Proteinase activity in slow lactic acid-producing variants of Streptococcus lactis

Variants of Streptococcus lactis that produce lactic acid slowly in milk were isolated by inducing plasmid loss in the wild type at 39 to 40 C. Such strains had lost most of their surface-bound proteinase activity and were designated prt(-). The specific proteinase activities of S. lactis C10 prt(+) whole cells and solubilized cell walls were 7 and 18 times, respectively, those of the prt(-) strain, but spheroplast lysates of prt(+) and prt(-) strains contained similar proteinase activity. S. lactis H1 showed a similar relative distribution of activity between prt(+) and prt(-) cellular fractions, although the overall level was lower. The limited growth in milk, characteristic of prt(-) strains, can be explained in terms of their low surface-bound proteinase activity.