Abstract:
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 plasrnids. These transposons contained an In4-type class 1 integron that canied a dfrA5 trimethoprim resistance gene cassette and the sull sulfonamide resistance gene. These integrons were located in the same position as 1n4 in Tn1696. The integron. from one isolate includes a large transposon-like structure containing four 1526 and the strAB, sul2, blaTEM, and aphAl 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 aphAl-containing transposon Tn4352 and a transposon, Tn6029, carrying genes derived from R5FIOIO 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 1S26 bounding it, is proposed as an intermediate.
