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 (PI versus PIII) 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 PI-type proteinase strain, whereas that of the PIII-type proteinase strain remained unaffected. The reduction in proteolytic activity of the PI-type proteinase strain (presumably resulting from an inhibition of the synthesis of the enzyme) due to the peptides released by the PIII-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 PIII-type proteinase strain.