Today the emergence and rapid spread of bacterial species with multidrug resistance (MDR) is a significant challenge in the field ofmedicine and treatment. Acinetobacteris an opportunistic pathogen that is considered as a threatening microorganism in treatment due to its high resistance to many antimicrobial drugs. Acinetobacter causes nosocomial infections such as bacteremia, secondary meningitis, urinary tract infections, wound and skin infections. It is also the most important cause of nosocomial pneumonia. The efflux pumps overexpression is one of the mechanisms of antibiotic resistance in Acinetobacter. This study aimed to identify presumed highly expressed efflux systems, responsible for the MDR phenotype in Acinetobacterspecies, using the EB-agar cartwheel method. The basis of this method is using different concentrations of ethidium bromide as the efflux pumps substrate to differentiate between an over-expressed efflux system in mutant and the intrinsic efflux system of the wild type through fluorescence activity. In this approach, the higher the concentration of ethidium bromide required to produce fluorescence of the bacterial mass, the greater the efflux capacity of the bacterial cells. Fifty Acinetobactersamples were isolated from patients in Shahid Kamyab hospital of Mashhad and after antibiotic susceptibility testing, MDR strains were selected and tested by the above method. The light produced by bacterial mass was analysed exposing to UV light after 16-18 h incubation in 37 ° C. Finally, the capacity of the efflux pumps for each sample was estimated and compared with others.

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