MIXED CULTURES IN MILK OF A PROTEINASE-POSITIVE AND A PROTEINASE-NEGATIVE VARIANT OF LACTOCOCCUS-LACTIS SUBSP LACTIS - INFLUENCE OF INITIALPERCENTAGE OF PROTEINASE-POSITIVE CELLS ON THE GROWTH-PARAMETERS OF EACH STRAIN AND ON THE RATE OF ACIDIFICATION

The interactive growth of a proteinase-positive (Prt+) and a proteinas e-negative variant (Prt-) of a strain of Lactococcus lactis subsp lact is has been studied in milk. Simplified models have been used to descr ibe the changes in population of each type of cells and in the acidifi cation rate of the milk. A clear interaction was observed between the 2 types of cells as soon as the non-protein fraction was exhausted. Th e population level of the mixed culture (ie Prt+ + Prt-) then correspo nded exactly to the population level at which the Prt+ starts its seco nd exponential growth phase or the Prt- stops growing exponentially, w hen these strains are grown separately. In the mixed culture, growth o f the Prt- variant 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 cultur e. On the other hand, the parental strain in the mixed culture showed a lower growth rate during the second exponential growth phase and a l ower final population level than the same strain in the pure culture. The extent of these effects depended upon the initial percentage of th e Prt+ strain in the mixture: the higher this percentage, the stronger the stimulation of the growth of the Prt- strain. On the other hand, the lower this percentage, the stronger the inhibition of the growth o f the Prt+ strain. As a result of this interaction, milk acidification rates were significantly lower under the critical limit of 20% Prt+ c ells in the inoculum. Consequently, the time taken to coagulate milk w as markedly longer.