The antibacterial activity of zinc oxide nanoparticles (ZnO NPs) against gram-positive and -negative bacteria has been extensively studied but the mechanism of this effect has not been revealed. The aim of this study was investigate mechanism of antibacterial activity of ZnO NPs against Acinetobacter baumannii. A. baumannii is one of the opportunistic pathogens responsible for nosocomial infections especially among immune compromised patients that resistance to most antibiotics. Environmental resilience and antibiotic resistance render it a difficult nosocomial pathogen and posing a serious threat to global public health. In the present study, ZnO NPs are synthesized by solvothermal method. The morphology, structure, and size of the synthesized NPs were characterized by SEM, XRD, and particle size analyzer. The preparation of stable ZnO nanofluids was carried out by mixing ZnO NPs with ammonium citrate and glycerol. After determining the minimum inhibitory concentration (MIC) of ZnO nanofluids by microdilution method, the mechanism of action of ZnO NPs was evaluated by DNA extraction, SEM microscopy, fluorescence microscopy and DAPI staining methods. The results of this study showed that the morphology of bacteria treated with ZnO nanofluids was changed. The possible reasons for the morphological changes may be direct interaction between ZnO NPs and the external membrane surface to increase membrane permeability or accumulation of ZnO NPs in cytoplasmic regions, and induction of DNA damage.

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