Exposure to arsenic and cadmium promotes conjugative transfer of plasmid-borne antibiotic resistance genes among soil microbiota

The spread of antibiotic resistance genes (ARGs) via horizontal gene transfer (HGT) poses a major global health threat. While cadmium (Cd) is known to influence ARG transfer in pure cultures, the effects of arsenic (As) and As-Cd co-exposure on plasmid-mediated conjugation, especially in soil, remain unclear. Here, we show that individual As (100-250 µM) or Cd (100-1000 µM) promoted the conjugative transfer of ARGs carried by plasmids R388 and RP4 in liquid medium, increasing frequency by 1.4- to 3.5-fold. In contrast, the As-Cd combination achieved similar promotion at only 25-50 µM each for the RP4 plasmid. In soil, the As-Cd mixture at concentrations slightly above or equal to environmentally relevant levels enhanced RP4 plasmid transfer to soil bacteria by 1.7- to 3.0-fold over 10 days, whereas individual metal(loid)s at the same concentrations had no significant effect. Metal(loid) exposure altered the composition of the soil transconjugant community, with some enriched taxa exhibiting potential metal(loid) resistance and pathogenic traits. Mechanistically, As exposure induced oxidative stress, SOS response, membrane damage, and viability reduction primarily in the donor (Escherichia coli SM10λπ), while Cd triggered the same four mechanisms mainly in the recipient (Pseudomonas putida KT2440). Their combination synergistically affected both strains at lower concentrations, eliciting a coordinated stress response encompassing all four pathways. Our findings demonstrate that both individual and combined As and Cd stress promote ARG dissemination, but their co-exposure achieves this promotion at substantially lower concentrations, highlighting a heightened risk in co-contaminated soils.