Large scale production and incorporation of titanium dioxide nanoparticles (NP-TiO2) in consumer products leads to their potential release into the environment and raises the question of their toxicity. The bactericidal mechanism of titanium dioxide nanoparticles (NP-TiO2) under UV light is known to involve oxidative stress due to the generation of reactive oxygen species. In the dark, several studies revealed that NP-TiO2 can exert toxicological effects. However, the mode of action of these nanoparticles is still controversial. In the present study, we used a combination of fluorescent probes to show that NP-TiO2 causes E. coli membrane depolarization and loss of integrity, leading to higher cell permeability. Using both transcriptomic and proteomic global approaches we showed that this phenomenon translates into a cellular response to osmotic stress, metabolism of cell envelope components and uptake/metabolism of endogenous and exogenous compounds. This primary mechanism of bacterial NP-TiO2 toxicity is supported by the observed massive cell leakage of K+ / Mg2+ concomitant with the entrance of extracellular Na+, and by the depletion of intracellular ATP level.This article is protected by copyright. All rights reserved