Genome plasticity among related ++Lactococcus strains: identification of genetic events associated with macrorestriction polymorphisms

The genomic diversity of nine strains of the Lactococcus lactis subsp. cremoris (NCDO712, NCDO505, NCDO2031, NCDO763, MMS36, C2, LM0230, LM2301, and MG1363) was studied by macrorestriction enzyme analysis using pulsed-field gel electrophoresis. These strains were considered adequate for the investigation of genomic plasticity because they have been described as belonging to the same genetic lineage. Comparison of ApaI and SmaI genome fingerprints of each strain revealed the presence of several macrorestriction fragment length polymorphisms (RFLPs), despite a high degree of similarity of the generated restriction patterns. The physical map of the MG1363 chromosome was used to establish a genome map of the other strains and allocate the RFLPs to five regions. Southern hybridization analysis correlated the polymorphic regions with genetic events such as chromosomal inversion, integration of prophage DNA, and location of the transposon-like structures carrying conjugative factor or oligopeptide transport system.