Mixed Cultures of Proteinase-Positive and Proteinase-Negative Strains of Lactococcus lactis in Milk

Abstract

Twenty-four mixed cultures of nonisogenic proteinase-positive and proteinase-negative strains of Lactococcus Zactis (initial ratio, 1:9) were grown in milk. The proteinase-negative strains were systematically stimulated, regardless of the proteinase-positive strain used. Conversely, the effect of the association on the growth of the proteinase-positive cells depended on the type of proteinase (i.e., PI or PIII) and the level of proteolytic activity. When the proteinasepositive strain had low PI proteolytic activity, competition existed for the oligopeptides released from caseins by the proteinase, and the growth rate of the proteinase-positive strain was lowered. When the proteinase-negative strain was grown in presence of a highly proteolytic strain of PI, a commensalism occurred, and the growth rate of the proteinase-positive strain was unaffected. Furthermore, for proteinasepositive cells of PIII type with low proteolytic activity, a commensalism interaction also took place.