Effect of inactivation of gtf genes on adherence of Streptococcus downei

The activity of glucosyltransferases (GTF), a group of enzymes that synthesize water-soluble and -insoluble glucans from sucrose, significantly contributes to the cariogenicity of mutans streptococci. Streptococcus downei produces 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 now examined its interaction with other enzymes by constructing mutants which were gtfS, gtfS/gtfT, gtfS/gtfI and gtfI. The mutants were tested for their ability to accumulate on wires and on plastic microtiter trays in the presence of sucrose. The gtfS mutant displayed a reduced ability to adhere compared 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 drastic reduction in adherence and when gtfI alone was inactivated, bacteria were 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 amount and structure of this glucan is dependent upon the availability of soluble glucans to act as primer molecules.