Fructan polymer, synthesized from
sucrose by the extracellular
fructosyltransferase of Streptococcus mutans, is thought to contribute to the progression of
dental caries. It may serve as an extracellular storage
polysaccharide facilitating survival and
acid production. It may also have a role in adherence or accumulation of bacterial cells on the tooth surface. A number of clinical isolates of S. mutans which produce large, mucoid colonies on
sucrose-containing
agar as a result of increased production of
fructan have been discovered. By using eight independent isolates, we sought to determine if such
fructan-hyperproducing strains represented a genetically homogeneous group of organisms. Restriction fragment patterns of total cellular
DNA were examined by using pulsed-field and conventional gel electrophoresis. Four genetic types which appeared to correlate with the serotype of the organism and the geographic site of isolation were evident. Southern blot analysis of several genetic loci for extracellular
enzymes revealed some minor differences between the strains, but the basic genomic organizations of these loci were similar. To evaluate whether the excess
fructan produced by these strains enhanced the virulence of these organisms in the oral cavity, it was of interest to create mutants deficient in fructosidase (FruA), the extracellular
enzyme which degrades this
polymer. The fruA gene was inactivated by allelic exchange in two
fructan-hyperproducing strains as well as in S. mutans GS5, a strain which does not hyperproduce
fructan. All of the fruA mutant strains were devoid of
fructan hydrolase activity when
levan was used as a substrate. However, the
fructan-hyperproducing strains retained the ability to hydrolyze
inulin, suggesting the presence of a second fructosidase with specificity for
inulin in these strains.