The effect of nanoscale zero-valent iron (nZVI) particles on bacteria from different growth phases was studied. Four bacterial strains including Escherichia coli strains JMI09 and BW25 l l 3, and Pseudomonas putida strains KT2440 and Fl were experimented. The growth characteristics of these strains were detennined. Their cells were harvested based on predetermined time points corresponding to different growth phases and exposed to nZVL The cell viability was determined by a plate count method. The cells in lag and stationary phases showed higher resistance to nZVI for all four bacterial strains, whereas the cells in exponential and decline phases were less resistant and were rapidly inactivated when exposed to nZVI. Bacterial inactivation increased with the concentration of nZVI. Furthermore, less than 14% reduction in viability was observed when the cells were exposed to the leachate of nZVI suspension suggesting that the physical interaction between nZVI and the cells is critical for bacterial inactivation. To understand the physiology that underlines these phenotypes, the responses from various oxidative stress gene knockout strains of E. coli BW25 l l 3 to nZVI were examined. For each of these mutant strains, cells from different growth phases were collected and exposed to nZVI. The viability of the cells was determined by a plate count method. All of the mutant strains exhibited higher susceptibility to nZVI when compared to the wild type strain. The results also indicated that different knockout strains exhibited different levels of susceptibility to nZVI. Strain lacking RpoS, a global stress regulator, showed the highest susceptibility. Among different defensive enzyme mutants, sodA and sodB mutants exhibited the highest vulnerability whereas sodC mutant revealed much less susceptibility, suggesting that nZVI may induce oxidative stress inside the cells via superoxide generation. The inducibility of catalase (hydroperoxidase I) was also investigated by exposing the cells to nZVI and measuring a related gene expression using quantitative polymerase chain reaction. Results suggested that nZVI repressed the expression of this enzyme.