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Salmonella entericaserovar Typhimurium phage type 9 isolates resistant to streptomycin and sulfonamide have been recovered from both bovine and human sources in Australia. This study aimed to identify the resistance genes and their location. Polymerase chain reaction was used to screen for resistance genes and sul2(sulphonamide resistance) and strAand strB(streptomycin resistance) were detected. A small streptomycin and sulfonamide resistance plasmid carrying the three resistance genes was recovered from these isolates by transformation and was shown to be essentially identical to the small IncQ plasmid RSF1010. The sequences of one plasmid, pSRC15, and RSF1010 differed at only a few positions that may be errors in the older sequence. RSF1010 has been recovered from many species and in many countries since its first isolation in the early 1970s. We conclude that this plasmid has persisted unchanged in the environment for over 30 years. The antibiotic resistance gene cluster containing strA, strB, and sul2genes has clearly arisen from other known entities by a combination of transposition and homologous recombination using a short segment of homology. This resistance gene cluster is now widely distributed in plasmids and genomic islands in a number of contexts. [ABSTRACT FROM AUTHOR]

Conjugative IncHI2 plasmids carrying tetracycline, trimethoprim, and sulphonamide resistance genes were recovered from two multiply antibiotic resistant Salmonella entericaserovar Typhimurium isolates from Australian food-producing animals. Transposons related to the mercury resistance transposon Tn1696were identified in both IncHI2 plasmids. These transposons contained an In4-type class 1 integron that carried a dfrA5trimethoprim resistance gene cassette and the sul1sulfonamide 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 IS26and the strAB, sul2, blaTEM, and aphA1genes 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 Tn4352and a transposon, Tn6029, carrying genes derived from RSF1010 and from Tn2. However, Tn4352and Tn6029overlap, sharing one IS26copy. 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 IS26bounding it, is proposed as an intermediate. [ABSTRACT FROM AUTHOR]