Proteolytic activity of microorganisms isolated from freshwater fish

A raw fish-juice was prepared and sterilized through the use of (60)Co gamma-irradiation. It was evaluated for suitability in an agar medium for testing the proteolytic activity of bacteria isolated from fish. Microorganism proteolytic activity was also detected by conventional methods with skim milk-agar. We tested 1,145 isolates from fresh and spoiling irradiated (0.0, 0.3, and 0.6 Mrad) yellow perch fillets for proteolytic activity, by the use of both media. Most isolates that showed proteolytic activity exhibited this activity in both media. A few isolates showed proteolytic activity only in one medium or the other. Proteolysis was found mainly among bacteria isolated from nonirradiated perch fillets. Nonproteolytic organisms were slightly more abundant than were proteolytic ones throughout refrigerated storage (6 days); the latter constituted 48% of the total organisms. Irradiation eliminated essentially all proteolytic bacteria when the fillets were stored at 1 C. However, some proteolytic bacteria survived for a few days after irradiation when the fillets were stored at 5 C.