Effect of inactivation of gtf genes on adherence of Streptococcus downei

The activity of glucosyltransferases (GTF), a group of enzymes that synthes ize water-soluble and -insoluble glucans from sucrose, significantly contri butes to the cariogenicity of mutans streptococci. Streptococcus downei pro duces four glucosyltransferases, GTFI, which produces insoluble glucan, and GTFS, GTFT, and GTFU, which synthesize soluble glucans. We have previously reported that inactivation of gtfS results in altered adherence and have n ow examined its interaction with other enzymes by constructing mutants whic h were gtfS, gtfS/gtfT, gtfS/gtfI and gtfI. The mutants were tested for the ir ability to accumulate on wires and on plastic microtiter trays in the pr esence of sucrose. The gtfS mutant displayed a reduced ability to adhere co mpared to the wild type but there was no further reduction of adherence in a gtfS/gtfT mutant. In contrast, the gtfS/gtfI double mutant showed a drast ic reduction in adherence and when gtfI alone was inactivated, bacteria wer e unable to adhere to a hard surface. The results confirmed that insoluble glucan is required for strong adherence to a smooth surface but that the am ount and structure of this glucan is dependent upon the availability of sol uble glucans to act as primer molecules.