Release and bacterial transformation activity of plasmid-borne antibiotic resistance genes adsorbed on iron nanoparticles as induced by sulfide reduction.

Dissolved sulfides are widely distributed in anoxic soils and sediments and can readily reduce the ubiquitous iron(hydro)oxide nanoparticles (IONPs), which strongly adsorb extracellular antibiotic resistance genes (eARGs) and thus inhibit their transformation activity. Here, we investigated whether and to what extent sulfide-induced reductive dissolution of IONPs affects the release and transformation potential of adsorbed eARGs. As the concentration of Na2S increased from 0.05 to 5 mmol/L, the release ratios of adsorbed plasmid from hematite nanoparticles (HNPs) and goethite nanoparticles (GNPs) increased from 0.8 % and 0.6 % to 91 % and 75 %, respectively, with the presence of Pahokee Peat Humic Acid (PPHA, 10 mg C/L). However, in the absence of PPHA, no plasmid was released regardless of the concentration of Na2S. Remarkably, increasing sulfide concentration concurrently reduced the activity of released plasmid to transform Escherichia coli DH5α. This observed much lower transformation activity was accounted for by sulfide-induced deactivation reaction of released plasmid. Intriguingly, PPHA played dual and opposing roles in mediating the transformation activity of released plasmid: an accelerator that prevented re-adsorption of released plasmid back to IONPs and to newly formed FeS, and a suppressor that facilitated electron transfer from sulfides to plasmid to exacerbate its deactivation. Considering the ubiquitous presence of sulfides in anoxic environments and their crucial role in iron cycling, the sulfide-induced release of IONPs-bound eARGs is particularly important and deserves serious consideration when assessing eARGs' fate and the potential for horizontal transfer to bacteria.