In order to assess the potential hazards of nanoparticles (NPs) releasing, better knowledge about their toxicity to microbes is required. However, it remains controversial whether NPs could exert particles pecifictoxicity. In this study, the toxic impacts of four kinds of rare earth oxides (REO) NPs (La(2)O(3),CeO(2), Gd(2)O(3), and Yb(2)O(3)) on gram-negative Escherichia coli (E. coli) pBR322 were examined. The results indicate that all the tested NPs possessed cytotoxicity against E. coli. To evaluate the ion-related toxicity of REO NPs, the NPs dissolution in the presence of test organisms was quantitatively measured using X-ray absorption fine structure (XAFS) spectroscopy. Our results suggest that NPs-cell contact could facilitate the dissolution of NPs, and the additional ionic release at the particle-cell interface might result in a substantial increase in the ion-related toxicities towards the test organisms. Therefore, the ion-related toxicity of NPs might be grossly underestimated if the additional dissolution of NPs caused by particle-cell contact was overlooked, further leading to a false interpretation of particle-specific toxicity. To the best of our knowledge, this is the first determination of the total NPs dissolution after particle-cell contact. These findings are helpful to advance mechanistic understanding of the toxicity exerted by dissolvable metal-based NPs.