The Fate of Antibiotic Resistance Genes Following the Application of Municipal Wastewater Solids to Agricultural Soil.

Municipal wastewater solids (a.k.a. sewage sludge) are a significant reservoir of antibiotic resistant bacteria and of antibiotic resistance genes (ARGs).  In this study, the fate of ARGs was studied following the application of treated wastewater solids to agricultural soils.  Wastewater solids were first treated by a myriad of different technologies and then applied to soils in microcosms at a ratio of 40 mg treated solids/g soil.  Three tetracycline resistance genes (tet(A), tet(W), and tet(X)), the integrase of a class 1 integron (intI1), an erythromycin resistance gene (erm(B)), a sulfonamide resistance gene (sul1), and the 16S rRNA gene were tracked for six months using real-time PCR.  The initial concentrations of intI1 and all resistance genes were greater in microcosms receiving treated residual solids (>10⁶ copies/g) than in control microcosms.  These relatively high gene concentrations declined over the course of the experiment, and decay rates were fit to a first-order kinetic model.  The half-lives of intI1 and ARGs determined from first-order kinetic coefficients varied between 10 and 100 days.  This research demonstrates that ARGs decay over time in soil, but the rates of decay depend on the specific gene and the technology used to treat the ARG prior to application to soil.