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 lactis (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 proteinase-positive 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 proteinase-positive cells of PIII type with low proteolytic activity, a commensalism interaction also took place.