Bacterial exposure to ZnO nanoparticles facilitates horizontal transfer of antibiotic resistance genes

Bacterial exposure to ZnO nanoparticles (nZnO) at sublethal concentrations (1 to 10 mg/l) for 24 h significantly increased the conjugative frequency of antibiotic resistance plasmid RP4. A 24.3-fold increase was observed in E. coli pure cultures, and an 8.3-fold increase occurred in a mixed culture of indigenous aquatic microbiota. In addition, nZnO increased by three fold the transformation efficiency of E. coli via the uptake of naked plasmid pGEX4T-1, which was confirmed by confocal fluorescence microscopy. Enhanced horizontal transfer of resistance plasmids was a nanoparticle-specific effect, since 5.3 mg/l Zn2 + (representing the maximum release of Zn2 + from 10 mg/l nZnO) did not enhance RP4 transfer or pGEX4T-1 uptake frequency. This phenomenon was attributed to increased bacterial cell permeability after exposure to nZnO (but not to Zn2 +), which was confirmed by flow cytometry with propidium iodide staining. Overall, this study forewarns that accidental or incidental release of nZnO may facilitate environmental dissemination and propagation of antibiotic resistance genes through horizontal gene transfer.