Effect of Swine Manure Application Timing on the Persistence and Transport of Antibiotic-Resistant Enterococcus and Resistance Genes.

Swine manure applied to agricultural fields may lead to the transport of antibiotic resistant bacteria and antibiotic resistance genes to freshwater systems. Enterococci were studied because they are fecal indicator bacteria associated with manure. Resistance genes include genes from live cells, dead cells, and free-floating DNA in the environment. This objective of this study was to investigate if manure application timing, crop rotation, and tillage practice influence the rate of persistence of enterococci resistant to tetracycline or tylosin, and the macrolide resistance genes ermB, and ermF in tile drained crop systems. Soils were sampled from September 2016 to June 2017 after early fall, late fall, and spring manure applications. Antibiotic resistance gene concentrations were measured using qPCR on soil DNA extracts; enterococci were measured using membrane filtration on agar containing tetracycline or tylosin at breakpoint resistance concentrations. First order decay models were used to calculate half-lives and decay coefficients. Enterococci resistant to tetracycline persisted at higher concentrations than enterococci resistant to tylosin among all treatments. Half-lives of tetracycline resistant enterococci were 4.0 and 14.3 days after spring and late fall manure applications, respectively. Tylosin resistant enterococci had half-lives of 2.9, 14.2 days after spring and late fall manure applications, respectively. Enterococci in all treatments returning to background concentrations within six months after manure application. Results from this study will supplement understanding of the fate and transport of antibiotic resistant bacteria and antibiotic resistance genes in the environment.