EFFECT OF CITRATE ON GROWTH OF LACTOCOCCUS-LACTIS SUBSP LACTIS IN MILK

The effect of citrate on the growth of Lactococcus lactis subsp. lacti s var. diacetylactis in milk has been investigated. Five strains of La ctococcus lactis subsp. lactis var. diacetylactis were compared to the ir citrate-negative variants, which lack the plasmid coding for citrat e permease. In most cases, acidification kinetics and the final bacter ial concentration of pure cultures of parental and variant strains did not differ significantly. Go-cultures of parental and variant strains , however, systematically tended towards the predominance of parental strains. Citrate metabolism is responsible for this change, since the predominance of citrate-positive strains was not observed in the absen ce of citrate. Continuous culture in milk enabled the difference in gr owth rates between the parental strain Lactococcus lactis subsp. lacti s var. diacetylactis CDI1 and its citrate-negative variant to be quant ified by following changes in the populations of the two co-cultured s trains. At 26 degrees C, the growth rate of the parental strain was 7% higher than that of its citrate-negative variant. These results show that citrate metabolism slightly stimulates the growth of lactococci i n milk.