Copper Ions Facilitate the Conjugative Transfer of SXT/R391 Integrative and Conjugative Element Across Bacterial Genera

Abstract

Copper can persist stably in the environment for prolonged periods. Except for inducing antibiotic resistance in bacteria, copper ions (Cu²⁺) can facilitate the horizontal transfer of plasmid DNA. However, whether and how Cu²⁺ can accelerate the conjugative transfer of SXT/R391 integrative and conjugative element (ICE) is still largely unknown. In this study, <i>Proteus mirabilis</i> ChSC1905, harboring an SXT/R391 ICE that carried 21 antibiotic resistance genes (ARGs), was used as a donor, and <i>Escherichia coli</i> EC600 was used as a recipient. Cu²⁺, at subinhibitory and environmentally relevant concentrations (1-10 μmol/L), significantly accelerated the conjugative transfer of SXT/R391 ICE across bacterial genera (from <i>P. mirabilis</i> to <i>E. coli</i>) (<i>p</i> < 0.05). The combined analyses of phenotypic tests and genome-wide sequencing indicated that reactive oxygen species (ROS) production and cell membrane permeability were critical in the enhanced conjugative transfer of SXT/R391 ICE. Furthermore, the expression of genes related to cell adhesion and ATP synthesis was also significantly upregulated on exposure to Cu²⁺ at a concentration of 5 μmol/L. This study clarified the potential mechanisms of Cu²⁺ to promote the conjugative transfer of SXT/R391 ICE, revealing the potential risk imposed by Cu²⁺ on the horizontal transfer of SXT/R391 ICE-mediated ARGs.