In the United States thousands of tons of antibiotics are annually administered in low doses to livestock, both as prophylactics and growth promoters. Mounting evidence has shown that the overuse of antibiotics in livestock might render them, or their structural analogs, less effective when needed in veterinary as well as human medicine. To examine antibiotic resistance trends in feedlot soil, we collected soil samples from pens at ten beef cattle feedlots across western Kansas. Soil samples were screened for bacteria resistant to the human broad spectrum antibiotics ciprofloxacin, chloramphenicol, and tetracycline, by plating on them on Mueller-Hinton agar containing antibiotic concentrations of 100 μg mL−1. Resistant organisms were subcultured until axenic cultures were obtained. Bacteria were subsequently identified by submitting them to a commercial laboratory that utilizes 16S rRNA sequencing. We further assayed the organisms for resistance to more than one antibiotic. Some resistant isolates showed single drug resistance while others showed resistance to more than one agent.
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Vol. 111 • No. 1