Mediators

Overview

Figure 1: Principle of electron transfer from bacteria to anode via mediators as electron shuttle.

The efficiency of a Microbial Fuel Cell (MFC) depends on many factors. One of the known bottle necks is the electron transfer between a bacterium and the anode of the MFC. There are three mechanisms how this process occurs, through direct contact between bacterium and anode, through so-called nanowires and through a substance called a mediator. Mediators are usually small water-soluble molecules, that are able to undergo redox transformations. The mediator acts as an electron shuttle, enhancing the kinetics of the electron transfer. This approach has been proven to be generally quite successful and many substances were tested for their potential as electron shuttle.

Escherichia coli, the organism of our choice for use in the MFC, is not naturally able to transfer its electrons to the anode, so the usage of a mediator is crucial in our case. Due to their origin, mediators can be divided into two classes, the ones that are produced by a bacterium itself, so-called endogenous mediators, and the ones that are chemically synthesized and are added externally (exogenous mediators). The latter are very expensive, not completely degradable and to some extent even toxic for the environment. In some cases a special treatment is needed in order to catalyze their degradation process (Du et al., 2007). In spite of this, used in a MFC environment, exogenous mediators have to be added to the system repeatedly, because the active form of the molecule is short-lived. All these aspects prove why we decided to create an E. coli strain that will produce its own mediator. An endogenous mediator is environmentally neutral and doesn’t have to be added externally, making it an ultimate economic advance by cost savings.

We have chosen different targets for a genetic optimization, overproduction of glycerol dehydrogenase GldA for NADH generation and endogenous riboflavin synthesis (vitamin B2). We have also tried an overproduction of phenazine-1-carboxylic acid (PCA), but we had to realize that this substance is mainly known for its antibiotic properties. That, of course, raised an uncertainty in the biosafety question, so we resigned from this sub-project in the early stages.

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References

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