Characterization of plasmids from Listeria sp

Out of 139 isolates of Listeria sp. (mainly L. monocytogenes) 107 (78%) contained extrachromosomal DNA. Plasmids from 51 of these isolates were investigated further and covered a range of 8 different-sized molecules as shown by agarose gel electrophoresis. Only one of the 107 isolates contained more than one plasmid. With the exception of one plasmid from an isolate of L. seeligeri, restriction analysis and hybridization experiments showed a high degree of homology between the different plasmids.