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Abstract
The toxicity effect due to chronic exposure of ZnO nanoparticles (NPs) was systematically studied by repeatedly treating different lower concentrations of ZnO nanoparticles with culture media of E. coli strain. The chronic exposure of ZnO NPs of concentrations below minimum inhibitory concentration (MIC) exhibited higher toxicity than the single exposure of higher concentrations. Most striking result was 57% inhibition of growth corresponding to chronic exposure of 0.06 mg/mL of ZnO NPs which was two folds more than that exhibited by single exposure of 0.30 mg/mL ZnO NPs. The toxicity of ZnO NPs in E. coli was studied in the light of formation of reactive oxygen species (ROS), measured as malondialdehyde (MDA) equivalent by thiobarbituric acid-ROS (TBARS) assay, and effect of Zn dissolution from ZnO NPs. Higher inhibition of growth for the chronic exposure batches were correlated with higher ROS generation, which subsequently contributed to cause membrane lipid peroxidation, confirmed from observation of cell wall deformation by scanning electron microscopy study and energy dispersive X-ray analysis showed adherence of ZnO NPs on cell wall. The possibility of membrane lipid peroxidation was addressed by revealing in vitro oxidation of oleic acid, which is a monounsaturated fatty acid. Further in this study we have shown that the dissolution of ZnO NPs at pH 7.4 was not significant to cause Zn-induced toxicity.
Acknowledgments
This work has been carried out under the BRNS project 2007/37/47/BRNS and authors thank BRNS for the support. Mr. N. Bhavani Prasad Naik is thankful to MHRD, Govt. of India, for awarding senior research fellowship. We would like to thank Dr. R. Acharya, Bhabha Atomic Research Centre, Mumbai, for his keen interest in this work. We thank Institute Instrumentation Centre of IIT Roorkee for allowing us to use the instrumental facilities.