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ZnO nanoparticles (ZnO-NPs) can be synthesized by chemical, physical and biological methods; many studies have indicated that biologically synthesized ZnO-NPs have ecofriendly benefits over physically and chemically synthesized ZnO-NPs. This study was carried out to evaluate the differences in characteristics and effects on pathogenic bacteria between ZnO-NPs synthesized by a chemical method and a biological method using the marine green alga Ulva fasciata. Additionally, the synergistic or antagonistic effects of loading ZnO-NPs synthesized by chemical and biological methods on antibiotics are investigated. The results show that there are differences in the characteristics of ZnO-NPs synthesized by the various methods, as determined by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM). ZnO-NPs had antibacterial activity against Staphylococcus aureus, Salmonella enterica subsp. salamae (Em.1-EGY015), Aeromonas hydrophila, Escherichia coli O157 (KY797670) and Bacillus cereus SH06; moreover, green-synthesized ZnO-NPs were more effective against pathogenic bacteria than chemically synthesized ZnO-NPs. The minimum inhibitory concentration (MIC) of ZnO-NPs synthesized by the green method was less than that of ZnO-NPs synthesized by the chemical method. The results demonstrated that the synergistic or antagonistic effects of ZnO-NP loading on antibiotics (ampicillin/sulbactam, tobramycin, flucloxacillin, chloramphenicol, amoxicillin, cephalexin, of loxacin, and neomycin) varied according to the type of pathogenic bacteria, antibiotic, and method of synthesis of ZnO-NPs. [ABSTRACT FROM AUTHOR]