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Dextran degrading activity of the human oral microflora was detected by culturing in TYD broth (Tryptose 1.0%, Yeast extract 0.3%, Dextran T-150 (Pharmacia, MW. 150,000) 0.15 %). All of the plaue and saliva samples collected from 10 subjects showed a dextran degrading activity, both cultured aerobically and anaerobically, while the anaerobic culture was more active than the aerobic. Furthermore, some individual differences were observed in their activity. Crude enzyme (s) was extracted from the supernatant of a mixed culture of plaque sample by adding ammonium-sulfate to 0.6 and 0.8 saturation (called E 1 and E-2, respectively). E-1 contained 2 dextran-degrading enzymes, one being thought to be an endoenzyme, with the optimal pH being 5.0 and the other an exo-enzyme with the optimal pH being 7.0-7.5. On the other hand, E-2 contained 1 enzyme of the endo-type. Thirteen strains producing dextranase were isolated rfom the plaque and were identified as Bacteroides oralis-like organisms. Several other organisms were thought to produce dextranase, although we failed to isolate them in this experiment.

The distribution of dextran-degrading microorganisms in the saliva and plaque samples from the human oral cavity was assayed on 9 subjects. Approximately 2/ 3 of the saliva samples degraded Dextran T-150 (Pharmacia, M.W. 150,000) and 1/10 the Blue Dextran (Pharmacia), while 2/5 and 1/8 of the plaque samples degraded Dextran T-150 and Blue Dextran, respectively. Thirty-seven strains of the Blue Dextran degrading bacteria were isolated from the saliva and plaque samples and were classified into 6 groups by their morphology, gram staining and oxygen tolerance. The 24 strains from the 37 isolates, more or less, were shown to degrade the insoluble glucan extracted from Streptococcus mutans FA-1 on the agar plate. The 8 strains, selected from each group, were tested for their activity against the insoluble glucan extracted from Str. mutans (FA-1, HS-6, BHT, CHT, GS-5, LM-7 and PK-1) and Str. salivarius (HHT). The strains belonging to Groups I, II, IV and V showed activity against the insoluble glucan used. Among them, the strains of Group IV (gram negative facultative cocci) and Group V (gram negative strict anaerobic rod) were the most active against the insoluble glucan.