Physical and biochemical studies of Streptococcus mutans sediments suggest new factors linking the cariogenicity of plaque with its extracellular polysaccharide content.

Cultures of Streptococcus mutans MFe28 (serotype h) were grown with differing extracellular polysaccharide (EPS) content. Biochemical and physicochemical characteristics considered relevant to caries were measured. Acid production parameters measured in a pH-stat were: Vm = 0.76 +/- 0.14 mumol/g/sec (wet weight); apparent Km (acid production) = 100 mumol/L; molar yield = 1.97 +/- 0.25 mol acid/mol glucose. Acid anion inhibition of acid production was also noted. Buffering by the pure washed bacterial residue required approx. 112 mumol of base/g (wet weight) of residue to change the pH from 4 to 6.5, and this dropped almost to zero as the EPS content increased to 100%. Diffusion coefficients (D) in the residues were independent of EPS content over a wide range. When the effusion method was used, De (glucose) and De (acetate) were (3.26 +/- 0.6) and (5.05 +/- 0.8) x 10(-6) cm2/sec, respectively. The extracellular fluid fraction, measured by inulin exclusion, increased from 0.33 for the pure bacteria to 0.78 for the pure EPS. It is shown how, by these factors alone, and without any need for diffusion restriction, plaque EPS may lead to a lower pH at the tooth surface, thus increasing the cariogenic challenge.