The release of Zn2+ antimicrobial ions has been suggested as a reasonable hypothesis about the toxicity of ZnO against S. cerevisiae (Kasemets et al., 2009). According to this author, the toxicity of ZnO nanoparticles could result from the solubility of Zn2+ ions in the medium containing the microorganisms.However,the solubility of metaloxides such as ZnO and Al2O3, is a function of their concentration and time (Wang et al., 2009). Thus, low concentrations of solubilized Zn2+ can trigger a relatively hight olerance by the microorganism. In the case of yeast, labile Zn2+ rapidly accumulates in dynamic vesicular compartments (vacuoles and zincosomes), which are an important cellular defense system to buffer both zinc excess and deficiency (Devirgiliis et al., 2004)