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The interactive growth of a proteinase-positive (Prtt) and a proteinase-negative variant (Prr) of a strain of Lactococcus lactis subsp lactis has been studied in milk. Simplified models have been used to describe the changes in population of each type of cells and in the acidification rate of the milk. A c1earinteraction was observed between the 2 types of cells as soon as the non-protein fraction was exhausted. The population level of the mixed culture (ie Prt+ Prr) then corresponded exactly to the population level at which the Prt+ starts its second exponential growth phase or the Prrstops growing exponentially, when these strains are grown separately. In the mixed culture, growth of the Prrvariant was stimulated, which resulted in the appearance of a second exponential growth phase for this variant and an increase in its final number of cells, when compared with that of the pure culture. On the other hand, the parental strain in the mixed culture showed a lower growth rate during the second exponential growth phase and a lower final population level than the same strain in the pure culture. The extent of these effects depended upon the initial percentage of the Prl+ strain in the mixture: the higher this percentage, the stronger the stimulation of the growth of the Prrstrain. On the other hand, the lower this percentage, the stronger the inhibition of the growth of the Prtstrain. As a result of this interaction, milk acidification rates were significantly lower under the critical limit of 20% Prt+ cells in the inoculum. Consequently, the time taken to coagulate milk was markedly longer.

le lait

Mixed cultures in milk of a proteinase-positive and a proteinase-negative variant of <i>Lactococcus lactis </i>subsp <i>lactis: </i>influence of initial percentage of proteinase-positive cells on the growth parameters of each strain and on the rate of acidification

Mixed cultures in milk of a proteinase-positive and a proteinase-negative variant of Lactococcus lactis subsp lactis: influence of initial percentage of proteinase-positive cells on the growth parameters of each strain and on the rate of acidification