Contributions of Three Glucosyltransferases to Sucrose-dependent Adherence of Streptococcus mutans

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Abstract
Streptococcus mutans produces 3 types of glucosyltransferase (GTF), whose cooperative action is considered to be essential for its cellular adherence to the tooth surface. However, the precise mechanisms for synthesizing adhesive glucans and the specific roles of each GTF in cellular adherence to smooth surfaces have not been elucidated. In the present study, seven types of isogenic mutants of S. mutans MT8148 lacking GTFB, GTFC, and/or GTFD activities were constructed by inactivation of the genes encoding GTFB, GTFC, and/or GTFD. Furthermore, recombinant GTFB, GTFC, and GTFD were prepared from Escherichia coli cells harboring recombinant plasmids containing each of the gtf genes. Using these GTF-deficient mutants and rGTFs, we reconstituted sucrose-dependent adherence of S. mutans resting cells and examined the role of each GTF in vitro. The highest level of sucrose-dependent adherence was found at the ratio of 20 rGTFB:l rGTFC:4 rGTFD in both the resting cells of GTF-deficient mutants and insoluble glucan synthesized by rGTFs. Moreover, when rGTFC and rGTFD were both present at concentrations of 1.5 mU and 6 mU, respectively, the insoluble glucan synthesized from sucrose by the rGTFs showed a high level of adhesiveness to smooth surfaces, even without rGTFB. These results suggest that the presence of all three GTFs at the optimum ratio is necessary for sucrose-dependent adherence of S. mutans, and that GTFC and GTFD may play significant roles in the synthesis of adhesive and insoluble glucan from sucrose.