Stability analysis of the lactococcus lactis drc1 lactose plasmid using pulsed-field gel electrophoresis

Pulsed-field agarose gel electrophoresis of Sma1 digests of genomic DNA was used to examine lactose plasmid copy number and stability in Lactococcus lactis. In L. lactis strain DRC1, the plasmid was found to exist as a single-copy plasmid. Transconjugants of strain HID113 carrying this plasmid were unstable. Variants were isolated with improved phenotypic stability resulting from improved maintenance of the lactose plasmid or from integration of part of the plasmid into the lactococcal chromosome. © 1993 Academic Press, Inc.