Overlooked volcanic effect during transmission of antibiotic resistance genes induced by copper and zinc

Adding heavy metals such as copper and zinc to animal feeds is common practice to promote growth, but meanwhile has side consequence of enhancing spread of antibiotic resistance genes (ARGs) in soil. This presents a global challenge to food security and human health. We in this study investigated the transmission of typical ARGs, i. e. β-lactamase genes (β-RGs), in dairy farm environments where dietary Cu and Zn were present in a wide range of concentration. The β-RGs were demonstrated to be highly prevalent across environmental media, with a relative abundance of 94.55%, dominated by mechanisms of antibiotic deactivation (93.75%) and cellular protection (6.25%). More importantly, we first found the transmission of ARGs to be highly dependent on the overlooked volcanic effect, i. e. low-concentration Cu (12–22 mg/kg) and Zn (45–80 mg/kg) acted as micronutrients necessary for microbial growth but facilitated ARGs transfer, whereas higher-concentration Cu (22–39 mg/kg) and Zn (80–153 mg/kg) became toxic to microbial communities and gene expression patterns. Notably, the specific microbial phyla Proteobacteria (2.28–82.94%), Bacteroidetes (0.02–56.48%) and Actinobacteria (1.62–12.92%) exhibited resistance at low concentration of Cu and Zn, which enhanced the transmission of β-RGs. However, this process was inhibited at higher concentration due to inactivation of microbes by Cu and Zn. The increase in resistance was first observed in class Gammaproteobacteria (2.02–88.51%) and Alphaproteobacteria (0.68–10.1%) with increased Cu and Zn concentration. This resulted in heightened transfer of ARGs by tnpA-07 (80.35%) due to protection of thicker cell membrane by chelation with Cu and Zn. This study not only offers mechanistic insights into the volcanic effect of dietary metals on dissemination of ARGs, but also has important implications for safe management of agricultural settings.