Antibiotic resistance genes and residual antimicrobials in cattle feedlot surface soil

Authors: BRIGHT, DAVID - University Of Florida; Miller, Daniel; Durso, Lisa; Spiehs, Mindy; Woodbury, Bryan; SNOW, DANIEL - University Of Nebraska; O'CONNOR, GEORGE - University Of Florida

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 6/7/2017
Publication Date: 8/13/2017
Citation: Bright, D., Miller, D.N., Durso, L.M., Spiehs, M.J., Woodbury, B.L., Snow, D., O'Connor, G. 2017. Antibiotic resistance genes and residual antimicrobials in cattle feedlot surface soil [abstract]. 4th International Symposium on the Environmental Dimension of Antibiotic Resistance. Paper No. 82.

Interpretive Summary: Antibiotic residues and resistant bacteria in cattle feedlot manure may impact antibiotic resistance in the environment. This study investigated common antimicrobials and resistance genes in cattle feedlot soils over time and compared those results with animal diets and other feedlot soil parameters. Feedlot soil samples were collected on consecutive years where animals were fed two diets. Soils were initially tested for the presence of 17 different resistance genes (prevalence ranged from 0 to 100%). In a follow up study, three tetracycline resistance genes, tetracycline antibiotics, and monensin were quantified. Genes ranged from 102 to 106 genes per gram; total tetracyclines and monensin varied from 40 to 12000 and 0 to 510 ng per gram soil, respectively. Moderate correlations between manure/soil parameters and genes or residual antimicrobials were observed. While antimicrobials were related to each other and resistance genes were related to one another, genes did not related directly to antimicrobials. Between years, antimicrobials decreased or didn’t change; two genes increased slightly, but the other did not change. Between the two diets, tetracycline content was 50% higher in pen soils where a 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.

Technical Abstract: Cattle feedlot soils receive manure containing both antibiotic residues and antibiotic resistant bacteria. The fates of these constituents are largely unknown with potentially serious consequences for increased antibiotic resistance in the environment. Determine if common antimicrobials (tetracyclines and monensin) and associated resistance genes in cattle feedlot soil change over time, vary with diet, and trend with other feedlot soil parameters. Soil samples were collected from12 locations in 10 pens on consecutive years where animals were fed two diets (distiller’s byproduct or corn diet). PCR screening for erm B, multiple tet (A, B, C, D, E, G, K, L, M, O, P, Q, S, X), and two sul (I and II) genes was done followed by qPCR of tet A, Q, and X genes and quantification of tetracyclines and monensin by LC/MS/MS in a subset of 50 samples. An initial 30-randomly selected sample screen detected various resistance genes in 0 to 100% of samples—tet B, D, G, and S were not detected, while tet C, sul I & II was >90%. In a second screen of all samples (n=240), tet P, Q, X, erm B, tet A, sul I was found in 33, 41, 47, 78, 80, and 95% of the samples, respectively. In 50 select samples, qPCR found 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). However, correlations between monensin and total tetracyclines (r=0.746) and 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.