No abstract

AbstractpUO-STmRV1 is an IncC plasmid discovered in the Spanish clone of the emergent monophasic variant of Salmonella enterica serovar Typhimurium, which has probably contributed to its epidemiological success. The sequence of the entire plasmid determined herein revealed a largely degenerated backbone with accessory DNA incorporated at four different locations. The acquired DNA constitutes more than two-thirds of the pUO-STmRV1 genome and originates from plasmids of different incompatibility groups, including IncF (such as R100 and pSLT, the virulence plasmid specific of S. Typhimurium), IncN and IncI, from the integrative element GIsul2, or from yet unknown sources. In addition to pSLT virulence genes, the plasmid carries genes conferring resistance to widely-used antibiotics and heavy metals, together with a wealth of genetic elements involved in DNA mobility. The latter comprise class 1 integrons, transposons, pseudo-transposons, and insertion sequences, strikingly with 14 copies of IS26, which could have played a crucial role in the assembly of the complex plasmid. Typing of pUO-STmRV1 revealed backbone features characteristically associated with type 1 and type 2 IncC plasmids and could therefore be regarded as a hybrid plasmid. However, a rooted phylogenetic tree based on core genes indicates that it rather belongs to an ancient lineage which diverged at an early stage from the branch leading to most extant IncC plasmids detected so far. pUO-STmRV1 may have evolved at a time when uncontrolled use of antibiotics and biocides favored the accumulation of multiple resistance genes within an IncC backbone. The resulting plasmid thus allowed the Spanish clone to withstand a wide variety of adverse conditions, while simultaneously promoting its own propagation through vertical transmission.

Salmonella enterica serovar Infantis (S. Infantis) is a broiler-associated pathogen which ranks in the fourth position as a cause of human salmonellosis in the European Union. Here, we report a comparative genomic analysis of two clinical S. Infantis isolates recovered in Spain from children who just returned from Peru. The isolates were selected on the basis of resistance to cefotaxime, one of the antibiotics of choice for treatment of S. enterica infections. Antimicrobial susceptibility testing demonstrated that they were resistant to eight classes of antimicrobial agents: penicillins, cephalosporins, phenicols, aminoglycosides, tetracyclines, inhibitors of folate synthesis, (fluoro)quinolones and nitrofurans, and one of them was also resistant to fosfomycin. As shown by whole-genome sequence analysis, each isolate carried a pESI-like megaplasmid of ca. 300 kb harboring multiple resistance genes [blaCTX-M-65, aph(4)-Ia, aac(3)-IVa, aph(3′)-Ia, floR, dfrA14, sul1, tet(A), aadA1 ± fosA3], as well as genes for resistance to heavy metals and disinfectants (mer, ars and qacEΔ1). These genes were distributed in two complex regions, separated by DNA belonging to the plasmid backbone, and associated with a wealth of transposable elements. The two isolates had a D87Y amino acid substitution in the GyrA protein, and truncated variants of the nitroreductase genes nfsA and nsfB, accounting for chromosomally encoded resistances to nalidixic acid and nitrofurantoin, respectively. The two S. Infantis isolates were assigned to sequence type ST32 by in silico multilocus sequence typing (MLST). Phylogenetic analysis revealed that they were closely related, differing only by 12 SNPs, although they were recovered from different children two years apart. They were also genetically similar to blaCTX-M-65-positive ± fosA3 isolates obtained from humans and along the poultry production chain in the USA, South America, as well as from humans in several European countries, usually associated with a travel history to America. However, this is the first time that the S. Infantis blaCTX-M-65 ± fosA3 MDR clone has been reported in Spain.