Dependence of the antibacterial behavior on ZnO (TM-doped and surface-modified) nanoparticles on Escherichia coil cells has been investigated. ZnO nanoparticles that differ in size, activator ion and in the microenvironment in which these nanoparticles are embedded were used. Comprehensive antibacterial studies of these ZnO nanoparticles owing to their size and surface defects are carried out against E. coli cells. These studies have been carried out both in Luria-Bertani medium and on solid agar medium in the presence and absence of light. The differences in antibacterial effect have been quantified in terms of minimum inhibitory concentration, minimum bactericidal concentration, colony forming unit counts, and qualitatively evaluated by growth curves and disk diffusion tests. The difference in antibacterial activities of the ZnO nanoparticles may be attributed to the enhanced or reduced oxygen vacancies and defect states and could be attributed to the increased or decreased surface defects.