Construction of Streptococcus lactis subsp. lactis Strains with a Single Plasmid Associated with Mucoid Phenotype.

Lactose-fermenting mucoid (Lac Muc) variants of plasmid-free Streptococcus lactis subsp. lactis MG1614 were obtained by protoplast transformation with total plasmid DNA from MucS. lactis subsp. cremoris ARH87. By using plasmid DNA from these variants for further transformations followed by novobiocininduced plasmid curing, Lac Muc MG1614 strains containing only a single 30-megadalton plasmid could be constructed. This plasmid, designated pVS5, appeared to be associated with the Muc phenotype.