Effects of copper and cadmium stress on resistance genes in farmland soil

BACKGROUND: Current farmland soils are increasingly affected by contamination from antibiotic resistance genes (ARGs) and heavy metals, with the evolution of microbial antibiotic resistance potentially influenced by metal stress. The distribution effects of ARGs in farmland soils under cadmium (Cd) and copper (Cu) combined pollution remain unclear. RESULTS: Cu and Cd exerted significant main effects on bacitracin, fosfomycin, sulfonamide, trimethoprim-β-lactam, polymyxin, tetracycline, trimethoprim, and vancomycin resistance genes. Interactions between Cd/Cu and fosmidomycin, Macrolide-Lincosamide Streptogramin (MLS), multidrug resistance, sulfonamide, and vancomycin abundance are significant. Co-occurrence network analysis identifies Pseudomonadota, Actinobacteriota, and Acidobacteriota as primary potential host groups for ARGs. Structural equation modeling analysis further demonstrates that Cd exhibits more pronounced effects on ARGs abundance compared to Cu. In contrast to some previous studies, bacterial alpha-diversity and mobile genetic elements (MGEs) did not act as primary drivers of ARGs variation, whereas metal resistance genes (MRGs) showed a strong positive association with ARGs abundance. CONCLUSION: In our study, MGEs did not act as primary drivers of ARGs variation but were positively associated with ARGs abundance through their linkage with MRGs, suggesting that metal exposure may increase the mobility potential of resistance determinants. These findings hold substantial implications for assessing potential ecological risks in soil metal pollution and provide new insights into the co-selection mechanisms between Cd and Cu in shaping ARGs dynamics.