98 N. Egypt. J. Microbiol. Vol. 46, January, 2017. EFFECT OF RESVERATROL ON QUORUM SENSING AND SOME VIRULENCE CHARACTERISTICS OF PSEUDOMONAS AERUGINOSA ISOLATED FROM MANOURA UNIVERSITY HOSPITALS BY Areej M. El-Mahdy FROM 1 Department of Microbiology and Immunology, Faculty of Pharmacy, Mansoura University, Mansoura 35516, Egypt 2 Department of Pharmaceutical Sciences, College of Pharmacy, Princess Nourah bint Abdulrahman University, Riyadh 11671, Saudi Arabia ABSTRACT Plant polyphenols possess diverse antimicrobial properties, such as antimicrobial action, synergism with other antimicrobials and reduction of bacterial virulence. Resveratrol (3, 5, 4’-trihydroxy-trans-stilbene) is a natural polyphenol belonging to the stilbene class. Resveratrol can be obtained from trees, few flowering plants, in peanuts, and in grapes. Several in vitro studies discovered various biological activities of resveratrol. These effects include the antioxidative, anti- inflammatory, and estrogenic effects, in addition to anticancer activities. The widespread of antibiotic-resistant Pseudomonas aeruginosa represents a global problem that necessitate the introduction of new antibiotics or non-antibiotic strategies. This study proved that resveratrol has activity against Egyptian isolates of Ps. aeruginosa, by examining its effect on quorum sensing, motility and some virulence factors including: protease, pyocyanin and lecithinase production. All tests were determined with and without resveratrol. It was observed that resveratrol have quorum sensing inhibitory (QSI) activity and can suppress the motility and the tested virulence factors without affecting the growth rate of the tested isolates. It can be concluded that resveratrol could be used as a virulence inhibitor in Ps. aeruginosa and this could be an alternative strategy for treating infections caused by Ps. aeruginosa. INTRODUCTION Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen that could be found in different environmental conditions. Ps. aeruginosa frequently enhances rates of morbidity and mortality due to its readiness for exhibiting resistance to different antimicrobials. Moreover, its ability to secrete toxins and virulence factors, including pyocyanin, protease and elastase (Hog et al., 2010) and facilitates its existence in the host tissues during early stages of infection. (Rada and Leto, 2013). Pseudomonas aeruginosa is motile organism and this is obviously associated with its virulence (Inoue et al., 2008). Pseudomonas aeruginosa secretes vast array of virulence factors that wriggle the immune system using various adaptive mechanisms. Multidrug resistance and several virulence factors produced by Pseudomonas make the treatment more complicated