A STREPTOCOCCUS-MUTANS MUTANT THAT SYNTHESIZES ELEVATED LEVELS OF INTRACELLULAR POLYSACCHARIDE IS HYPERCARIOGENIC IN-VIVO

We used the streptococcal transposon, Tn916 to identify and isolate mu tants of Streptococcus mutans with altered intracellular polysaccharid e (IPS) accumulation. We report on the isolation and characterization of S. mutans SMS202, a transposon mutant which accumulated the glycoge n-like IPS in excess of wild-type levels. Southern blot analysis confi rmed a single Tn916 insertion into the SMS202 chromosome. Moreover, qu antitative ultrastructural analysis revealed significantly increased c oncentrations of IPS in SMS202 relative to those of the wild-type prog enitor strain, UA130. The activities of ADPglucose pyrophosphorylase ( GlgC) and glycogen synthase (GlgA), enzymes required for the biosynthe sis of bacterial IPS, were also elevated in the IPS excess mutant. Fur thermore, SMS202 was significantly more cariogenic on the molar surfac es of germ-free rats than the wild type (P < 0.01), thus confirming a central role for IPS in S. mutans-induccd caries formation. We propose that the increased cariogenic potential of SMS202 is due to constitut ive expression of genes which encode glycogen biosynthesis in this ora l pathogen. The coordinate expression of GlgC and GlgA along with the results of ongoing nucleotide sequence analysis and Northern hybridiza tion experiments support an operon-like arrangement for the glg genes of this oral pathogen.