Efficient mobilization of a resistance derivative of pSLT, the virulence plasmid specific of Salmonella enterica serovar Typhimurium, by an IncI1 plasmid

• pUO-StVR2 is a virulence-resistance plasmid of S. enterica serovar Typhimurium.
• pStR12 is an IncI1 resistance plasmid which co-exists with pUO-StVR2.
• pStR12 has a high frequency of conjugation and can efficiently mobilize pUO-StVR2.

pUO-StVR2 is a derivative of pSLT, the virulence plasmid specific of Salmonella enterica serovar Typhimurium, which confers multidrug resistance. This plasmid is widespread among closely related isolates of S. Typhimurium, and often coexists with other plasmids like pStR12. The latter belongs to incompatibility group IncI1, was assigned to ST48 by pMLST (plasmid multilocus sequence typing), and confers resistance to streptomycin/spectinomycin, chloramphenicol, trimethoprim and sulphonamides, with the responsible genes (aadA1/aadA2, cmlA1, dfrA12 and sul3) located on a sul3-class 1 integron. When using clinical isolates of S. Typhimurium containing one (pUO-StVR2) or both (pUO-StVR2 and pStR12) plasmids as donors and Escherichia coli K12 J53 resistant to rifampicin as recipient, the conjugation frequencies of pUO-StVR2 and pStR12 were 10−8 and 10−3–10−5 transconjugants/donor, respectively, while the transfer frequency of pUO-StVR2 increased 102 up to 105 times through mobilization by pStR12, depending on the donor strain and experimental conditions. Mobilization of pUO-StVR2, a plasmid which encodes virulence and resistance functions, by compatible plasmids which coexist in the same bacterium can facilitate the spread of these properties in S. Typhimurium, one of the most common serovars of S. enterica.