As the host range of E. amylovora also includes pear trees, we further investigated selleck screening library the virulence of the wild type and its acrD-deficient mutant on immature pear fruits (cv. ‘Bartlet’) with the conclusion that AcrD is not involved in the interaction of the fire

blight pathogen with this host. Additionally, we studied the expression levels of the AcrAB and AcrD efflux pumps in vitro and in planta, respectively. The activity of the acrA promoter was lower in planta than in LB medium (Table 3). However, it is possible that growth of the bacteria in LB broth may increase expression of the AcrAB pump. A similar induction of the RND-type efflux system MexAB-OprM in Pseudomonas syringae was observed during growth in complex King’s B medium [40]. Specific components of the complex media might induce the expression of these RND efflux systems. Alternatively, the efflux pumps may play a role in the secretion of metabolites during exponential growth of bacteria in complex medium. PP2 The level of acrD expression was low during growth in LB medium (Figure 1B), whereas it was slightly induced in planta (Table 3) indicating that plant-derived compounds are able to induce the AcrD pump. The nature of these

compounds remains to be elucidated. Several multidrug transporters are induced in response to the presence of toxic substances [18]. We identified the substrates deoxycholate, naringenin, tetracycline, novobiocin, fusidic acid, tannin and zinc as inducers of acrD in E. amylovora. In prokaryotes, the expression of drug transporter genes is frequently mediated by transcriptional regulatory proteins, whose genes are often located adjacent to those encoding

the transport system. However, no local transcriptional regulator was identified flanking the acrD gene in E. amylovora, suggesting that expression of acrD may be subject to regulation at the global level. The acrD gene belongs to the regulon of the envelope stress response, two-component system BaeSR in E. coli and Salmonella enterica. A baeSR-deficient mutant of E. amylovora Ea1189 has previously been Org 27569 evaluated for virulence on immature pears, and exhibit full-virulence, as that of wild type, on immature pear fruits [41]. The core regulon of BaeSR consists of spy, encoding a protein chaperon, and the RND efflux pump genes acrD and mdtABC[42]. Interestingly, we identified a partial overlap between the compounds inducing expression of acrD in E. amylovora and baeR in E. coli, e.g., flavonoids (naringenin), zinc, and tannin [24, 42]. Accordingly, the contribution of the two-component system BaeSR to regulation of the acrD gene in E. amylovora became of particular interest to us. In E. coli and S. enterica, BaeR, upon activation by phosphorylation through BaeS, binds to the upstream promoter region of mdtA and acrD[19, 35].