Recent studies have shown that nano-bio interfaces are the most complex and the least understood. Notably, nanotoxicity of these nanoparticles is not even well recognized. In this work, we examined the toxic effects of different nanoparticles on bacteria cells (Nitrosomonas europaea). The four nanoparticles involved are: 25 nm anatase TiO2, 200 nm anatase TiO2, ZnO and CeO2 particles. These particles will have different electrical charges in the cell cultivating media. It has been observed that even with only 4 h of dosing, all of the particles caused apparent morphological damage to the cells. Experimental results suggest that ZnO particles exert the stress on cells by its dissolution and releasing of Zn2+ ions. The TEM and AUC (analytical ultracentrifuge) result suggest that cells become heavier in presence of CeO2 and TiO2 particles. No visible clear intrusions of bulk nanoparticles were observed. However, both the analytical ultracentrifuge and TEM results show that cells are heavier when being damaged. (C) 2010 Elsevier Inc. All rights reserved.