Abstract

The diagnostic accuracy of an integron PCR method (Int-PCR) for detecting class 1 integrons (1,000, 1,200, and 1,600 bp) in the identification of antibiotic-resistant Salmonella strains was evaluated using 730 Salmonella isolates from pen floor samples collected from four swine production systems in Illinois. Three integron groupings were detected: 1,000 bp only, 1,600 bp only, and both 1,000 and 1,200 bp. The presence of any of the three class 1 integron groupings was associated with four-drug resistance (streptomycin, spectinomycin, sulfisoxazole, and tetracycline [St Spc Su Tet]). In addition, the presence of both the 1,000- and 1,200-bp integrons added resistance to ampicillin (Amp) and chloramphenicol (Cm), and the 1,600-bp integron added resistance to gentamicin (Gen) and kanamycin (Kan). DNA sequencing of integrons confirmed the presence of the aminoglycoside adenyl transferase (aadA) gene, conferring St Spc resistance in the 1,000-bp integron; the beta-lactamase gene, conferring Amp resistance in the 1,200-bp integron; and the aadA and aadB genes, conferring St Spc Gen Kan resistance in the 1,600-bp integron. The 1,600-bp integron appears to have the 1,000-bp intergron as its core, with additional genetic material conferring additional antibiotic resistance. The diagnostic accuracy of Int-PCR in detecting resistance to individual antibiotics was limited by the presence of phenotypic resistance in isolates without integrons. However, Int-PCR had high diagnostic accuracy (sensitivity and specificity) in detecting multidrug resistance: 0.98 and 0.92, respectively, for St Spc Su Tet; 0.95 and 1.0 for Amp Cm St Spc Su Tet; and 1.0 and 0.99 for Gen Kan St Spc Su Tet. Thus, Int-PCR can be valuable in epidemiological surveys as a screening tool for the detection of multidrug-resistant Salmonella strains.