Engineered ZnO and TiO2 nanoparticles induce oxidative stress and DNA damage leading to reduced viability of Escherichia coli

Extensive use of engineered nanoparticle (ENP)-based consumer products and their release into the environment have raised a global concern pertaining to their adverse effects on human and environmental health. The safe production and use of ENPs requires improvement in our understanding of environmental impact and possible ecotoxicity. This study explores the toxicity mechanism of ZnO and TiO2 ENPs in a gram-negative bacterium, Escherichia coli. Internalization and uniform distribution of characterized bare ENPs in the nano range without agglomeration was observed in E. coli by electron microscopy and flow cytometry. Our data showed a statistically significant concentration-dependent decrease in E. coli cell viability by both conventional plate count method and flow cytometric live–dead discrimination assay. Significant (p < 0.05) DNA damage in E. coli cells was also observed after ENP treatment. Glutathione depletion with a concomitant increase in hydroperoxide ions, malondialdehyde levels, reactive oxygen species, and lactate dehydrogenase activity demonstrates that ZnO and TiO2 ENPs induce oxidative stress leading to genotoxicity and cytotoxicity in E. coli. Our study substantiates the need for reassessment of the safety/toxicity of metal oxide ENPs.