Background and Aim: When the size of a particle is reduced from micrometer to nanometer, its properties may change dramatically. For example, electrical conductivity, hardness, active surface area, chemical and biological activities are all known to be altered. The bactericidal effectiveness of metallic nanoparticles is believed to be due to both, their size and high surface-to-volume ratio. Reducing the particle size of the materials is an efficient and reliable tool for improving biocompability. Magnesium oxide Nano particles (MgO NPs) among inorganic antibacterial agents exhibit good bactericidal performance in aqueous environments due to producing freeradicals such as O2-. Silver nanoparticles (Ag NPs) are the most important antibacterial nanomaterials because of having high bactericidal activity and also good biocompability. In this study Mg (1-x) Ag (x) O (x=0.03) Nano composites was prepared by Sol - gel method and its antibacterial properties was investigated by spectrophotometrical methods against Escherichia coli and Bacillus subtilis. Also the structure and morphologies of MgO and Mg0.97Ag0.03O were characterized using X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. Material and Methods: Mg0.97Ag0.03O nanocomposite was prepared with Sol-gel method. It was dispersed in nutrient Broth with concentration of 250g/mL, 550g/mL and 750g/mL, each concentration was done three times, then they were autoclaved at 100C for 30min. After that 1mL of bacterial suspension (1McFarland) was added to each Erlenmeyer flask. In addition, positive control Erlenmeyer flask containing the same amounts of bacterial cell and nutrient broth. Negative control Erlenmeyer Flasks containing only nutrient broth. All the Erlenmeyer Flasks were incubated in a reciprocal shaker with a shaking speed of about 200 rpm/min at 37 C and the ODs was measured with spectrophotometer each hour during 24 hours and Growth curve was drawn. Results: The crystallite size of MgO and Ag in of Mg 0.97 Ag 0.03O composition were 27nm and 47nm, respectively using Scherrer's formula. For E. Coli as a gram negative bacteria indicator, Mg 0.97 Ag 0.03 O nanocomposite showed high antibacterial activity especially in 550g/mL and 750g/mL concentration while had a poor activity in 250g/mL. Similar results were observed for gram positive B. Subtilis in which bacterial growth was completely inhibited at presence of 550 and 750 g/ml Mg 0.97 Ag 0.03 O. Also the growth of both bacteria was slightly affected but not significantly with 250 g/ml NPs in comparison with positive control, where no NPs were present. Conclusion: To summarize, the Mg 0.97 Ag 0.03 O was obtained by a sol–gel method, and the antibacterial properties were examined using E. coli as a gram negative bacteria and B.subtilis as a gram positive bacteria, while the latter was found to be more sensitive because gram-positive and gram-negative bacteria are different in their membrane structure, the most distinctive of which is the thickness of the peptidoglycan layer.

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