Effect of Nanosilver and Antibiotic Loading on Fate of Antibiotic Resistance Genes in Thermophilic and Mesophilic Anaerobic Digesters

Wastewater treatment plants (WWTPs) have been identified as vehicles for the conveyance of pharmaceuticals and nanosilver contaminants as well as antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) from anthropogenic sources to the environment. Additionally, WWTPs have high nutrient loadings, dense microbial populations, and trace antibiotic concentrations that create “breeding ground” conditions for the selection, transfer and dissemination of ARGs among various bacteria. The use of nanosilver in consumer products is on the rise because of its antimicrobial properties and, as such, nanosilver may incur a selective pressure similar to antibiotics.The purpose of our studies was to investigate the fate of ARGs in mesophilic and thermophilic anaerobic digestion under conditions of artificially%applied antibiotic and nanosilver selection pressures, possible co%selection of multiple ARGs via integrons, and the impact of nanosilver and antibiotic loading on thermophilic anaerobic digestion performance.This study suggests that anaerobic digestion is an effectively reduces ARG loading to the environment up to one or two orders of magnitude even in the presence of contaminant selection pressures. The fact that ARG reductions remained consistent in spite of the application of selection pressures suggest that the digester operating conditions themselves are the major factor that governs the composition of the bacterial community and thus the prevalence of ARGs. In addition, this work did not find any impact of nanosilver at environmentally relevant concentrations (0.01 to 1.0 mg/L; ∼0.7 to 70 mg%Ag/kg TS) on thermophilic anaerobic digestion.