Translocation of integron-associated resistance in a natural system: acquisition of resistance determinants by Inc P and Inc W plasmids from Salmonella enterica Typhimurium DT104.

Salmonella enterica Typhimurium DT104, 961368, a veterinary field isolate that encodes a chromosomal cluster of resistance genes as well as two integrons, was used to study the mobility of resistance cassettes (aadA2 and pse-1) and nonintegron-associated resistance determinants (chloramphenicol and tetracycline). A range of natural plasmids was used as targets for the translocation of resistance. Plasmids that acquired resistance from the DT104 chromosome were segregated by conjugation into Escherichia coli K12. Plasmids R751, R388, and RP4::Tn7 acquired several combinations of resistance determinant (including single cassettes) at frequencies comparable with transposition. RP4 and pOG660 did not acquire any determinants from DT104. Phenotypic and PCR-based analysis of all the transconjugants that were translocated-both cassettes and more complex combinations of determinants-was carried out to determinate the genetic content. Translocation to R751 and R388 was associated with the loss of the indigenous trimethoprim cassette to both plasmids and also acquisition of sulfonamide resistance by R751 and RP4::Tn7, which indicated movement of the 3' terminus of one or both of the DT104 integrons. Sequencing of the R751 transconjugants confirmed these findings and showed that the translocation of streptomycin and ampicillin cassettes was associated with the precise excision of dhfrIIc and orfD cassettes. Furthermore, the translocation of multiple determinants occurred by at least two mechanisms, one of which was likely to involve a circular intermediate analogous to a composite cassette. Instability was detected in some of the transconjugants. The implication of the findings for the dissemination of resistance among clinical isolates is discussed.