Understanding the Properties of Phenazine Mediators That Promote Extracellular Electron Transfer in Escherichia coli

Abstract

Establishing an efficient extracellular electron transfer (EET) between microorganisms and electrode surfaces plays a critical role in the development of mediated microbial electrochemical technologies. The two main EET mechanisms are (1) direct electron transfer (DET) from membrane-bound redox proteins and (2) indirect, mediated electron transfer (MET) via exogenous or endogenous electron shuttles (mediators). In DET, anodes are in physical contact with functional motifs (redox-active proteins) on the cellular surface, facilitating EET. However, most microbial-based systems do not have these redox surface proteins. Therefore, the use of artificial, redox-active mediator systems is necessary to facilitate and/or increase electron transfer. Here, we describe a phenazine-based mediator system to facilitate electron transfer from the model microbe Escherichia coli. Recognized for its extensive synthetic biology toolkit for bioelectrochemical purposes, E. coli offers various advantages, such as fast growth rates, easy culturing methods, and the ability to metabolize different substrates. However, detailed information on electron transfer mechanisms in E. coli with electrode surfaces and its interaction with redox mediators remain unclear. Phenazine redox mediators were experimentally evaluated, demonstrating distinct mediated current densities, dependent on the mediator structure. Our results show that the choice of a mediator with the appropriate redox potential is not the single aspect to consider in the rational design and development of future mediated EET systems.