AbstractWe performed a comprehensive investigation of the toxicity of ZnO and TiO₂ nanoparticles using Escherichia coli as a model organism. Both materials are wide band gap n-type semiconductors and they can interact with lipopolysaccharide molecules present in the outer membrane of E. coli, as well as produce reactive oxygen species (ROS) under UV illumination. Despite the similarities in their properties, the response of the bacteria to the two nanomaterials was fundamentally different. When the ROS generation is observed, the toxicity of nanomaterial is commonly attributed to oxidative stress and cell membrane damage caused by lipid peroxidation. However, we found that significant toxicity does not necessarily correlate with up-regulation of ROS-related proteins. TiO₂ exhibited significant antibacterial activity, but the protein expression profile of bacteria exposed to TiO₂ was different compared to H₂O₂ and the ROS-related proteins were not strongly expressed. On the other hand, ZnO exhibited lower antibacterial activity compared to TiO₂, and the bacterial response involved up-regulating ROS-related proteins similar to the bacterial response to the exposure to H₂O₂. Reasons for the observed differences in toxicity and bacterial response to the two metal oxides are discussed.