Abstract
The broad host range antibiotic resistance plasmid pAM beta 1 was transferred from Streptococcus faecalis to 9 of 15 Listeria monocytogenes strains by conjugation. L. monocytogenes transconjugates could transfer the plasmid either among L. monocytogenes strains or back to S. faecalis. Transfer between the various strains occurred without any detectable plasmid DNA rearrangements. The pAM beta 1 replicon was stable in L. monocytogenes--it was retained without antibiotic selection when the bacteria were grown in culture media or passed in mice--and the presence of pAM beta 1 had no major effect on L. monocytogenes virulence. These data suggest that pAM beta 1 or its derivatives might serve as useful L. monocytogenes cloning vehicles. The data presented also demonstrate that pAM beta 1 is compatible with two different native L. monocytogenes plasmids and that Listeria species harbor native plasmids in addition to the 38.5-megadalton plasmid pRYC16 previously reported by Pérez-Díaz et al. (J. C. Pérez-Díaz, M. F. Vicente, and F. Banquero, Plasmid 8:112-118, 1982). Of 29 L. monocytogenes strains screened, 7 contained plasmid DNA. Four strains had similar if not identical plasmids that were 34 megadaltons in size, whereas three other strains contained either a 53-, 44-, or 32-megadalton plasmid; none of these plasmids has the same restriction patterns as pRYC16. DNA homology experiments indicate that the various plasmids are related and suggest that there may be a common set of sequences present in all of the plasmids examined.