Dokument

Summary:
Bacterial growth and survival critically hinges on the ability to rapidly adapt to ever-changing environmental conditions. Elaborated stress response systems allow bacteria to sensitively detect and adequately respond to fluctuations in environmental conditions, such as pH, temperature, osmolarity, or the concentrations of nutrients and harmful substances. Often, bacterial stress responses towards a specific stressor involve multiple interconnected mechanisms - controlled by a sophisticated network involving signal-transduction cascades, metabolic pathways and gene expression regulation. In this thesis, bacterial stress responses towards two different environmental stressors are analysed; mainly focussing on the regulatory mechanisms that give rise to the overall cellular response. The first part of this thesis addresses the heme stress response in Corynebacterium glutamucim. Heme is an essential cofactor and alternative iron source for almost all bacterial species but can cause severe toxicity when present in elevated concentrations. Consequently, heme homeostasis needs to be tightly controlled. Therefore, one important challenge is to understand how bacteria regulate heme stress responses to both benefit from heme while simultaneously eliminating the associated toxicity. It is shown that C. glutamicum induces a heme detoxification mechanism (mediated via the heme exporter HrtBA) and a heme utilization mechanism (mediated via the heme ogygenase HmuO) in a temporal hierarchy, with prioritisation of detoxification over utilization. A combined approach of experimental reporter profiling and computational modelling reveals how differential biochemical properties of the two two-component systems that sense heme in C. glutamicum - ChrSA and HrrSA - and an additional regulator (the global iron-regulator DtxR) control this hierarchical expression of the two stress response modules. This analysis sheds light on the multi-layered heme stress response that contributes to overall heme homeostasis in C. glutamicum and adds on to the understanding of bacterial strategies to deal with the Janus-faced nature of heme. The second part of this thesis focusses on bacterial response strategies towards cell wall antibiotics, which play a key role in bacterial antibiotic resistance. To combat resistance evolution, it is important to understand how cell wall antibiotics affect bacterial cell wall biosynthesis and how bacteria orchestrate stress response mechanisms to protect themselves from cell wall damage. The first question is addressed through a comprehensive mathematical model describing the bacterial cell wall synthetic pathway - the lipid II cycle - and its systems-level behaviour under antibiotic treatment. It is found that the lipid II cycle features a highly asymmetric distribution of pathway intermediates and that the efficacy of antibiotics in vivo scales directly with the abundance of targeted pathway intermediates: The lower the relative abundance of a lipid II intermediate within the lipid II cycle, the lower the in vivo efficacy of an antibiotic targeting this intermediate. This leads to the formulation of a novel principle of ‘minimal target exposure’ as an intrinsic bacterial resistance mechanism and it is demonstrated that cooperativity in drug-target binding can mitigate the associated resistance. The development of new drugs to counteract antibiotic resistance clearly benefit from these insights. The second question is then addresses by an experimental-based expansion of the model, which allows the analysis of the interplay between multiple stress response mechanisms that protect against a single antibiotic - focussing here on the well-studied response of Bacillus subtilis towards the cell wall antibiotic bacitracin. This study reveals that the properties of the lipid II cycle itself control the interaction between the primary bacitracin stress response determinant BceAB mediating bacitracin detoxification, and the secondary determinant BcrC, which contributes to cell wall homeostasis under bacitracin treatment. By elucidating regulatory mechanisms of the multi-layered response towards bacitracin, this analysis contributes to an advanced understanding of bacterial antibiotic resistance.

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