Conjugative IncHI2 plasmids carrying tetracycline, trimethoprim, and sulphonamide resistance genes were recovered from two multiply antibiotic resistant Salmonella enterica serovar Typhimurium isolates from Australian food-producing animals. Transposons related to the mercury resistance transposon Tn1696 were identified in both IncHI2 plasmids. These transposons contained an In4-type class 1 integron that carried a dfrA5 trimethoprim resistance gene cassette and the sul1 sulfonamide resistance gene. These integrons were located in the same position as In4 in Tn1696. The integron from one isolate includes a large transposon-like structure containing four IS26 and the strAB, sul2, bla(TEM), and aphA1 genes conferring resistance to streptomycin, sulphonamides, ampicillin, kanamycin, and neomycin, respectively. This structure is flanked by an 8-bp duplication, but it includes both the aphA1-containing transposon Tn4352 and a transposon, Tn6029, carrying genes derived from RSF1010 and from Tn2. However, Tn4352 and Tn6029 overlap, sharing one IS26 copy. This suggests that they do not move by a standard transpositional mechanism. A circular intermediate, carrying only the region containing the resistance gene(s) and one of the IS26 bounding it, is proposed as an intermediate.
- CLASS-1 INTEGRONS