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Escherichia coli variants for gas and indole production at elevated incubation temperatures

Two strains of Escherichia coli were subjected to heat and cold-storage treatments to determine the stability of the fecal E. coli characteristics of gas production from lactose and indole production at elevated incubation temperatures. No variants were detected with repeated sublethal heat treatment. A high incidence of variants was observed with extended cold storage of the organisms in liquid and semisolid media, especially with poor nutrient composition, and in the absence of cryoprotective agents. The indole characteristic at elevated temperature was more stable than the production of gas from lactose. The critical temperature at which both gas production from lactose and the indole characteristic were lost was 44.5 degrees C. It appeared that the variants resulted from increased temperature sensitivity of the formic hydrogen lyase and tryptophanase enzymes, respectively.