Effect of Manure Application Rate and Rainfall Timing on the Leaching of Antibiotic-Resistant Bacteria and Their Associated Genes

In this study, we investigate the effect of application rate and timing of liquid swine slurry on leaching of antibiotic-resistant bacteria (ARB) and their antibiotic resistance genes (ARGs) through soil columns. Swine slurry was added to laboratory soil columns at rates of 5000 or 30,000 gal ac^?1. For both application rates, rainfall was applied at either 1, 7, or 21 days after slurry application. Column effluent and the top centimeter of soil in the columns were sampled post-rainfall for cultivable bacteria and quantitative PCR was used to quantify tetracycline, methicillin, β-lactam, and erythromycin resistance genetic determinants. We also conducted similar experiments using swine lagoon slurry spiked with antibiotic-resistant E. coli and Salmonella. We found that the concentration of ARB and ARG recovered in the column effluent following application of the swine lagoon slurry generally decreased with increasing lag time between slurry application and simulated rainfall, though most of these decreases were not statistically significant. Moreover, no statistically significant differences in CFU or GU concentration in the column leachate were observed between the low and high slurry application rates. In the experiments using swine slurry spiked with E. coli and Salmonella, concentrations of both microorganisms eluted from fine sand columns were affected by both slurry application rate and lag time; recovery of ARGs, however, was mostly unaffected, but some differences were observed. In columns packed with loamy sand, no recovery was detected in the column effluent for either organism and recovery of ARG was unaffected by manure application rate or rainfall lag time.