Selected antimicrobial resistance during composting of manure from cattle administered sub-therapeutic antimicrobials
Received for publication December 10, 2007. Composting is being increasingly employed for the recycling of nutrients in manure from the livestock industry. However, composting manure from animals fed antimicrobials has not been well characterized. In this study, compost windrows were prepared using manure collected from cattle (Bos Taurus L.) fed tylosin (TY), chlortetracycline-sulphamethazine (TS), and control cattle (no antimicrobials). The objectives of the 18-wk trial were to quantitatively assess the survival of total E. coli, E. coli resistant to ampicillin (Ampr) and tetracycline (Tetr), and select tetracycline (tet) and erythromycin resistance methylase (erm) genes. We found that while compost windrows did not reach the recommended temperature of 55°C for 15 d, composting reduced high initial levels of total, Ampr, and Tetr E. coli as early as Week 2. A significant antimicrobial effect on total (P = 0.04) and Ampr (P = 0.03) E. coli was observed. Significant antimicrobial x time interactions were observed from Week 0 to Week 3 (Total E. coli: P = 0.04; Ampr: P = 0.02; Tetr: P = <0.001). Low absolute abundance of tet and erm genes (<106 copies g–1) was found and the resistance genes displayed different dynamics; tet(A,C) and erm(A) increased marginally at Week 11 relative to Week 0 and 5 and the remaining genes (tet(G), RPP tet, erm(B), erm(C), erm(F), erm(T), and erm(X)) decreased for most time points and treatments. These results indicate that even though composting reduces antimicrobial resistant E. coli, tet and erm genes could still be detected. Our experiments reiterate advantages of polymerase chain reaction (PCR)-based quantitative assays over cultivation-based methods for the rapid identification of composting effectiveness in eliminating resistance genes before land application.