Effects of cations and nanoparticles on the antibiotic-resistance gene transformation of Escherichia coli at different time scales

Abstract

Antibiotic resistance genes (ARGs) can be horizontally transferred when bacterial cells take up environmental DNA and gain the antibiotic resistance, a process referred to as transformation. This study investigated the effects of two major cations (monovalent Na⁺, divalent Ca²⁺) and two representative organic (polystyrene nanoparticles, PSNP) and inorganic (silver nanoparticles, AgNP) nanoparticles, on the natural transformation of E. coli. In nanoparticle-free systems, both Na⁺ and Ca²⁺ promoted transformation. The effect was stronger at higher ion concentration and in Ca²⁺ than in Na⁺. Ion-induced plasmid conformation change, reduced electrostatic interaction, and cation bridging (for Ca²⁺) were identified as dominant mechanisms leading to the increased plasmid uptake and transformation, despite decreased membrane permeability. Interestingly, AgNP and PSNP exhibited time-dependent, opposing roles in transformation. At 1.5 hours, AgNP promoted transformation by increasing membrane permeability, while PSNP inhibited it by binding plasmids and preventing uptake. However, over 24 hours, these effects reversed: AgNP inhibited transformation due to accumulating toxicity of dissolved Ag⁺, whereas PSNP enhanced transformation by increasing membrane permeability through PSNP accumulation. These findings underscore the complex, material-specific, and time-dependent influences of nanoparticles on bacteria transformation, revealing their potential environmental and health implications.