Antibiotic Resistance Genes and Residual Antimicrobials in Cattle Feedlot Surface Soil.

Antibiotic residues and resistant bacteria in cattle feedlot manure may impact antibiotic resistance in the environment. This study investigated common antimicrobials (tetracyclines and monensin) and associated resistance genes in cattle feedlot soils over time. Animal diets and other feedlot soil parameters were analyzed for correlations with residues and resistance genes. Feedlot soil samples were collected from 10 pens on consecutive years where animals were fed either a byproduct or corn-based diet. Soils were initially screened for the presence of 17 different erm, tet, and sul genes using PCR, and the range of detection ranged from 0 to 100% (tet B, D, G, and S were not detected, while tet C, sul I & II was >90%). In a follow up analysis of 50 samples, tet A, Q, and X genes and tetracyclines and monensin were quantified. Tet A, Q, and X ranged from 10² to 10⁶ genes gm^-1 soil; total tetracyclines and monensin varied from 40 to 12000 and 0 to 510 ng gm^-1 soil, respectively. Moderate correlations between manure/soil parameters (volatile solids, nitrogen content) and tet genes or residual antimicrobials were observed, but the correlations between genes and antimicrobials were very weak (-0.147 to 0.179). Correlations between monensin and total tetracyclines (r=0.746) and between tet A, Q and X (0.405 to 0.760) were stronger. Between years, antimicrobials decreased or didn’t change, while tet Q & X increased, but tet A did not change. Between the two diets, the total tetracycline content was 50% higher in pen soils where distiller’s byproduct diet was fed but there were no gene differences. Although feedlot soils contained a diverse pool of resistance genes, no relationship was found with residual antimicrobial content. Various parameters associated with manure correlated strongest with resistance and antimicrobial content. Diet and annual differences were not as important.