Mechanisms of acetate resistance in acetic acid bacteria

Abstract Weak organic acids, of which acetate is the most prominent member, are produced by microbes as metabolic by-products or overflow metabolites. Since many of these low molecular weight products have cytotoxic effects and retard growth and product formation by microbes in many biotechnological relevant processes, identification of potential resistance mechanisms is of scientific and applied interest. Vinegar producing acetic acid bacteria (AAB) exhibit pronounced resistance and, thus, are presumed to possess acetate specific resistance mechanisms. For this thesis, wild-type Acetobacter aceti was chosen as a model for AAB. The focus of this thesis was identification and characterization of acetate resistance mechanisms in A. aceti by molecular genetic, proteomic, and physiological analyses. Since acetate resistance in acetic acid bacteria is known to be an unstable phenotype, we investigated acquisition of acetate resistance in the moderately acetate resistant wild-type strain by long-term evolution under increasing acetate challenge (Chapter 2). Physiological results from evolved cultures revealed significant improvements in acetate resistance when compared to the wild-type strain. We show that evolved A. aceti can actively decrease the intracellular concentration of acetate, presumably by an active acetate export system. Besides reduction of intracellular acetate concentration, physiological data suggests that other mechanisms add to the overall acetate resistance of A. aceti. Global changes in protein expression during adaptation of wild-type A. aceti to high acetate concentrations are analyzed by proteome analysis (Chapter 3). The complex response of A. aceti to acetate challenge results in the discovery of 19 so-called acetate adaptation proteins (Aap). Due to the lack of genome data, deduced N-terminal protein sequences of these Aaps were used to identify the corresponding genes by a reverse genetics approach (Chapter 4). Thereby, two Aaps are isolated and identified as oxidoreductases that may be involved in acetate resistance by cofactor recycling or by synthesis of membrane components. For more detailed molecular characterization of the identified Aaps and for alternative strategies to identify and isolate acetate resistance mechanisms (genes), several genetic tools were modified or newly developed for A. aceti (Chapter 5). Lastly, to identify acetate resistance mechanisms that are not related to the identified Aaps, we used functional expression and selection in E. coli, which is transformed with genomic libraries of A. aceti and the Gram-positive, acetate resistant Staphylococcus capitis (Chapter 6). Plasmids, containing the gene for the ATP-dependent helicase RecG were reproducibly selected in E. coli under weak organic acid challenge. Overexpression of RecG from A. aceti but also from E. coli conferred weak organic acid resistance to E. coli. Our data indicate that weak organic acids negatively affect DNA replication or repair, presumably by reduction of the intracellular pH or ATP levels. Thus, we show that the RecG-conferred resistance is not specific for acetate resistant bacteria but is a general protection mechanism that can be transferred to other, industrially relevant organisms. Overall, this thesis reveals different mechanisms that are involved in acetate resistance of A. aceti and emphasizes the complexity and redundancy of bacterial resistance to weak organic acids.