PLASMID PROFILES AND STABILITY OF INDUSTR IAL STRAINS OF LACTOCOCCI

Industrial lactococci strains, usually cultivated on agar with hydroly zed milk, are characterized by an increased instability with respect t o their ability to utilize lactose on enriched M21 medium. Plasmid pro files of 14 industrial strains of Lactococcus lactis subs. lactis and of a large number of segregants phenotypically differing in sugar-ferm enting ability and phage sensitivity were examined. Four out of 14 str ains segregated Lac(-) Gal(-)clones, i.e., simultaneously lost the abi lity to utilize lactose and galactose. All phage-sensitive segregants retained systems of DNA restriction and modification. Segregants sensi tive to the phage lethal effect were found. These segregants possibly retained the defense mechanism against phages manifested as abortive i nfection. In the majority of Lac(-) segregants, large plasmids of 40 t o 55 kb were lost, although changes in plasmid profiles of many Lac(-) segregants were not detected.