Antibiotic Resistance Gene Concentrations in Soils from Cattle Feedlot: A Three Year Study.

Antibiotic resistance (AR) is a common trait that allows bacteria with AR genes to compete against organisms that produce and/or have susceptibilities to antibiotics. Antibiotics are commonly used in livestock production to promote growth and combat disease. However, as much as 80% may pass through the animal and into the soil where selective pressure may favor survival of AR bacteria. This study was conducted to determine concentrations of AR genes in soils around a beef cattle feedlot operation. Samples were taken while the livestock were on-site and for two years following their removal. Microbial populations with AR genes for tetracycline, erythromycin and sulfonamides were quantified using quantitative, real-time (qPCR) analysis. Concentrations of bacteria with AR genes were highly variable across the feedlot landscape, but in general initial concentrations averaged between 2.5 and 3.3 orders of magnitude  (OM) higher in the feeder area than in grassy areas.  Two years after removal of animals from the site, concentrations of bacteria with genes for erythromycin (ermB and ermF), tetracycline (tetQ and tetW) and sulfonamide (sulI) resistance were at least one OM (90%) lower. The highest concentration of residual AR genes in soils (2.1 ± 3.2 X 10⁹ copies per gram) and background in grass (4.8 ± 3.5 X 10⁶ copies per gram) were for sulfonamide resistance. These results suggest that the concentration of bacteria with AR genes increases in soils where livestock congregate, but are effectively restricted to those areas by the presence of grass filter strips down-gradient. The concentration of AR genes remained elevated for two years after livestock were removed from the site. Future research should determine which AR populations remain in soils and to identify intervention strategies to limit their impact outside of the agricultural environment.