Fate of antimicrobials and antimicrobial resistance genes in simulated swine manure storage.

The behavior of three antibiotics (bacitracin, chlortetracycline, and tylosin) and two classes of antibiotic resistance genes (ARGs), tet and erm, were monitored in swine manure slurry under anaerobic conditions. First-order decay rates were determined for each antibiotic with half-lives ranging from 1 day (chlortetracycline) to 10 days (tylosin). ARGs were monitored in the swine manure slurry, and losses of approximately 1 to 3 orders of magnitude in relative abundance were observed during the 40 day storage period. First-order degradation profiles were observed for chlortetracycline and its corresponding resistance genes, tet(X) and tet(Q). Tylosin was degraded to approximately 10% of the starting concentration by day 40; however, the relative abundance of erm(B) remained at 50-60% of the initial relative abundance while the relative abundance of erm(F) decreased by 80-90%, consistent with tylosin. These results indicate that tet resistance genes respond primarily to chlortetracycline antimicrobials, and may be lost when the parent tetracycline compound is degraded. In contrast, erm(B) resistance gene may respond to a range of antimicrobials in animal manure, and may persist despite losses of tylosin.