Salmonella enterica serotype Typhimurium carrying hybrid virulence-resistance plasmids (pUO-StVR): A new multidrug-resistant group endemic in Spain

The epidemiological impact in Spain of an emerging group of multidrug-resistant Salmonella enterica serotype Typhimurium, characterized by the presence of virulence-resistance hybrid plasmids (termed pUO-StVR) that are related to the S. Typhimurium virulence plasmid pSLT, was evaluated. Adscription to the group was based on detection of the blaOXA−1 gene (encoding ampicillin resistance) by PCR, and identification of a pUO-StVR plasmid through hybridization with specific probes for virulence (spvC) and resistance (blaOXA−1) genes. In this way, 57 out of 134 ampicillin-resistant clinical isolates of S. Typhimurium, collected over 2002–2004 in 21 Spanish cities, were assigned to the group, which can be already regarded as endemic. Most isolates (>89%) shared the following features: (i) resistance to ampicillin, chloramphenicol, streptomycin/spectinomycin, sulfonamides, and tetracycline, encoded by blaOXA−1-catA1-aadA1-sul1-tet(B); (ii) a class 1 integron (InH) with the blaOXA−1-aadA1 gene cassettes within its variable region of ca. 2000 bp; (iii) the spvC, rck, samA, oriT, traT, traX, repA (RepFIIA), and parA/B genes (but not rsk and pefABCD) of pSLT; (iv) a hybrid plasmid of ca. 125 kb, termed pUO-StVR2, where the resistance and virulence genes are located. However, intra-group diversity was also detected, since a total of four resistance phenotypes, five resistance genotypes, two integron profiles, five plasmid variants (pUO-StVR2, 4-7, differing in size, restriction profile and/or resistance pattern), 15 XbaI-BlnI combined macrorestriction profiles, and five phage types were identified. Each hybrid plasmid was revealed as a distinctive BlnI band, through hybridization with pUO-StVR2. The genetic markers used, together with the knowledge generated in the present study, could be applied to epidemiological surveillance of S. Typhimurium pUO-StVR worldwide.