Tehran University of Medical Sciences Journal of Medical Bacteriology 2251-8649 2 3-4 2015 10 12 32 40 EN Mohammad Reza Pourmand Department of Pathobiology, School of Public Health, Tehran University of Medical Sciences, Tehran, IR Iran. Hooman Sadighian Urology Research Center, Sina Hospital, Tehran University of Medical Sciences, Tehran, IR Iran AND Department of Microbiology, School of Medicine, Zahedan University of Medical sciences, Zahedan, IR Iran. Mahmood Naderi Medical Biotechnology, School of Medical Sciences, Tarbiat Modares University, Tehran, IR Iran AND Department of Molecular Biology and Genetic Engineering, Stem Cell Technology Research Center, Tehran, IR Iran. 2015 10 12 2015 10 12 Background: Quorum-sensing systems regulate expression of several virulence factors in Pseudomonas aeruginosa. This study investigated the relation between expression of the las quorum-sensing system and expression of mexY and ampC in35 clinical isolates of P. aeruginosa. Methods: Antibiotic susceptibility was determined by the disc diffusion method according to the Clinical and Laboratory Standards Institute (CLSI) guidelines. Expression of genes including lasI, lasR, mexY, ampC and ampR was assessed by real time RT-PCR. Results: Twenty eight isolates with elevated expression of mexY and ampC (compared to  P. aeruginosa PAO1) were resistance to ceftazidime, imipenem, ciprofloxacin and amikacin. Also these isolates demonstrated increased expression of lasI and lasR. The remaining seven isolates showed intermediate resistance to ciprofloxacin and amikacin. These seven isolates with elevated ampR expression demonstrated decreased expression of mexY, ampC, lasI and lasR. Conclusion: In contrast to previous study, current study demonstrated that the expression pattern of ampC was identical to the lasI expression pattern among clinical isolates. Furthermore according to previous study we expected that AmpR positively regulated ampC expression but in our study, some of the isolates with elevated ampR expression showed decreased ampC expression. The expression pattern of lasR and mexY was identical in all of the isolates. It seems there was a direct or indirect relation between expression of lasR and mexY expression in P. aeruginosa. https://jmb.tums.ac.ir/index.php/jmb/article/view/49 https://jmb.tums.ac.ir/index.php/jmb/article/download/49/44