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A multidrug-resistant Salmonella enterica serotype 4,5,12:i:- clone carried a class 1 integron harboring dfrA12 and aadA2 gene cassettes and bla(TEM-1), aac(3)-IV, cmlA1, and tetA genes located in large plasmids of about 140 kb (carrying spv) or 120 kb (lacking spv). Several segregants, lacking multidrug resistance, contained a plasmid smaller than the parental one and no longer hybridized with probes for the lost resistances. The genes mediating resistance to ampicillin, chloramphenicol, and tetracycline in the 4,5,12:i:- clone are different from those found in the pentadrug-resistant serotype Typhimurium DT104 clone.
Guerra et al. (Sun,) studied this question.