Field-based evidence for copper contamination induced changes of antibiotic resistance in agricultural soils

Bacterial resistance to antibiotics and heavy metals are frequently linked, suggesting that exposure to heavy metals might select for bacterial assemblages conferring resistance to antibiotics. However, there is a lack of clear evidence for the heavy metal-induced changes of antibiotic resistance in a long-term basis. Here, we used high-capacity quantitative PCR array to investigate the responses of a broad spectrum of antibiotic resistance genes (ARGs) to 4-5 year copper contamination (0-800 mg kg-1) in two contrasting agricultural soils. In total, 157 and 149 unique ARGs were detected in the red and fluvoaquic soil, respectively, with multidrug and b-lactam as the most dominant ARG types. The highest diversity and abundance of ARGs were observed in medium copper concentrations (100-200 mg kg-1) of the red soil and in high copper concentrations (400-800 mg kg-1) of the fluvo-aquic soil. The abundances of total ARGs and several ARG types had significantly positive correlations with mobile genetic elements (MGEs), suggesting mobility potential of ARGs in copper-contaminated soils. Network analysis revealed significant co-occurrence patterns between ARGs and microbial taxa, indicating strong associations between ARGs and bacterial communities. Structural equation models showed that the significant impacts of copper contamination on ARG patterns were mainly driven by changes in bacterial community compositions and MGEs. Our results provide field-based evidence that long-term Cu contamination significantly changed the diversity, abundance and mobility potential of environmental antibiotic resistance, and caution the un-perceived risk of the ARG dissemination in heavy metal polluted environments.