Effect of manure application rate and rainfall timing on the leaching of antibiotic-resistant bacteria and their associated genes

Authors: Bolster, Carl; Brooks, John; Cook, Kimberly - Kim

Submitted to: Water, Air, and Soil Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2018
Publication Date: 3/29/2018
Citation: Bolster, C.H., Brooks, J.P., Cook, K.L. 2018. Effect of manure application rate and rainfall timing on the leaching of antibiotic-resistant bacteria and their associated genes. Water, Air, and Soil Pollution. https://doi.org/10.1007/s11270-018-3781-6.

Interpretive Summary: Antibiotics are used in swine production for therapeutic and for growth promotion purposes. It is well established that as much as 50 to 90% of antibiotics administered to animals are passed in the feces or urine in an unchanged form and this may lead to development of antibiotic-resistant strains of bacteria in the manure. Because land application is the most common method of disposing of swine lagoon effluent, there exists the potential threat of contaminating the underlying groundwater with antimicrobial-resistant bacteria (ARB) and their antibiotic-resistant genes (ARG). Our study centered on improving our scientific understanding on the effect of liquid swine manure application rate and timing on soil leaching of ARB and associated genes. We hypothesized that two commonly used manure management practices – manure application rate and timing – will have significant effects on the leaching potential of ARB and ARG through soil. Swine manure solutions were added to laboratory soil columns at rates of 5,000 or 30,000 gallons/acre. For both manure application rate, three columns were randomly selected to have rainfall applied at 1, 7, or 21 days after manure application. Column effluent and the top section of soil in the columns were sampled for cultivable bacteria and quantitative PCR (qPCR) was used to analyze and quantify tetracycline, methicillin, ß-lactam and erythromycin resistance genetic determinants. Our findings show a clear effect of time interval between slurry application and leaching event. This suggests the need to avoid swine slurry application to fields when significant rainfall event is forecasted in the near future.

Technical Abstract: 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 (ARG) through soil columns. Swine slurry was added to laboratory soil columns at rates of 5,000 or 30,000 gallons acre-1. For both application rates, rainfall was applied at 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, ß-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 amount of ARB and ARG recovered in the column effluent following application of the swine lagoon slurry decreased with time interval, though most of these decreases were not statistically significant. Moreover, no statistically significant differences in CFU or GU recovery in the column leachate were observed between the low and high manure application rates. In the experiments using swine slurry spiked with E. coli and Salmonella, recovery of both microorganisms eluted from fine sand columns was affected by both manure application rate and time interval between manure application and rainfall event; 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 timing.