ELUCIDATING THE ROLE OF METABOLITES AS QUORUM SENSING SIGNALS USING PHASE PLANE ANALYSIS: THE CASE OF INDOLE IN ESCHERICHIA COLI

Abstract

Quorum sensing signals were initially restricted to those molecules whose only role is to sense density. Nevertheless Indole is a quorum sensing stationary phase signal secreted by Escherichia coli which is an intermediate in tryptophan synthesis. Moreover, it is known to regulate several genes such as astD, tnaB, and gabT. Hence, the main function that carries out indole in E. coli has not been clearly elucidated. Here we evaluated its role through computational and experimental techniques utilizing a flux balance analysis model. A metabolic phase plane was constructed for E. coli growth by calculating the shadow prices vector, which regards the biomass objective function sensibility upon modifications of the exchange and synthesis indole fluxes. Based on the metabolic phase plane, we found that cells do not prefer a specific role for this metabolite as both, intracellular synthesis and external signal concentration could exercise and effect on its physiological state. We experimentally corroborated our results by measuring growth rate when altering tnaA transcription rate and extracellular indole concentration hence synthesis and extracellular indole fluxes, respectively. Growth rate significantly varies when crossing the direction of the exchange flux (zero flux) as the extracellular indole variation goes from a positive to negative slope and this occurs when augmenting the indole concentration from 0.4 to 0.6 mM at the [6, 10] µM IPTG interval. Regarding the inductor concentration we found a significant variation of the growth rate when crossing the 8 µM concentration for all extracellular indole concentrations.