Ultrastructural localization of sucrases in Streptococcus mutans GS-5 and an extracellular polysaccharide mutant: a comparative cytochemical and immunocytochemical study

Synthesis of glucan on the cell surface of Streptococcus mutans: chemical and scanning electron microscopic studies

The appearance and continuing growth of extracellular material on Streptococcus mutans HS6 cells in sucrose-containing Merthiolated buffer was observed in a scanning electron microscope and was found to be related to the glucan synthesis on the cell and to adherence of the cell to a smooth surface. Cells grown in broth completely deprived of sucrose by invertase (HS6-IV) had a characteristic, slightly rugged surface structure. On incubation of HS6-IV in the sucrose-containing buffer, a few small globular particles appeared on the surface and grew to an irregular shape (globular to fibrilar) after several hours. The increase in the total glucan content of the cells paralleled the growth of the globular material, to which ferritin-conjugated anti-dextran globulin was found to bind. On the cell surface of cells harvested from conventional broth, both small globular and irregular structures, which possibly formed from sucrose in the broth, existed originally and continued to grow during incubation, along with the material newly appearing on the surface. The accumulation of glucan on the cells resulted in their adherence to a glass surface. The inhibition of growth of the extracellular material on the cells by trypsin, dextranase or anti-glucosyltransferase corresponded to the decrease in glucan synthesis and the loss of adhering ability. These results indicated that the material growing on the cell surface was glucan synthesized by glucosyltransferases.

Localization of Streptococcus mutans GS-5 glucosyltransferases and intracellular invertase

Antibodies directed against Streptococcus mutans GS-5 intracellular invertase and glucosyltransferase fractions capable of synthesizing primarily water-soluble or insoluble glucans were used to ultrastructurally localize the enzymes by means of the unlabeled antibody peroxidase-antiperoxidase method. This immunocytochemical procedure revealed that the intracellular invertase was associated primarily with the cytoplasmic membrane of the cariogenic organism. The glucosyltransferase complex responsible for insoluble glucan synthesis was localized as aggregates attached to the cell surface or extracellular polysaccharides of strain GS-5. In contrast, the glucosyltransferase activity synthesizing primarily water-soluble glucans was distributed uniformly over the cell surface or in association with extracellular polysaccharides. These results are discussed relative to the sucrose-metabolizing ability of Streptococcus mutans.

Comparison of the glucosyltransferases of Streptococcus mutans gs-5 to a non-cariogenic mutant

The extracellular glucosyltransferase (GTF) enzymes of Streptococcus mutans GS-5 and avirulent mutant GS-511 were fractionated using agarose and DEAE-cellulose columns. GTF of GS-5 produced both water-soluble and -insoluble glucans, while those of GS-511 made soluble products almost exclusively. Nearly all (98%) of the small amount of insoluble GTF enzyme made by GS-511 was bound to the cell wall.

Biology, immunology, and cariogenicity of Streptococcus mutans

Images

Resolution of the glycosyltransferase activities from two strains of Streptococcus mutans by polyacrylamide gel electrophoresis in the presence of Tween 80

The glycosyltransferase complex from Streptococcus mutans can be resolved by polyacrylamide gel electrophoresis following treatment with Tween 80. The enzyme complex was treated with Tween 80 and electrophoresis was performed in the presence of Tween 80. Discrete bands of glycosyltransferase activity were observed when the gels were incubated in buffered sucrose. The enzyme aggregation that occurs with the exocellular glycosyltransferase may involve lipophilic interactions that are disrupted by the Tween 80.

Effect of sucrose in culture media on the location of glucosyltransferase of Streptococcus mutans and cell adherence to glass surfaces

Streptococcus mutans strain B13 (serotype D) almost exclusively produced free glucosyltransferase (GTase) in the culture supernatant when grown in sucrose-free TTY broth medium, which was composed of Trypticase (Baltimore Biological Laboratory [BBL] Cockeysville, Md.), tryptose (Difco Laboratories, Detroit, Mich.), yeast extract (BBL), salts, and 1% glucose. Organisms grown in sucrose-free TTY broth retained very weak cell-associated GTase activity and did not adhere significantly to glass surfaces in the presence of exogenous sucrose. If sucrose was added to TTY broth, however, GTase was found on the cell surface where cell-bound, water-insoluble glucans were synthesized. Most commercially available products of Todd-Hewitt broth were found to contain trace amounts of sucrose, as did Trypticase soy broth (BBL), whereas brain heart infusion broth (Difco and BBL) was found to be essentially free of sucrose. Almost all detectable GTase activity was cell associated when S. mutans B13 was grown in Todd-Hewitt or trypticase soy broth. Heat-treated B13 cells grown in Todd-Hewitt broth and cell-free, water-insoluble glucans bound free GTase and produced marked adherence in the presence of sucrose. Experiments strongly suggest that the binding sites for free GTase are the surface glucans, and cell-associated and extracellular GTases are most likely alternate states of the same enzyme protein.

Interaction of Streptococcus mutans glucosyltransferases with antibodies

1) The GTF-A and GTF-B activities of serotype (c), (e), and (f) organisms are antigenically more closely related to each other than to the comparable enzymes from organisms of the other four S. mutans serotypes. 2) The adherence-inhibiting antibody in anti-GTF-A appears to be mediated against a heat sensitive cell surface antigen of S. mutans - most likely GTF activity. However, unequivocal proof of this conclusion awaits the preparations of homogeneous GTF-A preparations.

Genetic alterations of Streptococcus mutans' virulence

The use of mutants defective in caries-associated traits has enabled the genetic dissociation of agglutination from adhesion, the demonstration of serotype-specific contributions of IPS to virulence, the importance of glucanohydrolase to virulence to a greater degree than to plaque formation, and the apparent lack of importance of agglutination to virulence. We have also been able to demonstrate the ability of plaque formation-defective mutants and other variants both to infect and to emerge, yet not to cause disease. Additional mutants, currently under study in our laboratory include fructanase, invertase, and sucrose permease-defectives. Ultimately, the identification of key, probably surface-associated virulence factors will offer more potent and specific antigens for directed immune responses by the host.