Spatial Distribution of Antibiotic Resistance in Soils Receiving Beef Feedlot Runoff

A study was conducted to provide new insight on the potential contribution to antibiotic resistance from the land application of beef feedlot runoff to soil. This study reports the distribution and quantity of antibiotic resistant bacteria (ARBs), fecal indicator bacteria (FIB), and antibiotic resistance genes (ARGs) in soil from (i) a field receiving long-term application of beef feedlot runoff holding pond effluent and (ii) a cool-season pasture with no history of supplemental manure application. Soil samples were collected June 2015 at the U.S. Meat Animal Research Center near Clay Center, Nebraska. A response surface sampling design (RSSD) model based on apparent soil electrical conductivity (ECa) measured using electromagnetic induction (EMI) was used to identify six independent sampling locations in each field representing varying degrees of manure accumulation. At each location, intact soil cores were collected to a depth of 2.0 m, subsampled, and analyzed for ARBs (cefotaxime, erythromycin, and tetracycline resistance), FIB, and ARGs (erm and tet). Methods included culture-based, disc diffusion, Etest, and qPCR. Results suggest the long-term application of beef feedlot runoff increased the soil microbial population, erythromycin resistant bacteria, erm(C), and tet(Q). The abundance of three cultured ARBs and erm(C) significantly decreased with depth in soil. Areas of high manure deposition had a positive correlation with erythromycin resistant bacteria. The data produced will contribute to the body of knowledge impacting decisions and future research efforts of scientists, researchers, and policy-makers who are striving to effectively address the potential contribution to antibiotic resistance in humans from agricultural practices. Advisor: Amy M. Schmid