Indirect interaction in milk between proteolytic and isogenic non-proteolytic strains of Lactococcus lactis. I. Effect of pre-culturing by a non-proteolytic variant

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The indirect interaction between proteolytic and lsogenic non-proteolytic strains of Lactococcus lactis was studied by performing sequential cultures in skim milk. After pre-culturing with a non-proteolytic strain for specifie periods of time, the milk was pasteurized, re-inoculated with a second strain and then re-incubated. This second strain was either the same non-proteolytic strain, or the parental proteolytic strain. ln the control milk, bath proteolytic parental and non-proteolytic strains grew exponentially at the same rate. Whereas the non-proteolytic variant stopped growing, its parental strain continued to grow exponentially, but at a lower growth rate than in the first exponential growth phase. When grown in milk pre-cultured by the non-proteolytic strain, the same strain exhibited a slower growth rate and a lower maximum population than when grown in control milk. While abundant growth of the pre-culture strain led to lower maximum populations of the second culture, the combined cell populations in the first and second cultures were constant. Furthermore, this sum was equal to the maximum population attained by the non-proteolytic strain in control milk. When grown in pre-cultured milk, the proteolytic strain exhibited biphasic exponential growth and attained the same maximum population as when cultured in control milk. While the growth rates during the 2 exponential phases were not affected, the shift from the first to the second growth rate occurred earlier in the pre-cultured than in the control milk, with the shift in growth rates occurring earlier in milk containing higher populations of pre-culture organisms. This apparent inhibitory effect of pre-culturing milk with a non-proteolytic variant was suppressed by adding sorne non-proteic nitrogen sources to the milk, thus indicating that the pre-cultured milk was depleted in non-proteic nitrogen sources. The result of this depletion was an indirect competition in milk between isogenic proteolytic and non-proteolytic strains for non-proteic nitrogen. This indirect competition was also observed using 11 other isogenic pairs of proteolytic and non-proteolytic strains belonging to the species L lactis.