Current control efforts are rather rare and rely primarily on antibiotic applications (e.g., streptomycin or oxytetracycline) to protect flowers. However, the use of antibiotics for the management of fire blight is highly controversial due to the potential risk of promoting the emergence and spread of antibiotic resistance [5]. Gram-negative bacteria often possess multidrug efflux transporters within the cytoplasmic

membrane, which have been found to recognize and expel a broad range of structurally unrelated compounds from the cell [6, 7]. Among the multidrug efflux pumps, members of the resistance-nodulation-cell division (RND) family appear to be the most effective efflux systems in Gram-negative bacteria. RND transporters form a tripartite complex, consisting of an inner membrane pump that recognizes and captures the substrates, a periplasmic membrane MK-8931 in vivo fusion protein (MFP) and an outer membrane channel [8, 9]. AcrAB is the major multidrug MLN2238 manufacturer efflux pump in E. coli and shows high conservation among Gram-negative bacteria [8, 10–12]. AcrD, a close homolog of AcrB, is an RND-type efflux pump characterized as a Selleckchem BI 2536 transporter of aminoglycosides, a highly

hydrophilic class of molecules, and as a transporter of several amphiphilic compounds [13, 14]. Typically, the inner membrane pump and the periplasmic MFP are co-transcribed in tandem on polycistronic mRNA molecules [15]. Interestingly, this is

not the case for acrD, which appears as a single gene and seemingly functions in concert with AcrA, a MFP co-transcribed with AcrB [14]. Both AcrAB and AcrD efflux systems are also present in the plant pathogen E. amylovora. AcrAB has already been Thalidomide characterized as an efflux system required for virulence of E. amylovora in resistance towards apple phytoalexins and for successful colonization of the host plant [16]. AcrAB of E. amylovora showed a similar substrate spectrum as AcrAB of E. coli[17]. In this study, the substrate specificity of AcrD from E. amylovora was characterized and its contribution to virulence in apple and pear analyzed. As it was found that acrD is expressed only at low levels under in vitro conditions, we were interested in investigating whether the expression of the AcrD transporter in E. amylovora is induced in planta. Multidrug transporters are often expressed under control of local, as well as, global transcriptional regulators [18]. Current data show that expression of acrD in E. coli can be induced by the two-component regulatory system BaeSR [19]. Two-component systems (TCS) play an important role in the regulation of physiological processes in response to environmental or cellular parameters and enable bacterial cells to adapt to changing environmental conditions.