bTo develop applicable and susceptible models to evaluate the toxicity of nanoparticles, the antimicrobial effects of CuO nanoparticles (CuO-NPs) on various Saccharomyces cerevisiae (S. cerevisiae) strains (wild type, single-gene-deleted mutants, and multiple-gene-deleted mutants) were determined and compared. Further experiments were also conducted to analyze the mechanisms associated with toxicity using copper salt, bulk CuO (bCuO), carbon-shelled copper nanoparticles (C/Cu-NPs), and carbon nanoparticles (C-NPs) for comparisons. The results indicated that the growth inhibition rates of CuO-NPs for the wild-type and the single-gene-deleted strains were comparable, while for the multiple-gene deletion mutant, significantly higher toxicity was observed (P < 0.05). When the toxicity of the CuO-NPs to yeast cells was compared with the toxicities of copper salt and bCuO, we concluded that the toxicity of CuO-NPs should be attributed to soluble copper rather than to the nanoparticles. The striking difference in adverse effects of C-NPs and C/Cu-NPs with equivalent surface areas also proved this. A toxicity assay revealed that the multiple-gene-deleted mutant was significantly more sensitive to CuO-NPs than the wild type. Specifically, compared with the wild-type strain, copper was readily taken up by mutant strains when cell permeability genes were knocked out, and the mutants with deletions of genes regulated under oxidative stress (OS) were likely producing more reactive oxygen species (ROS). Hence, as mechanism-based gene inactivation could increase the susceptibility of yeast, the multiple-gene-deleted mutants should be improved model organisms to investigate the toxicity of nanoparticles. With recent developments in nanotechnology around the world, there are mounting concerns regarding the potential environmental and human health risks caused by exposure to engineered nanomaterials (ENMs). Relative to the rapid development of nanotechnology, ecotoxicology and environmental hazard assessments have obviously fallen behind and still represent huge knowledge gaps (1). It is thus urgent to establish approaches to speed up safety analysis, and the insights gained from these approaches could give us a better understanding of the hazardous effects ENMs at the biological level and assist the development of safe-design approaches.As one of the increasingly used metal oxide nanoparticles (NPs), CuO-NPs have been applied to superconducting materials, sensing materials, glass, and ceramics and in other primarily antimicrobial applications (2). The extensive production and usage of CuO-NPs increase the harmful effects to organisms. Investigators have demonstrated that CuO-NPs are toxic to Escherichia coli HB101 (3, 4), prokaryotic alga Microcystis aeruginosa (5), airway epithelial (HEp-2) cells (6), the crustacean Thamnocephalus platyurus (7), Lemna gibba (8), mice (9), and other organisms. A considerable amount of research has been done about the toxicity of CuO-NPs, and comparative interspecies sensitivity was found to vary...