Chemotaxis of Escherichia Coli Towards Norepinephrine Metabolites
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The co-existence of ~1014 commensal bacteria and host cells in the human gastrointestinal (GI) tract creates an environment rich in molecules produced by both. The close-proximity of different signals and cells in the GI tract is thought to lead to inter-kingdom (IK) signaling where bacteria and human cells recognize and respond to signals produced by each other. One such IK signaling molecule abundant in the GI tract is Norepinephrine (NE), which is known to increase the virulence and pathogenesis of GI tract pathogen, enterohemorrhagic E. coli (EHEC). It has also been shown that NE is a potent chemoattractant for EHEC as well as for non-pathogenic E.coli. While the effects of NE on virulence are well studied, its role as a chemoeffector is not fully understood. The overall goal of this work is to comprehensively characterize the chemotaxis response of E. coli toward NE and to elucidate the underlying mechanisms. A part of this work is also aimed at developing a probabilistic model to simulate the bacterial migration towards attractants. We showed that attraction of E. coli RP437 towards NE requires prior exposure to a lower concentration of NE during cell growth, and that de novo expression of two enzymes - TynA and FeaB - is required for E. coli chemotaxis towards NE. We discovered that NE is not the actual chemoattractant but the molecule that E. coli RP437 responds to is dihydroxymandelic acid (DHMA) generated from NE. We observed that chemotaxis to DHMA requires the Tsr chemoreceptor and the minimum concentration required for a detectable chemotaxis response was ~5 nM. We also observed that the chemotaxis response to DHMA was significantly reduced at concentrations greater than 50 μM and concluded that negative cooperativity between the two serine-binding sites resulted in attenuation of chemotaxis response. We investigated the molecular mechanism underlying the conversion of NE to DHMA in E. coli RP437, and identified that it requires the QseC histidine kinase and its cognate response regulator QseB, and to a lesser extent, the response regulator QseF. We also determined that the feaR transcription factor is required for tynA and feaB expression. This work is significant as it suggest that host-derived signals such as NE can be converted by commensal bacteria to a potent chemoattractant, which can then recruit pathogens that possess Tsr-like receptors to the site of infection. A probabilistic model was also developed to simulate the chemotaxis behavior of bacteria in microfluidic devices. The time-dependent distribution of bacteria in the chemotaxis chamber was simulated using MATLAB®. We determined that the time dependent bacterial migration in the microfluidic device is influenced by bulk motion of the fluid and existing concentration gradient of chemoeffector. The probabilistic model can be used to reduce the experimental space required to test the response of an unknown chemoeffector in the microfluidic device.
Pasupuleti, Sasi Kiran (2017). Chemotaxis of Escherichia Coli Towards Norepinephrine Metabolites. Doctoral dissertation, Texas A & M University. Available electronically from