Transfer of class 1 integron-mediated antibiotic resistance genes from Salmonella enterica of farm fly origin to susceptible Escherichia coli and Salmonella strains

Integrons in Salmonella are critical genetic elements with antibiotic resistance genes that could be disseminated through horizontal gene transfer, imposing risks to public health. This study was undertaken to determine the structure of integrons in Salmonella isolated from flies on cattle farms and examine the transferability of the integrons through conjugation. Results showed that 2 out of 606 isolated Salmonella, 438 and 442, harbored class 1 integrons. Integron gene cassette in Salmonella 438 carried a single gene of aadA7, and Salmonella 442, drfA12-orfF-aadA2. The two integrons were transferrable through conjugation on microbiological media to a Salmonella strain isolated from fly and to E. coli C600 at efficiencies ranging from 1.47 x 10(-6) to 6.29 x 10(-4). However, only Salmonella 442 was able to transfer its integron to the recipient cells in/on 3 out of 8 farm samples, with conjugation efficiencies ranging from 4.26 x 10(-10) to 1.36 x 10(-8). Antibiotic resistance genes not carried by integrons, e.g. genes encoding resistance to tetracycline and chloramphenicol, were co-transferred with integron-mediated antibiotic resistance genes. The data suggests that some Salmonella isolated from flies of cattle source carry integrons which could disseminate antibiotic resistance genes through horizontal gene transfer in the farm environment.