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Zusammenfassung:
The lifecycle of Escherichia coli is biphasic. The primary environment of this organism is the lower intestinal tract of warm-blooded animals and reptiles. Once excreted from an animal host, this bacterium enters its secondary habitat: external environment, such as water, sand or soil. It was traditionally assumed that the viable enteric bacteria cannot last long extraintestinally, with the presence of E. coli in soil or water being only a result of a recent fecal contamination. However, there is increasing evidence that despite all environmental stress factors at least some strains of E. coli can survive and reproduce in the secondary environment for prolonged periods of time. While previous research mostly focused on external factors influencing E. coli persistence outside the host, such as nutrient scarcity, osmotic shifts, predation etc, genetic determinants such as the stress response pathway are also known to play an important role. In this work we aimed to systematically identify other genes crucial for E. coli to survive in its secondary environment, using the lake water as a model. Our results demonstrate and expand the importance and dualism of stress response pathway for E. coli growth outside the host, with the surprising finding that the mutations in the general stress response sigma factor σS being beneficial for growth in the lake water. On top of this, we demonstrated that the defects in membrane integrity are detrimental for growth. Finally, we identified mutations in two neighboring poorly characterized genes, hdfR and maoP, that are beneficial for E. coli growth in the lake water. We demonstrated that the mutations in hdfR significantly inhibit σS-dependent functions, such as biofilm formation. Subsequent analysis of the HdfR and MaoP pair showed that they form a novel regulatory system that controls several cellular functions, including flagella expression, plasmid copy number and chromosome organization. HdfR was previously described as a negative transcriptional regulator of flagellar master operon flhDC, but we demonstrated that its effect on flhDC expression is not direct but rather mediated by MaoP that is itself transcriptionally regulated by HdfR. Besides, this system affects the copy number of ColE1 plasmid and influences the organization of E. coli chromosome.
