Short communicationTransposition and homologous recombination drive evolution of pUO-StVR2, a multidrug resistance derivative of pSLT, the virulence plasmid specific of Salmonella enterica serovar Typhimurium

•We show the structure of 5 variants of a virulence-resistance plasmid of Typhimurium.•Several transposable elements played an active role in the generation of the variants.•Lost or gain of resistance genes were observed in most variants.•Most variants lacked an iron uptake system carried by the original plasmid.•Variants were infrequent despite the structural instability of the original plasmid.

Five variants of a resistant derivative of pSLT (termed pUO-StVR2) were detected in clinical isolates of Salmonella enterica serovar Typhimurium recovered in Spain. The structure of these variants revealed the involvement of IS1, IS26 and Tn21-like transposition, as well as homologous recombination in the generation of deletions, inversions and insertions which, depending on the variant, affected an orf of unknown function, genes encoding a possible iron acquisition system, and/or resistance properties. These variants, which appeared at a relatively low frequency, can be used as a model to understand the co-selection mechanisms that are helping to maintain multidrug resistance in bacterial pathogens, despite the structural instability of the responsible DNA.