Background: Increasing use of antibiotics in the treatment of bacterial infections has increased resistance against them. The development of new antimicrobial drugs cannot overcome the continuous and rapid spread of this resistance.

Aim: To determine the antimicrobial efficacy of green zinc oxide nanoparticles against gram-positive and gram-negative bacteria.

Methods: In this study, Escherichia coli (ATCC25922), Staphylococcus aureus (ATCC25923), Pseudomonas aeruginosa (ATCC27853) were tested as microorganisms. High purity zinc oxide nanoparticles were prepared by green and environmentally friendly method. In order to identify the target bacteria, biochemical tests were performed for gram-negative bacteria and specific tests were performed for gram-positive bacteria. Then, the antimicrobial properties of zinc oxide nanoparticles were investigated by MIC method, disk diffusion (measuring the diameter of growth inhibition zone) and exposure method.

Results: The results of MIC showed that zinc oxide nanoparticles with green synthesis method have antimicrobial effect against Escherichia coli, Staphylococcus aureus and also the antibacterial activity of zinc oxide nanoparticles increases with increasing concentration and decreasing nanoparticle size. The results of nanoparticle impregnated disks showed that Escherichia coli was more susceptible than Staphylococcus aureus. The results of exposure showed that after counting the bacterial colonies, the number of Escherichia coli and Staphylococcus aureus colonies decreased and this was a confirmation of the antimicrobial properties of zinc oxide nanoparticles.

Conclusion: Today, researchers believe that zinc oxide nanoparticles can be used as an alternative to antibiotics and even in synergism with antibiotics to treat bacterial infections.

Keywords: Zinc oxide nanoparticles with green synthesis, Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa