Polymers of D-
fructose produced by a variety of oral bacteria are believed to function as extracellular
carbohydrate reserves. Degradation of these
polysaccharides in plaque following exhaustion of
dietary carbohydrates is thought to contribute to the extent and duration of the
acid challenge to the tooth surface and thus to the initiation and progression of
dental caries. Streptococcus mutans produces a
fructanase, the product of the fruA gene, which is capable of degrading beta(2,6)- and beta(2,1)-linked
fructans that are commonly synthesized by
dental plaque microorganisms. To evaluate the role of the FruA
protein in exopolysaccharide metabolism and to assess the contribution of this
enzyme to the pathogenic potential of S. mutans, a
fructanase-deficient strain of S. mutans was constructed. Inactivation of a cloned fruA gene was accomplished in Escherichia coli by using a mini-Mu dE transposon, and then an isogenic mutant of S. mutans UA159 was constructed by allelic exchange. Successful inactivation of fruA was confirmed through the use of biochemical assays, Western blotting (immunoblotting) with anti-recombinant FruA
antisera, and Southern hybridization. The data indicated that FruA was the only
fructan hydrolase produced by S. mutans UA159. Inactivation of fruA had no significant effects on
glucosyltransferase or
fructosyltransferase activity. In the rat caries model using animals fed a high-
sucrose diet and ad libitum, there were no significant differences in the number or severity of smooth surface, sulcal, or
root caries elicited by the fruA mutant and the wild-type organism.