Contribution of mexAB-oprM and mexXY (-oprA) efflux operons in antibiotic resistance of clinical Pseudomonas aeruginosa isolates in Tabriz, Iran. 
Overexpression of efflux pumps is one of the most important mechanisms that contributes to intrinsic and acquired resistance to antibiotics in Pseudomonas aeruginosa.  The present study evaluated the role of MexAB-OprM and MexXY (-OprA) efflux pump overexpression in antibiotics resistance of P. aeruginosa clinical isolates.  One-hundred clinical isolates of P. aeruginosa were obtained from four hospitals of Tabriz city in Northwest Iran.  Isolates were identified and evaluated by the disk diffusion method and agar dilution in order to determine antibiotic resistance.  Effect of Phenylalanine Arginine beta-Naphthylamide (PAβN) on susceptibility to various anti-Pseudomonas antimicrobials and expression levels of mexB and mexY using quantitative real-time PCR were determined in the clinical isolates.  Random Amplified Polymorphic DNA Typing (RAPD-PCR) was used for genotyping of the isolates.  The most and least effective antibiotics tested were colistin and ofloxacin, respectively.  Seventy-one percent of the isolates were found as multidrug resistant (resistant to at least three different classes of antibiotics).  Among ciprofloxacin and levofloxacin resistant isolates, 39.6% and 28.5% of them showed four-fold reduction in MIC with PAβN, respectively.  Sixty-two percent and 65% of isolates overexpressed mexB and mexY, respectively.  Sixty six isolates showed overexpression of both mexB and mexY efflux genes.  Moreover, 76% and 88.7% of MDR isolates were mexB and mexY overexpressed, respectively.  There were 30 different RAPD types in this study which were clustered into 6 clones.  The study indicated that there is a significant correlation between the expression of efflux pumps and the resistance to most anti-pseudomonal antibiotics.