Indole modifies the central carbon flux in the anaerobic metabolism of $\textit{Escherichia coli}$: application to the production of hydrogen and other metabolites

Abstract

Indole is a bicyclic signaling molecule with effects on both eukaryotic and prokaryotic cells. The majority of studies of indole action have been performed with bacteria cultured under aerobic conditions and little information is available about its effects under anaerobic conditions. Here the effect of the indole on anaerobic metabolism of $\textit{Escherichia coli}$ WDHL was studied. Indole in the range 0.5–8 mM was added to the culture medium and cell growth, hydrogen and metabolite production were compared to cultures lacking indole. Results showed that while 8 mM indole abolished growth completely, 4 mM indole had a partial bacteriostatic effect and the maximum optical density of the culture decreased by 44% compared to the control cultures. In addition, 4 mM indole had an important effect on anaerobic metabolism. Hydrogen production increased from 650 ± 115 to 1137 ± 343 mL H$_{2}$/L, and hydrogen yield increased from 0.45 ± 0.1 to 0.94 ± 0.34 mol H$_{2}$/mol glucose, compared to the control culture. Carbon flux was also affected and the composition of the final by-products changed. Lactate (41 mM) was the main metabolite in the control cultures, whereas ethanol (56.2 mM) and acetate (41.2 mM) were the main metabolites in the cultures with 2 mM indole. We conclude that the supplementation of $\textit{E. coli}$ cultures with exogenous indole is a simple and novel strategy to improve the production of hydrogen as well as other metabolites such as ethanol used as biofuels.