Antibacterial activity by ZnO nanorods and ZnO nanodisks: A model used to illustrate “Nanotoxicity Threshold”

Abstract

Many studies suggested that waste water treatment using metal oxide nanoparticles, in particular the zinc oxide nanoparticles, appears to be an economical and efficient solution. However, antibacterial activities reported from various research groups were often confusing and contradicting. The present study aimed to evaluate the antibacterial effects and the underlying mechanisms on the custom-engineered Zinc oxice (ZnO) nanorods and ZnO nanodisks. Firstly, antibacterial activities of ZnO nanorods and ZnO nanodisks were studied on both Gram positive and Gram negative bacteria. The antibacterial performance of these ZnO nanoparticles were bench marked with titanium dioxide nanoparticles (TiO2). In antibacterial susceptibility assay, both types of ZnO nanoparticles showed potent inhibition on Bacillus subtilis (B. subtilis) and Staphylococcus aureus (S. aureus) with minimal inhibitory concentration (MIC) at 128 and 256 ug/ml, respectively. Nevertheless, no inhibition was found against Gram negative bacteria. Interestingly, no inhibition was found against both Gram positive and Gram negative bacteria for TiO2 nanoparticles, and thus TiO2 nanoparticles were omitted in the subsequent study. In the time-kill kinetics assay, the inhibitory effect of both types of ZnO nanoparticles on B. subtilis was significant as early as 90min after treatment at 128μg/ml. Besides different types of bacteria, generation of reactive oxygen species (ROS), surface area, surface charges, polarity were also examined. Our study suggested that the antibacterial effects of ZnO nanorods and ZnO nanodisks were mediated through not one but multiple chemical and physical properties of nanoparticles. This is the first report introducing a new concept on ‘Nanotoxicity Threshold’, i.e. the maximal tolerance level in bacteria before any damage manifested. Our study warrants that a more holistic view on the mechanistic insights of nanoparticles is required to minimize adverse health effect on human.