EVALUATION OF HORIZONTAL AND VERTICAL GENE-TRANSFER AND STABILITY OF HETEROLOGOUS DNA IN STREPTOCOCCUS-THERMOPHILUS ISOLATED FROM YOGURT AND YOGURT STARTER CULTURES

In order to evaluate the potential of horizontal and vertical gene tra nsfer in S. thermophilus isolated from yogurt and yogurt starter cultu res, 18 strains were mated in vitro with Lactococcus lactis subsp. lac tis Bu2-60 harbouring plasmid pAMbeta1 from Enterococcus faecalis. pAM beta1 was transferred into 8 strains with transfer frequencies per rec ipient ranging from 4 x 10(-5) to 8 x 10(-9). In intraspecific crosses between S. thermophilus strains, transfer of pAMbeta1 was detected at transfer rates of 1 x 10(-4) to 1 x 10(-7), while retransfer of pAMbe ta1 into L. lactis subsp. lactis Bu2-60 always occurred at high freque ncies (2 x 10(-4) to 1 x 10(-4)). However, conjugal transfer of pAMbet a1 among S. thermophilus strains was not detected in fermented milk. T wo S. thermophilus strains could be transformed by electroporation wit h the lactococcal vector pNZ18 with low efficiency. A transfer of pNZ1 8 was not observed in S. thermophilus strains. Segregational stability of this heterologous DNA was studied during non-selective propagation in skim milk. Plasmid pNZ18 was stably maintained in more than 80% of the S. thermophilus cells after 144 generations at 42-degrees-C, and was still present in more than 60% of the cells after 126 generations (growth at 37-degrees-C) or 72 generations (growth at 32-degrees-C). G rowth temperature mediated an opposite effect on the stability of pAMb eta1 in S. thermophilus in which no loss was detected after 84 generat ions at 37-degrees-C, while a rapid loss occured at 42-degrees-C. Upon associative growth with Lactobacillus delbrueckii subsp. bulgaricus, maintenance of pNZ18 was enhanced in S. thermophilus, while a minor de crease in stability was detected for pAMbeta1.