Heavy metal copper accelerates the conjugative transfer of antibiotic resistance genes in freshwater microcosms

Recent studies have consistently demonstrated increasing abundances of antibiotic resistance genes (ARGs) in the absence of antibiotic use. There is a large amount of quantitative data that has correlated the elevated ARGs levels with the concentrations of heavy metals in environments with anthropogenic impact. However, the mechanisms by which heavy metals facilitate the proliferation and horizontal gene transfer of ARGs among environmental bacteria were still unknown. This study validated effects of four typical heavy metals (Cu, Cd, Pb, Zn) on the plasmid RP4 mediated conjugative transfer of ARGs in freshwater microcosms. The results suggested that the typical heavy metals including Cu, Pb and Zn would promote conjugative transfer of the plasmid RP4, and Cu (5.0 μg/L) had the greatest ability to increase conjugative transfer by 16-fold higher than the control groups. In conjugative transfer microcosms, the species of each cultivable transconjugant were isolated, and their minimum inhibitory concentrations (MICs) were assessed via antibiotic susceptibility testing. The mechanism of the increased conjugative transfer of Cu was that Cu induced cell damage and the reduced conjugative transfer of Cd was that Cd increased the content of extracellular polymers substances (EPS). This study confirms that heavy metal Cu facilitates the conjugative transfer of environmental-mediated plasmid RP4 by cell damage effect, therefore accelerating the transmission and proliferation of ARGs.