Fate and transport of antibiotic resistant bacteria and resistance genes in artificially drained agricultural fields receiving swine manure application
Thomas B. Moorman
The growing numbers of swine receiving antimicrobial additives in feed at sub-therapeutic levels as a prophylactic and growth promoter has led to increasing concerns regarding levels of antibiotics and antibiotic resistant bacteria in their excrement. Application of swine manure to agricultural fields as fertilizer creates a pathway for antibiotic resistant bacteria and their associated resistance genes to enter the environment. This study monitored enterococci, tylosin resistant enterococci and four genes known to confer macrolide antibiotic resistance (ermB, ermC, ermF and msrA) in soil and subsurface artificial drainage water. Manure concentrations for ermB, ermC and ermF were all >109 copy g-1. MsrA was not detected in manure, soil or water. The average enterococci concentration in manure was 1.76 x 105 CFUg-1, with 83% resistant to tylosin. The next highest concentrations of enterococci and tylosin resistant enterococci were located in soil from the manure injection band which contained median concentrations >200 CFUg-1 soil. Gene abundances of ermB, ermC and ermF in manured soil returned to levels identified in non-manured control plots by the spring following manure application. While enterococci and tylosin resistant enterococci concentrations in drainage water samples showed no trends between treatments, resistance genes ermB and ermF were found at significantly higher concentrations (p<0.01) in drainage water from manured plots when compared to non-manured plots gene concentrations. ErmB was found in 78% of drainage water samples from plots with manure treatment. ErmF was detectable in 44% of drainage water samples from manure amended plots. No significant differences (p>0.10) were identified due to tillage treatments for any of the genes detected. Although ermC was detected at the highest concentrations of the three genes in drainage water, concentrations in water from manure treated plots were not significantly greater (p>0.10) than the control plot concentrations. These results suggest a short-term increase in antibiotic resistant bacteria and resistance genes in soil from manure application. Additionally, this study is the first to report significant increases in resistance gene abundances in agricultural drainage water from soils receiving manure application.