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Background  This study was aimed to characterize the genetic diversity and expression of three putative resistance-nodulation-cell division (RND)-type efflux systems and their contribution to multidrug efflux in clinical isolates of  Acinetobacter baumannii .    Methods  Antimicrobial susceptibility testing of 95  A. baumannii  isolates was determined by Kirby-Bauer disk diffusion for 18 antibiotics and minimum inhibitory concentration (MIC) of colistin was determined by the broth microdilution method. Moreover, the MIC of five classes of antibiotics was assessed using E-test strips in the presence and absence of phenylalanine-arginine beta-naphthylamide (PAβN). Regulatory genes of the RND efflux pumps ( adeRS ,  adeL ,  adeN  and  baeSR ) were subjected to sequencing. The relative expression of  adeB ,  adeG  and  adeJ  genes was determined by quantitative real-time PCR (qRT-PCR).    Results  Overall, the majority of isolates (94%) were extensively drug-resistant (XDR). In the phenotypic assay, efflux pump activity was observed in 40% of the isolates against multiple antibiotics mainly tigecycline. However, we found no efflux activity against imipenem. Several amino acid substitutions were detected in the products of regulatory genes; except in AdeN. Of note, G186V mutation in AdeS was found to be associated with overexpression of its efflux pump. No insertion sequences were detected.    Conclusions  Our findings outlined the role of RND efflux pumps in resistance of  A. baumannii  to multiple antibiotics particularly tigecycline, and pointed out the importance of a variety of single mutations in the corresponding regulatory systems. Further studies are required to decipher the precise role of RND efflux pumps in multidrug-resistant clinical isolates of  A. baumannii .

antimicrobial resistance and infection control

Characteristics and diversity of mutations in regulatory genes of resistance-nodulation-cell division efflux pumps in association with drug-resistant clinical isolates of Acinetobacter baumannii