Cariogenicity of Streptococcus mutans strains with defects in fructan metabolism assessed in a program-fed specific-pathogen-free rat model

To define the role of dental plaque fructans and the enzymes involved in their metabolism in the initiation and progression of dental caries, we constructed otherwise-isogenic mutants of Streptococcus mutans defective in the ability to synthesize fructans, to degrade fructans, or to do both. The cariogenic potential of these organisms was evaluated in a specific-Pathogen-free rat model in which the feeding patterns of the animals were controlled by means of a König-Hofer programed feeder. Specifically, rats were infected with wild-type S. mutans UA159 or derivatives of this strain which contained an insertionally-inactivated fructanase (fruA) gene, fructosyltransferase (ftf) gene, or which had both genes inactivated. The animals were fed 17 meals per day of Diet 2000 containing 56% sucrose at 70-minute intervals for five weeks, and caries experience was evaluated. Animals infected with S. mutans with a mutated fruA gene only had statistically significant decreases in sulcal caries severity. Such a decrease was not observed in previous studies with ad libitum-fed animals (Wexler et al., 1992). The manifestation of diminished virulence in the programed feeding model, but not in ad libitum-fed animals, supports the concept that the primary contribution of FruA to virulence is through the utilization of fructans storage polysaccharides. Animals infected with strains carrying the ftf mutation or simultaneous mutations in ftf and fruA did not display decreased virulence, perhaps indicating that sucrose utilization pathways may compete for substrate in vivo, or that accumulation of fructans may affect the ecology or the physicochemical characteristics of dental plaque in such a way as to reduce its cariogenic potential. The results of this study also emphasize that the contribution of a particular virulence determinant to the caries process may be highly dependent on the experimental design, feeding regimen and diet, and the presence or absence of other enzymatic activities.