Tehran University of Medical Sciences Iranian Journal of Microbiology 2008-3289 8 1 2016 03 01 62 69 EN Hamid Reza Goli Drug Applied Research Center, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Mohammad Reza nahaei Drug Applied Research Center, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Mohammad Ahangarzadeh-Rezaee Infectious Diseases and Tropical Medicine Research Center , Faculty of Medicine, Tabriz University ofMedical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Alka Hasani Infectious Diseases and Tropical Medicine Research Center , Faculty of Medicine, Tabriz University ofMedical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Hossein Samadi Kafil Drug Applied Research Center, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. Mohammad Aghazadeh Drug Applied Research Center, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran AND Department of Medical Microbiology, Faculty of Medicine, Tabriz University of Medical Sciences, Tabriz, Iran. 2015 11 29 2016 02 02 Background and Objectives: The prevalence of multidrug resistant Pseudomonas aeruginosa is the main reason of new drugs resurgence such as colistin. The main objectives of this study were to determine the antibiotic resistance pattern and the rate of colistin resistance along with its correlation with overexpression of MexAB-OprM and MexXY-OprM efflux pumps among P. aeruginosa isolates. Materials and Methods: Hundred clinical isolates were collected from 100 patients during 6 months in 2014. Susceptibility to the eight antibiotics was investigated using Kirby-Bauer and agar dilution methods. The Quantitative Real-time PCR was used to determine the expression levels of efflux genes. Results: Resistance rates to various antibiotics were as follows: ticarcillin (73%), ciprofloxacin (65%), aztreonam (60%), ceftazidime (55%), gentamicin (55%), imipenem (49%), piperacillin/tazobactam (34%) and colistin (2%). In disk diffusion method, only two isolates were non susceptible to colistin, however in agar dilution method the two isolates were confirmed as resistant and two others were intermediate resistant. Sixty eight (68%) isolates were multi-drug resistant and 10 isolates were susceptible to all tested antibiotics. Both colistin resistant isolates showed overexpression of both efflux pumps, but two intermediate resistant isolates exhibited reduction of efflux genes expression. Conclusions: Emergence of colistin resistance is increasing in P. aeruginosa indicating great challenge in the treatment of infections caused by MDR strains of this organism in Iran. ParRS may promote either induced or constitutive resistance to colistin through the activation of distinct mechanisms such as MDR efflux pumps, and LPS modification. https://ijm.tums.ac.ir/index.php/ijm/article/view/826 https://ijm.tums.ac.ir/index.php/ijm/article/download/826/562