Characterization of tetracycline resistance in feedlot runoff applied to a vegetative treatment system

Authors: Durso, Lisa; Miller, Daniel; HENRY, CHRISTOPHER - University Of Arkansas; SNOW, DANIEL - University Of Nebraska

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/26/2015
Publication Date: N/A
Citation: N/A

Interpretive Summary: Human and agricultural wastes (feces and urine) are two ways that antibiotic drugs and resistant bacteria are introduced into the environment. Looking just at cattle operations, we already know that there are ways to treat manure to reduce pathogens. We are interested to see if these same techniques can also be used to reduce the number of antibiotic resistant bacteria. We looked at one specific type of manure management system, called a vegetative treatment system (VTS). In a VTS, runoff from a cattle feedlot is collected in a basin, then used to irrigate fields that are harvested for hay. While there are many different types of antibiotic resistance, we looked specifically at tetracycline resistance, since this type is often measured in environmental studies. Although tetracycline resistance was routinely detected in bacteria from beef cattle feedlot runoff, soil sampling indicated that these microorganisms did not survive long-term in the soil. This particular manure management strategy was therefore shown to be effective at reducing the number of tetracycline resistant bacteria originating in manure.

Technical Abstract: Introduction. In U.S. beef cattle operations, manure management systems effectively reduce survival and transport of zoonotic pathogens, and previous studies show that the taxonomic composition of a microbial community changes once feces is excreted from the animal. Our hypothesis is that since manure management systems change the community structure, the same manure management strategies that can reduce pathogens will also impact the number and types of antibiotic resistant (AR) bacteria. Objectives. The objectives were to evaluate whether a vegetative treatment system, where feedlot runoff is applied to fields containing cool season grasses, reduced the number of bacteria resistant to selected antibiotics, characterize these AR bacteria (ARB) spatially and temporally, and describe the relationship between phenotypic and genotypic resistance profiles observed in the isolates. Methods. The VTS consisted of a field that was divided into 8 application cells, separated from each other with earthen berms (Figure 1), allowing for replication of our measurements on a field scale. Measurements were taken in the spring and summer of three consecutive years from feedlot runoff applied at the top of the cells, and excess wastewater that collected at the bottom of each cell. ARB survival in soil was tracked, and the vertical transmission of ARB was evaluated. Baseline data were collected from ground deposited feces, rain water runoff, and the untreated berm areas. Tetracycline resistance was chosen because it is commonly used as a target in environmental samples. Isolates were screened for resistance to 12 antibiotics using standard disk diffusion methods, and characterized for the presence or absence of 11 tetracycline resistance genes. Results. There were significantly fewer tetracycline resistant bacteria (TRB) in the VTA rainwater samples, compared to feedlot runoff and excess wastewater, with feedlot runoff and excess wastewater samples typically 2 to 3 orders of magnitude higher in concentration of resistant bacteria. No discernible annual or seasonal trends were observed. Bacteria carrying tetracycline resistance could be cultured from soil samples immediately following feedlot runoff application, and numbers decreased by 0.5 log over two weeks. Berm samples were negative for TRB. Both single and multiple resistant E. coli strains were more likely to be found in the runoff, compared to excess wastewater and rain. Of 467 isolates screened for resistance to 12 antibiotics, 96% were resistant to less than three of the antibiotics assayed. Over 12,000 isolates were screened for 11 tetracycline resistance genes (TRG). Of these 77% had at least one, and 28% carried multiple TRGs. The maximum number of TRGs found in a single isolate was three. Low concentrations of several antibiotics, including tetracyclines, were measured in feedlot run-off and in soil samples collected from areas receiving wastewater from the feedlot. Conclusion. Although tetracycline resistance was routinely detected in bacteria from beef cattle feedlot runoff, soil sampling indicated that these microorganisms did not survive long-term in the soil. The VTA was therefore shown to be effective at reducing the number of tetracycline resistant bacteria originating in manure, when evaluated over the course of a one year application time.