More sensitive test agar for detection of dextranase-producing oral streptococci and identification of two glucan synthesis-defective dextranase mutants of Streptococcus mutans 6715.

A more sensitive test agar was developed to detect oral microbes with relatively low dextranase activity and to identify dextranase-negative mutants. Several oral streptococci that had previously been scored as dextranase negative readily decolorized the new, blue dextran-containing medium. To assess whether dextranase plays a role in glucan synthesis by oral streptococci, various glucan synthesis-defective mutants were tested for dextranase activity on the new medium. Mutants 4 and 27, which do not cause smooth-surface caries and which synthesize more soluble glucan than their parent, Streptococcus mutans 6715-13, were markedly deficient in these dextranase activity tests.