~~~~ ~Dextranase activity was determined in cell extracts and cell-free filtrates of Streptococcus mutans strains which had been grown in batch culture. Exo-dextranase activity was located chiefly in cell extracts, whereas endodextranase was mainly extracellular. Release of endo-dextranase began early in the exponential phase of growth, and ended when the concentration of residual sugar was low. Thus, dextranase expression was associated with rapidly growing cells, and the yield of dextranase was increased several fold when the initial concentration of D-glucose in the medium was changed from 0.5% to 2%. The endo-dextranase was not stable at pH 5 , and control of the pH of the culture was essential to preserve active dextranase during overnight growth. Strain Ingbritt (serotype c) and serotype d strains were the best dextranase producers; other strains (serotypes a, b, c, e and f) displayed much lower activity. The ability to produce endo-dextranase, and to synthesize a-D-glucans with a high proportion of (1 -3)-linked sequences, appeared to be related properties. The possibility is discussed that the release of two enzymes, namely endodextranase and the D-glucosyltransferase (GTF-I) that synthesizes ( 1 +3)-a-D-glUCan, are factors that contribute to the cariogenicity of S . mutans serotype d.
The production of extracellular beta-D-fructanase by several strains of Streptococcus mutans was studied in continuous culture. When glucose was the limiting nutrient, S. mutans K1-R and OMZ176 accumulated fructanase to maximum levels at low growth rates (dilution rate 0.05-0.10 h-1), due to the longer residence times of the bacteria in the culture vessel under these conditions. Extracellular fructanase activity was greater than has been previously reported for batch cultures. The rate of fructanase production for both S. mutans strains K1-R and OMZ176 increased with increasing growth rate when glucose was limiting. Under conditions of glucose sufficiency, the rate of fructanase production was always lower than in cultures where glucose was limiting, irrespective of the growth rate. Cultures of S. mutans Ingbritt (serotype c) grown with sorbitol- or glucose-limitation synthesized fructanase at a very low basal rate. When fructose was the limiting carbohydrate the enzyme was induced with a maximum rate of production occurring at a dilution rate of 0.40 h-1. Strains of S. mutans from other serotypes (a, d, d/g) were either not affected by changing the limiting sugar from glucose to fructose or else fructanase activity was slightly decreased in the fructose-limited medium. Fructanases from various strains of S. mutans readily hydrolysed (2----6)-beta-D-fructans, but all possessed the ability to hydrolyse (2----1)-beta-D-fructans to varying degrees.
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