Molecular differentiation of common promoters in Salmonella class 1 integrons

The integron is a mobile gene element which harbors antibiotic-resistance gene cassettes capable of site-specific integration. Among the four known types of integrons, the class 1 integron has been associated with multidrug-resistance in pathogenic bacteria. These gene cassettes have been the focus of a series of studies. The gene cassettes share a common promoter, and their expression levels are affected not only by their proximity to the promoter, but also by the strength (weak, hybrid and strong) of the common promoter, P1, as well as the presence of the additional promoter, P2. In this study, we developed molecular methods for the differentiation of promoter structures using PCR, restriction enzyme analysis, and polyacrylamide gel electrophoresis, and have applied them to the characterization of class 1 integrons in 33 non-typhoidal Salmonella serotypes in Korea. Class 1 integrons were detected in four serotypes: S. Derby (SD), S. Istanbul (SI), S. Paratyphi B (SPB), and S. Livingstone (SL), and the amplicon sizes were 1.0 Kb (SD, SI and SPB) and 2.0 Kb (SL). All of the 1.0 kb amplicons harbored gene cassettes (aadA1 or aadA2), but the 2.0 kb amplicon harbored three (dhfrXII-orfF-aadA2) gene cassettes, which conferred streptomycin/spectinomycin (aadA) and trimethoprim (dhfr) resistances. Our promoter structure study revealed three types of promoters; strong P1 (SD), weak P1 (SPB and SL), and weak P1 + P2 (SI). In conclusion, the class 1 integrons were detected in Korean NTS, and their promoter structures were found to be variable. Therefore, our methods may prove helpful in terms of our understanding of molecular diversity, as well as the transmission of class 1 integrons and phenotype-genotype relationships in antibiotic-resistance.