Concentration-dependent interfacial effects of mackinawite on conjugative transfer of antibiotic resistance genes in redox environments

The conjugative transfer (CT) of antibiotic resistance genes (ARGs) during dynamic conversion of mackinawite (FeS) have received little attention. We disentangled the individual contribution of minerals-mediated and •OH-mediated processes towards CT. Low concentrations of FeS promoted CT frequency with a volcano-shaped change trend, which was jointly driven by minerals-mediated and •OH-mediated processes, with their relative contributions shifting from equalling to prevailing by the former as FeS concentration increased. However, high concentrations of FeS manifested significant inhibition to CT owing to growth coercion caused by FeS itself. The key of promoting CT lied in a cascade of up-regulated bacterial responses against oxidatation, involving membrane permeability, ATP synthesis, extracellular polymeric substances secretion, and quorum sensing. The stress triggered by excessive FeS impaired membrane integrity and caused overexpression of bacterial responses, thereby suppressing CT. These findings provide valuable insights into effects of active minerals on ARGs dissemination and developing pollution control technologies. Low concentrations of mackinawite (5-50 mg/L) promoted conjugative transfer through mineral- and hydroxyl radical-mediated processes, while high concentrations (75−100 mg/L) inhibited transfer due to growth coercion and refractory cell damage, based on model conjugation experiments.