Characterization of VariantSalmonellaGenomic Island 1 Multidrug Resistance Regions from Serovars Typhimurium DT104 and Agona

Abstract
Strains of multidrug-resistantSalmonella entericaserovar Typhimurium DT104 (DT104) andS. entericaserovar Agona (Agona) have been found to harborSalmonellagenomic island 1 (SGI1), a 43-kb genomic region that contains many of the drug resistance genes. Such strains are resistant to ampicillin (pse-1), chloramphenicol/florfenicol (floR), streptomycin/spectinomycin (aadA2), sulfonamides (sul1), and tetracycline [tet(G)] (commonly called the ACSSuT phenotype). All five resistance genes are found in a 13-kb multidrug resistance (MDR) region consisting of an unusual class I integron structure related to In4. We examined DT104 and Agona strains that exhibited other resistance phenotypes to determine if the resistance genes were associated with variant SGI1 MDR regions. All strains were found to harbor variant SGI1-like elements by using a combination of Southern hybridization, PCR mapping, and sequencing. Variant SGI1-like elements were found with MDR regions consisting of (i) an integron consisting of the SGI1 MDR region with the addition of a region containing a putative transposase gene (orf513) anddfrA10located between duplicatedqacEΔ1/sulIgenes (SGI1-A; ACSSuTTm); (ii) an integron with either anaadA2(SSu) or apse-1(ASu) cassette (SGI1-C and SGI1-B, respectively); (iii) an integron consisting of the SGI1-C MDR region plus an orf513/dfrA10region as in SGI1-A (SGI1-D; ASSuTm; ampicillin resistance due to a TEM β-lactamase); and (iv) an integron related to that in SGI1 but which contains a 10-kb inversion between two copies of IS6100, one which is inserted infloR(SGI1-E; ASSuT). We hypothesize that the MDR of SGI1 is subject to recombinational events that lead to the various resistance phenotypes in theSalmonellastrains in which it is found.