Short summary

In the first module we aim to identify possible mediators that are capable for electron transport. We want to use electricity to chemically reduce these mediators and transport them into the cells. The process takes place in a bioreactor called "reverse microbial fuel cell" (rMFC). One important requirement for a suitable mediator is that its reduction potential is high enough to restore reduction equivalents, like NAD(P)H (nicotinamide adenine dinucleotide (phosphate)). These reduction equivalents enter the respiratory chain where ATP (adenosine triphosphate) is produced which will be used in the next module.

Here you will find the results of the rMFC.

Electrotrophes

There are several approaches to feed microorganisms with electrons in order to support microbial respiration. One promising feasibility is the direct transfer of electrons to microorganisms. Bacteria that can directly accept electrons from electrodes for the reduction of terminal electron acceptors are called electrotrophes or electrode oxidizing bacteria. The possibility of electron transfer to microorganisms was investigated for the first time by studies with Geobacter species. (Lovley, Derek R., 2011)
Normally the production of multi-carbon organic products relies on organic feedstocks (biomass) as electron donor. The use of biomass has the disadvantages that biomass production competes with food production and that the costs for the required carbon source are a major factor if a production process is profitable or not. That is why the possibility of powering microbial processes with electricity is very attractive. (Lovley, Derek R. & Nevin, Kelly P. 2013)
Microorganisms can be provided with electrons via two major principles: Direct- and indirect electron transfer.

Indirect electron transfer

Indirect electron transfer is mediated by soluble redox mediators that are freely moving in the media. There naturally expressed mediators like flavins, quinones and phenanzines, but structurally similar synthetic molecules are also suitable to serve as electron shuttles for numerous oxidation-reduction cycles. (Harnisch, F. & Freguia, S., 2012)

Figure 1: Electron flow into GRAM-negative bacteria cells.

Direct electron transfer

Direct electron transfer describes an extracellular electron transfer pathway without any diffusive electron mediators. It has been demonstrated that outer membrane c-type cytochromes are involved in the electron transfer between an electrode and bacterial cells. The detailed mechanism is still unclear.
Cytochromes are present in the respiratory chain and mediate electron transfer due to their heme groups which are located in the active centr for redox reactions. Heme groups are prostetic groups, consisting of metal-ions surrounded by highly conjugated porphyrin rings.( Qiao et al. 2010) Direct-electron transfer occurs through direct contact of the electrode with outer membrane cytochromes. (Harnisch, F. & Freguia, S., 2012)

Figure 2: Principle of direct electron transfer mediated by outer membrane cytochromes.

One special case of direct electron transfer is realized by conductive pili called microbial nanowires. These nanowires belong to the type IV pili and are formed in particular by Geobacter species. These pili are highly conductive and transfer electrons from the cell surface to the surface of Fe(III) oxides. These oxides function as the terminal electron acceptors for Fe(III) reducers.
The pilus apparatus is anchored in the periplasm and outer membrane of GRAM-negative cells and has the function to complete the circuit between various intermediate electron carriers. Besides that task it can also mediate the cell-to-cell electron transfer. (Reguera et al., 2005)
Due to the fact that Geobacter species are GRAM-negative and naturally competent to transfer electrons they provide very interesting solutions for electron transport into E. coli cells. The current situation is that there has been a lot of research on microbial fuel cells (mfc) where the aim is to produce electricity and convey electrons to an electrode. The mechanisms for the electron uptake has been investigated, but is not understood yet.

References

• Lovley, Derek R., 2011. Powering microbes with electricity: direct electron transfer from electrodes to microbes. In: Environmental Microbiology Reports 3 (1), pp. 27–35.
• Lovley, Derek R. & Nevin, Kelly P., 2013. Electrobiocommodities: powering microbial production of fuels and commodity chemicals from carbon dioxide with electricity. In: Current Opinion in Biotechnology, 24, pp. 385-390.
• Harnisch, F. & Freguia, S., 2012. A Basic Tutorial on Cyclic Voltammetry for the investigation of Electroactive Microbial Biofilms. In: Chemistry – An Asian Journal, 7 (3), pp. 466–475.
• Qiao, Y., Bao, S. & Li, C. M. (2010): Electrocatalysis in microbial fuel cells—from electrode material to direct electrochemistry. In: Energy Environ. Sci. 3 (5), pp. 544 - 553.
• Dantas, J. M., Tomaz, D. M., Morgado, L. & Salgueiro, C. A.(2013): Functional charcterization of PccH, a key cytochrome for electron transfer from electrodes to the bacteium Geobacter sulfurreducens. In: FEBS Letters 587, pp. 2662 - 2668.
• Reguera, G., McCarthy, K. D., Mehta, T., Nicoll, J. S., Tuominen, M. T., Lovley, D. R. (2005): Extracellular electron transfer via microbial nanowires. In: Nature 435 (7045), pp. 1098–1101.
• Lovley, D. R. (2012): Electromicrobiology. In: Annu. Rev. Microbiol. 66, pp. 391-409.