[Isolation and characterization of a plasmid pTS50, which encodes nisin resistance determinant in Lactococcus lactis...

Three nisin-resistant Lactococcus lactis strains were screened from 197 samples of fresh milk on a selective medium (M17) supplemented with nisin, lactose and bromocresol purple, and were confirmed to have the nisin resistance determinant (nsr) by PCR amplification. Physiological and biochemical tests as well as Lactococcus lactis specific 16S rDNA sequence analysis revealed that the three strains all belong to Lactococcus lactis subsp. lactis. A large plasmid, pTS50, was identified in L. lactis subsp. lactis TS1640 which encodes resistance to nisin following electroporation of the total plasmids DNA into L. lactis MG1363. The molecular weight of plasmid pTS50 was estimated to be 47 kb by restriction analysis of BamHI, EcoRI, HindIII, NcoI, PstI and nsr was localized on a 1.9 kb EcoRI fragment by Southern hybridization.