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

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 22C. Proteinase-positive cells produced from 6 to 27 times the cell mass of the proteinase-negative variants. Streptococcus crernoris proteinase-negative isolates produced a significantly larger ceil mass than did Streptococcus 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 two cell types were grown in the medium, from 10 to 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.