Isolation of Streptococcus lactis C2 Mutants Selected for Temperature Sensitivity and Potential Use in Cheese Manufacture

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Abstract
In an attempt to select streptococcal strains that could be used to accelerate the cheese ripening process, we used ethyl methanesulfonate treatment to isolate Streptococcus lactis C2 temperature-sensitive mutants exhibiting normal growth patterns at 30 to 32.degree. C but not at the highest cooking temperatures used in Cheddar cheese manufacture. Three classes of mutants isolated included: 1) mutants unable to grow and produce acid at 38 to 40.degree. C; 2) mutants unable to grow at 38 to 40.degree. C but that continued to produce acid at a rate similar to temperature-sensitive strains of S. cremoris; and 3) mutants that underwent lysis when the cells were shifted to 38 to 40.degree. C, as evidenced by a decrease in optical density of the cell suspension and the concomitant release of the intracellular enzyme, phospho-.beta.-galactosidase. The thermolytic variants could possibly be used to accelerate cheese ripening either by releasing their intracellular enzymes into the curd at an early stage in the cheese making process or by serving at a "delivery system" of ripening enzymes, the genes of which could be cloned into the thermolytic strains.