Infections caused by resistant pathogens result in significant morbidity and mortality and contribute to rising healthcare cost worldwide. The major mechanisms of resistance are permeability barrier of the outer membrane of Gram- negative bacteria as it slows down antibiotic entry into the periplasm. One approach to the restoration of antibiotic activity is to administer them in conjunction with non-antibiotic compounds that depress resistance mechanism. The present study aimed to study the effect of gallic acid, thymol, chitosan, sorbic acid, and EDTA on outer membrane permeability of irradiated and non-irradiated strains to enhance the activity of different antibiotics against multidrug resistance microorganisms. The plasmid profile analysis of some selected strains before and after irradiation was also determined. Various resistance rates were recorded for the tested antimicrobial agents. Percentage resistance of all the tested Gram-negative isolates to different beta-lactam antibiotics exceeded 50% in case of piperacillin (84.85%),cefoperazone / sulbactam (63.64%),cefoxitin (60.61%) while showed (100%) resistance in case of erythromycin.