In a preceding study, an index was given for bacteria with intermediate tolerances for oxygen which could express their aerobic to anaerobic potentials in a quantitative manner. The purpose of this paper is to examine to what extent changes in that index might relate with the oxidoreductase activities in those oral indigenous bacteria under different oxygen conditions. The oxidoreductases studied in bacteria grown under aerobic and anaerobic conditions include NADH peroxidase, peroxidase,

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... xide dismutase (SOD), and catalase. Isozyme of SOD was also studied in some bacteria using disc polyacrylamide elecrrophresis. It is suggested that NADH peroxidase and superoxide dismutase (SOD) play a dominative role in the grown of oral indigenous bacteria under several oxygen concentrations.