Background: The development of multidrug-resistant Acinetobacter species has created serious problems in nosocomial infections. Understanding the underlying resistance mechanisms and their significance in conferring resistance to different antibiotics is the first step to develop strategies for fighting or reversing the current resistance. Objectives: The aim of this study was to investigate the role of efflux pumps in decreasing susceptibility to amikacin inAcinetobacter clinical isolates. Methods: Forty-six clinical Acinetobacter isolates were collected from 2 teaching hospitals of Mashhad, Iran. Susceptibility testing was conducted by the disc diffusion method. Amikacin minimum inhibitory concentration (MIC) for resistant Acinetobacter isolates was determined according to the Clinical and Laboratory Standards Institute (CLSI) guidelines either with or without the efflux pumps inhibitor, carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Conventional polymerase chain reaction (PCR) was used to analyze the presence of pump genes. Results: Acinetobacter isolates were identified as 2 species; Acinetobacter baumannii and A. lwoffii. Susceptibility testing showed high levels of resistance to amikacin in 27 isolates, including both A. baumannii and A. lwoffii, among which 20 A. baumannii isolates showed a 2- to 524288-fold reduction in amikacin MIC in the presence of CCCP, while no reduction occurred in amikacin MIC in resistant A. lwoffii isolates. The PCR results showed high frequencies of adeB, abeM, and adeI genes in Acinetobacter isolates yet the adeE gene was not found in any of the isolates. Conclusions: The obtained results indicated the importance of efflux pumps in conferring resistance to amikacin in clinical isolates of A.baumannii, yet not in A. lwoffii.

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