Daniel Pletzer

• The emergence of multidrug resistant bacteria is a serious medical and economical problem with global impact. Bacteria have evolved energy-dependent multidrug efflux pumps to prevent intracellular accumulation of toxic compounds and to survive in different ecological niches. On the other hand, the uptake of essential nutrients, while at the same time preventing the internalization of toxic molecules, are accompanying circumstances that cause an enormous selection pressure on bacterial populations. In the first part of this thesis, three resistance-nodulation-cell division (RND)-type multidrug efflux pumps, AcrD, MdtABC, and MdtUVW, from the plant pathogen Erwinia amylovora were characterized in terms of their substrate specificity, induction upon exposure to various stressors, and pathogenesis on apple and pear. Results of this study show that AcrD is an efflux pump conferring resistance to a limited number of amphiphilic compounds. MdtABC and MdtUVW were found to contribute to disease development in apple rootstocks. Moreover, we identified global and local regulators in E. amylovora and found that the BaeSR two-component system is involved in the activation of the RND-type efflux pumps AcrD and MdtABC. In the second part, the dipeptide uptake ABC transporter, DppBCDF, and its associated substrate-binding proteins, DppA1-A5, from Pseudomonas aeruginosa were characterized by a high-throughput method. We were able to determine the substrate specificity of this uptake transporter using the Biolog phenotype MicroArray technology.