Isolation of salmonella from flies on cattle farms and horizontal transfer of integron-mediated antibiotic resistance genes on microbiological media and selected farm samples
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Flies can be transmission vehicles of Salmonella from cattle to human. This study determined the prevalence of Salmonella in/on flies captured from 33 dairy/beef cattle farms in Georgia and characterized antibiotic resistance profiles of, and integron structures in, isolated Salmonella. Salmonella was isolated from 26/33 cattle farms (79%) and 185/1,650 flies (11%). Among 185 selected Salmonella isolates, 29% were resistant to ampicillin, 28% to tetracycline, 21% to amoxicillin/clavulanic acid, 20% to cefoxitin, and 12% to streptomycin. Two out of the 185 isolates, 438 and 442 harbored class 1 integrons. Isolate 438 carried a gene cassette of aadA7 (ca. 1.1 kb), and 442 carried drfA12-orfF-aadA2 (ca. 2.0 kb). Both integrons were transferrable through conjugation on tryptic soy agar while the drfA12-orfF-aadA2 was transferrable on 3/8 farm samples used. Study suggests that flies could be vectors of antibiotic-resistant Salmonella on cattle farms to potentially disseminate resistant genes, imposing risks to public health.