It has also been reported that deletion of acrD does not cause hypersusceptibility to amphiphilic drugs [36, 37], which may be due to low expression levels during cellular growth [14]. We have been able to detect similar low expression levels of acrD in E. amylovora Ea1189 during growth in LB broth (Figure 1A). Moreover, we were unable to detect hypersusceptibility to any of the tested antimicrobial Batimastat in vivo compounds in an acrD-deficient mutant (Table 1). As noted for other bacteria, the overproduction of AcrD in an acrB-deficient host led to increased resistance towards detergents, novobiocin and fusidic acid [14, 35]. Overproduction of AcrD in an acrB-deficient

mutant of E. amylovora Ea1189 increased the selleck kinase inhibitor resistance to several antimicrobial compounds and heavy metals. It is noteworthy that expression of acrD under control of the lac promoter displayed only a minor effect on the resistance level compared to acrD expression driven SBI-0206965 price by a combination of the lac promoter and the native promoter (up to 16-fold changes in MICs, Table 1). It has previously been reported that strong overproduction of AcrD may interfere with normal activity of the pump [14]. In this study, we identified two new substrates, clotrimazole and luteolin, which increased the substrate spectrum of AcrD in enterobacteria. Clotrimazole is a derivative of imidazole, commonly

used in the treatment of fungal infections, and acts primarily by inhibiting the activity of cytochrome P450 mono-oxygenase [38]. Luteolin is one of the most common flavonoids present in many plant families. One of the functions of flavonoids in plants is their protective role against microbial invasion. Luteolin before was shown to inhibit bacterial N-acetyltransferase activity [39]. Since AcrD conferred resistance to aminoglycosides in E. coli[13], we hypothesized that AcrD of E. amylovora would display a similar substrate spectrum. However, overexpression of AcrD in E. amylovora Ea1189-3 did not increase the MICs of the aminoglycosides amikacin, gentamicin, streptomycin, and tobramycin. Although it is important to note that we observed

occasional, but not reproducible, 2-fold differences between the aminoglycoside MICs for different experiments (data not shown). While this result is contradictory to previous findings for E. coli[13], it may reflect a possible adaptation of the AcrD transporter to a particular physiological function during growth in the plant environment. To elucidate the role of AcrD in the plant environment, we analyzed whether this RND-type efflux pump is involved in pathogenesis of the plant pathogen. Previously, we have observed that disruption of the AcrB efflux pump in E. amylovora significantly reduced virulence on apple rootstock [16]. This prompted us to evaluate the effect of AcrD on the virulence of the fire blight pathogen by studying development of disease symptoms.