Monitoring metabolic pathway alterations in Escherichia coli due to applied potentials in microbial electrochemical system

Abstract

Redox potential is one of the key regulators in determining the fate of the metabolic pathways of biocatalysts and of their associated product synthesis in microbial electrochemical systems. In the present study, the influence of applied potentials on fermentation products and metabolic flux was investigated using isolated E. coli HP3 as a model organism using pyruvate as a substrate. To provide insights into metabolic shifts, electro-fermentative (EF) systems were constructed and poised at both positive and negative redox potentials of 0.2 V, 0.4 V, 0.6 V and 0.8 V (vs Ag/AgCl) at the anode. The relative expression of genes encoding lactate dehydrogenase (ldhA), pyruvate formate lyase (pflB), pyruvate dehydrogenase (aceF), hydrogenase (hycE) and NADH: oxidoreductase (nuoB) enabled assessment of metabolic shifts in addition to cyclic voltammograms and short chain fatty acid profiling. Results showed that poised conditions had a significant effect on product formation and observed up-regulation of key enzymes involved in pyruvate metabolism in comparison to controls. More specifically, EF poised at −0.8 V and −0.2 V enhanced H2 production by 7.9 folds and 5.3 folds respectively, whilst at +0.8 V poised operation enhanced lactate production by 1.9 folds compared to the control. Concomitantly, the key genes involved in the pathway for H2 production viz., plfB, hycE and nuoB were all up-regulated significantly in a reactor poised at −0.8 V compared with other conditions. Similarly, transcripts for gene ldhA were up-regulated significantly in the system poised with +0.8 V. The study elucidated the role of redox potential on the product formation with respect to the relative expression of the genes encoding key enzymes in the metabolic pathway in correlation with bio-electrochemical characteristics.