Similar results have been reported by Padmavathy and Vijayaraghavan (2008), indicating that the ZnO nanoparticle suspensions in lower concentrations (0.01–1 mM) seem to have less antimicrobial activity against E. coli, and the presence of soluble Zn2+ ions may act as nutrients for this microorganism. Thus, while metals and metallic oxides are known to be toxic at relatively high concentrations, ZnO has shown no toxicity at low concentrations, since the zinc element is an essential cofactor in a variety of cellular processes.