Dextranase activity of streptococcal isolates from human dental plaques

Streptococci were isolated from sixty human dental plaques. Of those isolates which exhibited dextranolytic activity on culture media containing dextran, three were selected for further investigation. Two of these isolates were identified as Streptococcus mitior and the other was a strain of Streptococcus mutans. The organisms were cultivated in a dialysed medium and enzyme activities isolated from culture supernatants by precipitation with ammonium sulphate, dialysis and lyophilization. Decolourized annuli surrounding wells in agar plates containing Blue Dextran 2000 provided evidence of dextranase activity present in culture supernatants, precipitated and redissolved proteins, dialysis retentates and lyophilized materials. The pH optima for the crude enzyme preparations were determined using the modified assay of Koh and Khouw (1970). Dextranases from the Streptococcus mitior isolates had optima at pH 6, and the optimal activity for the enzyme from the Streptococcus mutans isolate occurred at pH 5.5. Additional studies of the purified enzymes are necessary to increase understanding of the possible effects of these enzymes in human dental plaques.