Abstract
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::Tn
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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::Tn
7
, 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.