Interaction between proteolytic strains of Lactococcus lactis influenced by different types of proteinase during growth in milk

The influence of the type of cell envelope-located proteinase (P-I versus P-III) on the associative growth of Lactococcus lactis in milk was studied. Two genetically engineered strains, differing only by the type of proteinase, were first used as a model study. An interaction occurred during the second exponential growth phase of the mixed culture and resulted in a decrease in growth rate of the P-I-type proteinase strain, whereas that of the P-III-type proteinase strain remained unaffected. The reduction in proteolytic activity of the P-I-type proteinase strain (presumably resulting from an inhibition of the synthesis of the enzyme) due to the peptides released by the P-III-type proteinase was found to be partly responsible for this interaction. Extension of the study to wild-type proteinase-positive L. lactis strains showed a systematic imbalance of the mixture of the two strains in favor of the P-III-type proteinase strain.