Characterization and regulation of AcrABR, a RND-type multidrug efflux system, in Agrobacterium tumefaciens C58

Agrobacterium tumefaciens AcrR is the transcriptional repressor of the acrABR operon. The AcrAB efflux pump confers resistance to various toxic compounds, including antibiotics [ciprofloxacin (CIP), nalidixic acid (NAL), novobiocin (NOV) and tetracycline (TET)], a detergent [sodium dodecyl sulfate (SDS)] and a biocide [triclosan (TRI)]. The sequence to which AcrR specifically binds in the acrA promoter region was determined by EMSA and DNase I footprinting. The AcrR-DNA interaction was abolished by adding NAL, SDS and TRI. Quantitative real time-PCR analysis showed that induction of the acrA transcript occurred when wild-type cells were exposed to NAL, SDS and TRI. Indole is a signaling molecule that increases the antibiotic resistance of bacteria, at least in part, through activation of efflux pumps. Expression of the A. tumefaciens acrA transcript was also inducible by indole in a dose-dependent manner. Indole induced protection against CIP, NAL and SDS but enhanced susceptibility to NOV and TRI. Additionally, the TET resistance of A. tumefaciens was not apparently modulated by indole. A. tumefaciens AcrAB played a dominant role and was required for tolerance to high levels of the toxic compounds. Understanding the regulation of multidrug efflux pumps and bacterial adaptive responses to intracellular and extracellular signaling molecules for antibiotic resistance is essential. This information will be useful for the rational design of effective treatments for bacterial infection to overcome possible multidrug-resistant pathogens.