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

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.