococcus mutans aggregation

Intensity fluctuation spectroscopy was used to study dextran-induced aggregation of Streptococcus mutans bacteria. Smoluchowski's theory of colloidal flocculation provided a consistent model of the agglutination process. Our experiments indicated that aggregation was inhibited by the negatively charged surfaces of the cells, while dextran polymers effectively bound organisms together. Our experimental data were consistent with the quantitative predictions of a polymer bridge model of agglutination.

Effects of molecular weight of dextran on the adherence of Streptococcus sanguis to damaged heart valves

Dextran-producing streptococci such as Streptococcus sanguis are the organisms most frequently associated with infective endocarditis in humans. A series of experiments was designed to study how the molecular weight of dextrans affects the adherence of an endocarditis strain of S. sanguis to canine heart valves covered with platelets and fibrin. The data indicated that this adherence was dependent on dextrans of high molecular weight, such as dextran T-2000 or glucans isolated from S. sanguis or S. mutans. The adherence properties of the strain studied were not modified by prior exposure of the bacterial cells of valve leaflets to high-molecular-weight dextrans. Preexposure of bacterial cells or valve leaflets to low-molecular-weight dextrans decreased their adherence. Low-molecular-weight dextrans interfered with adherence of dextran-positive strains to damaged heart valves.

Biology, immunology, and cariogenicity of Streptococcus mutans

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Streptococcus mutans adherence: presumptive evidence for protein-mediated attachment followed by glucan-dependent cellular accumulation

Adherence of Streptococcus mutans to smooth surfaces has been attributed to the production of sucrose-derived d-glucans. However, several studies indicate that the bacterium will adhere in the absence of sucrose. The present data confirmed that S. mutans adherence to saliva-coated hydroxyapatite beads in the absence of sucrose is described by the Langmuir equation. The nature of the sucrose-independent adherence was studied with the Persea americana agglutinin as a selective adherence inhibitor. Pretreatment of the bacterium with P. americana agglutinin caused a 10-fold reduction in adherence, and the inhibition was not reversed with the addition of sucrose. Pretreatment of S. mutans with proteases also reduced adherence, regardless of the sucrose content, whereas periodate oxidation and glucanohydrolase treatment of the bacteria reduced sucrose-mediated adherence to the levels found for sucrose-independent adherence. The P. americana agglutinin, glucanohydrolase, and pepsin pretreatment of the cells did not eliminate sucrose-induced agglutination. Scanning electron microscopy showed that short streptococcal chains were bound to saliva-coated hydroxyapatite crystals in the sucrose-independent system, whereas the presence of sucrose caused larger bacterial clumps to be found. A two-reaction model of S. mutans adherence was developed from these data. It is proposed that one reaction is attachment to the tooth pellicle which is mediated by cell-surface proteins rather than glucans or teichoic acids. The other reaction is cellular accumulation mediated by sucrose-derived d-glucans and cell surface lectins. A series of sequential adherence experiments with P. americana agglutinin as a selective inhibitor provided presumptive evidence for the validity of our model of S. mutans adherence.