Sub-toxic effects of CuO nanoparticles on bacteria: Kinetics, role of Cu ions and possible mechanisms of action

The sub-toxic effects of CuO nanoparticles (nano-CuO) were evaluated using three recombinant luminescent Escherichia coli bacteria responding specifically to (i) reactive oxygen species (ROS), (ii) single-stranded DNA breaks and (iii) bioavailable Cu ions. Using these sensors we showed that nano-CuO induces the formation of superoxide anions, hydrogen peroxide and single-stranded DNA already at very low sub-toxic levels (0.1 mg Cu/L). The maximal sub-toxic response of all biosensors to nominal concentrations of nano-CuO, micro-CuO (size control) and CuSO4 (solubility control) occurred at ∼6, ∼600 and ∼0.6 mg Cu/L, respectively. According to the chemical analysis all the latter concentrations yielded ∼0.6 mg of soluble Cu/L, indicating that dissolution of CuO particles was the key factor triggering the ROS and DNA damage responses in bacteria. Cu-ions chelation studies also showed that CuO particles were not involved in these stress responses. The solubilization results were confirmed by Pseudomonas fluorescens Cu-ion sensor.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► New ROS and DNA-damage-inducible luminescent E. coli biosensors were constructed. ► Already sub-ppm levels of nano-CuO induced the ROS, DNA damage and Cu ion sensors. ► The bacterial ROS and DNA damage response to nano-CuO was triggered by Cu ions.