Cell Mass and Acid Production of Proteinase-Positive and Proteinase-Negative Lactic Cultures in Buffered Nonfat Milk

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Abstract
A sterile, nonfat, dry-milk substrate with .beta.-glycerophosphate was used to differentiate proteinase-positive and proteinase-negative lactic streptococci. Fifteen pairs of both cell types were incubated in this medium 24 h at 22.degree. C. Proteinase-positive cells produced from 6-27 times the cell mass of the proteinase-negative variants. Streptococcus cremoris proteinase-negative isolates produced a significantly larger cell mass than did S. lactis isolates. Cell mass production from proteinase-negative variants did not correlate with those from parent strains producing high cell mass. When mixtures of the 2 cell types were grown in the medium, from 10-50% of proteinase-positive cells were required for growth and acid production to equal that of 100% proteinase-positive cells. It was possible to detect 2% proteinase-positive cells in a proteinase-negative culture.