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

¹ Institut National de la Recherche Agronomique, Station de Recherches, Laitières, 78352 Jouy-en-Josas Cédex, France

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., P_I or P_III) and the level of proteolytic activity. When the proteinase-positive strain had low P_I 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 P_I, a commensalism occurred, and the growth rate of the proteinase-positive strain was unaffected. Furthermore, for proteinase-positive cells of P_III type with low proteolytic activity, a commensalism interaction also took place.

Key Words: Lactococcus lactis • proteinase • mixed culture • milk

Submitted on May 15, 1995
Accepted on February 20, 1996

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