Regulation of the AcrAB efflux system by the quorum-sensing regulator AnoR in Acinetobacter nosocomialis

Multidrug efflux pumps play an important role in antimicrobial resistance and pathogenicity in bacteria. Here, we report the functional characterization of the RND (resistance-nodulation- division) efflux pump, AcrAB, in Acinetobacter nosocomialis . An in silico analysis revealed that homologues of the AcrAB efflux pump, comprising AcrA and AcrB, are widely distributed among different bacterial species. Deletion of acrA and/or acrB genes led to decreased biofilm/pellicle formation and reduced antimicrobial resistance in A. nosocomialis . RNA sequencing and mRNA expression analyses showed that expression of acrA/B was downregulated in a quorum sensing (QS) regulator ( anoR )-deletion mutant, indicating transcriptional activation of the acrAB operon by AnoR in A. nosocomialis . Bioassays showed that secretion of N-acyl homoserine lactones (AHLs) was unaffected in acrA and acrB deletion mutants; however, AHL secretion was limited in a deletion mutant of acrR , encoding the acrAB regulator, AcrR. An in silico analysis indicated the presence of AcrR-binding motifs in promoter regions of anoI (encoding AHL synthase) and anoR . Specific binding of AcrR was confirmed by electrophoretic mobility shift assays, which revealed that AcrR binds to positions -214 and -217 bp upstream of the translational start sites of anoI and anoR , respectively, demonstrating transcriptional regulation of these QS genes by AcrR. The current study further addresses the possibility that AcrAB is controlled by the osmotic stress regulator, OmpR, in A. nosocomialis . Our data demonstrate that the AcrAB efflux pump plays a crucial role in biofilm/pellicle formation and antimicrobial resistance in A. nosocomialis , and is under the transcriptional control of a number of regulators. In addition, the study emphasizes the interrelationship of QS and AcrAB efflux systems in A. nosocomialis .