Antibiotics are used in swine production for both therapeutic and growth promotion purposes. Compounds are placed in feed or water and used in oral and injection treatments and encompass a broad range of antibiotic classes. It is well known that as much as 50 to 90% of administered antibiotics are passed in the feces or urine in an unchanged form and these may lead to development of antibiotic-resistant strains of bacteria in contaminated environments. Strains of antibiotic-resistant bacteria have been isolated from swine lagoon effluent from farms which used antibiotics. 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 associated genes.
1. The objective of our study was to determine the effect of liquid swine manure application rate and timing on soil leaching of ARB and their associated genes.
2. The general approach of this proposed study was to pass swine lagoon effluent known to contain ARB through soil at two manure application rates and three time intervals between manure application and first rainfall-induced leaching event using laboratory soil columns. We also conducted similar experiments using swine lagoon effluent spiked with two antibiotic resistant bacteria: E. coli and Salmonella. Time intervals between swine manure application and rainfall application were 1, 7, or 21 days. Transport and attachment behavior of the ARB and associated genes were evaluated in custom-made polycarbonate cylindrical columns packed with soil. Each treatment was conducted in triplicate using a full factorial design. Effluent from columns were plated onto tetracycline-, erythromycin-, and ampicillin-amended Reasoner’s 2 Agar (R2A) to determine the extent of transport of resistant bacteria that exhibit resistance to these antimicrobials. For the experiments using spiked swine lagoon effluent we used organism-specific media with the appropriate antibiotics added.