The study of the Escherichia coli BarA-UvrY two-component system and its ability to sense the environment

A bacterium always has to be prepared for rapid changes in its environment, e.g. a varying supply of nutrients. The rapid adjustment to new conditions largely relies on two-component systems (TCSs), typically consisting of a membrane-bound sensor protein communicating via phospho-transfer reactions with a cognate regulator protein inside the bacterium. The phosphorylated regulator acts at the transcriptional level and controls genes needed to adapt to the changing environment. The importance of TCSs for adaptation during the different steps of infection has consequently made them attractive targets for novel types of antimicrobial therapy. This thesis is focused on a TCS in the Gram-negative bacterium Escherichia coli. This bacterium causes a variety of common bacterial infections in humans and animals, including diarrhoea, septicemia, neonatal meningitis and urinary tract infections (UTI). The sensor protein BarA was earlier implied to be involved in the ability of E. coli to cause a UTI. Homologous Bar-TCSs identified in other species, e.g. Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa, Vibrio cholerae, Erwinina carotovora and Legionella pneumophila are also involved in the pathogenesis. Increased knowledge of the Bar-TCS would add to the understanding of the complex interaction between bacteria and the host during an infection. By phylogenetic, biochemical and genetic studies we identified the UvrY-protein as the cognate regulator to BarA. UvrY had previously no assigned function in the bacterium. We could also show that the BarA- UvrY TCS regulates a non-coding RNA, CsrB, which can form a ribonucleoprotein complex with the CsrA protein. Non-bound CsrA subunits are known to inhibit glycogen metabolism, gluconeogenesis and biofilm formation, whereas they activate glycolysis, acetate metabolism, motility and flagellar biosynthesis, all of which occurs at the post-transcriptional level. We could demonstrate an effect on motility and flagellar expression, by genetically manipulating the genes for either the sensor barA or the cognate regulator uvrY. Regulation by the BarA-UvrY TCS on motility was however only partly dependent on csrB, suggesting there are additional genes involved in this regulatory pathway. We could furthermore demonstrate the involvement of the BarA-UvrY TCS in the switching between different metabolic pathways. When the bacterium face new nutritional conditions, the most profitable carbon source will be consumed first. This leads to frequent switches between different metabolic pathways, implying the importance of their ability to sense the present carbon source. We propose that this ability is mediated by regulation of the Csr-system and that a stimulus sensed by the BarA sensor might originate from the metabolism of the bacterium. From the previously suggested coupling between BarA and UTIs, sensing via the BarA-UvrY TCS and the ability to switch between different metabolic pathways can confer a higher fitness during the colonization of the urinary tract. We propose that sensing via the BarA- UvrY TCS confers an advantage and contributes to the bacterium's ability to adapt to, and survive, in the urinary tract