Macrolide- and quinolone-resistant bacteria and resistance genes as indicators of antibiotic resistance gene contamination in farmland soil with manure application

The appearance of antibiotic resistance bacteria and genes (ARGs) poses a great threat to agricultural ecosystems. Here, to analyze the contamination of ARGs in the farmland soil with long-term manure application, quinolone- and macrolide-resistant bacteria, eight typical ARGs and two mobile gene elements (MGEs) were assessed as indicators. Compared with soil without manure, a large number of macrolide- and quinolone-resistant bacteria and resistance genes were found in the soil fertilized with manure. The resistant bacteria rates were decreased greatly with increasing doses of antibiotics, which showed a significant dose-effect. In addition, the bacterial resistance rates of tylosin (TYL) were higher than those of enrofloxacin (ENR). Further testing of the distribution characteristics of ARGs showed that the relative concentration of ermC was the highest. Moreover, significant correlations were found between the abundance of intI1 and qnrS, intI2 and ermB, respectively. The total contents of ermB and qnrA were positively correlated with the concentration of roxithromycin (ROX) residues, the total content of Cu and Zn as well as the cation exchange capacity (CEC) and moisture content (MC) of soil samples. Also, the abundance of ermC and qnrS was positively correlated with total and available contents of Cu and Zn, ROX residues dose and MC, OM in soils. The results of this study highlight the potential risks of antibiotics and ARGs in farmland soils.