Spread and evolution of natural plasmids harboring transposon Tn5

The presence of transposon Tn5 was studied in 730 Enterobacteriaceae strains from clinical and sewage origin. From these strains, twenty-five conjugative plasmids harboring transposon Tn5 were isolated. These plasmids were compared with pJR67 and pRYC119, the only previously studied plasmids harboring Tn5. A phylogenetic tree of the evolution of all different plasmids was proposed. Irrespective of their bacterial host and geographical place of isolation, some of the plasmids were shown to be identical. All of them can be included in only eight different prototypical plasmid species. Twenty-two plasmids (88%) carried an IncI1 incompatibility determinant as judged form DNA hybridization experiments. The presence of some other common resistance genes suggested that these plasmids are descendants of a common ancestor. These IncI1 plasmids could be grouped in six prototypical species. The results presented here suggest that Tn5 spread in nature may be dependent on the conjugative ability of the IncI plasmids harboring the transposon, rather than on the efficiency of Tn5 transposition between different replicons.