1. Microbial Fuel Cells Powered by Geobacter sulfurreducens
Ihsan Tria Pramanda
BM 3204 Bacteriology

2. Introduction
The development of processes to generate biofuels and bioenergy has been of special interest lately
Microbial Fuel Cells (MFCs)  collects the electricity generated by microbes when they metabolize substrates
Considered to be one of the most efficient energy sources:
No burning is required to produce energy
The only raw materials needed to power fuel cells are simple organic compounds or even waste materials from other reactions
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3. Principles of the MFCs
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4. Electron Transfer Mechanisms
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5. Microbial Fuel Cells
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6. Geobacter sulfurreducens
Comma shaped gram-negative, anaerobic bacteria that are one of the predominant metal-reducing bacteria
Generates electricity by oxidizing compounds and reducing the anode
Has been shown to generate substantial amounts of energy due to:
multiple mechanisms of transporting electrons to extracellular  pili or c-type cytochromes.
Formation of biofilms on the anodes  all the cells are involved in electron transport to the anode
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7. Simplified Microbial Anatomy
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8. C-type Cytochromes
Geobacter sulfurreducens has 111 different genes that code for c-type cytochromes  more than any other organism
Most important ones that encode for c-type cytochromes are omcB, omcE, omcS and omcZ
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9. Biofilms
Biofilms formed around the surfaces which it uses as an electron acceptor  as thick as 50 µm
The formation of biofilms is possible because of pili
Gene involved  pilA
Wild type cells that express the gene are able to form biofilms on almost any surface.
Mutants that have a pilA deletion can adhere to different surfaces but are not able to either express pili or form thick biofilms
Complemented pilA mutants (having a pilA gene reinserted) are once again able to express pili and form biofilms
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10. Biofilm of Geobacter sulfurreducens
Confocal scanning laser microscopy of G. sulfurreducens on anode surfaces.
(A to C) Wild-type biofilms producing 1.4 mA (A), 2.2 mA (B), and 5.2 mA (C).
(D and E) Biofilms of a pilin-deficient mutant (D) and the genetically complemented mutant strain (E) when
current production was nearing maximum (ca. 1 mA and 3 mA, respectively).
Live cells are green, while dead cells are red.
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11. Why do Strains that Form Biofilms are Able to Produce More Energy?
First Explanation  pili can act as microbial nanowires
Pili do not have the chemical composition or functional groups that are required for this process
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12. Why do Strains that Form Biofilms are Able to Produce More Energy? (2)
Second Explanation  c-type cytochromes
c-type cytochromes can interact with the same proteins from other cells and electrons can be passed from one cell to another.
In cases where there are no electrons acceptors near the cell  c-type cytochromes can even act as electron sinks and store electrons until a source to which they can be transferred is available.
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13. Gene Manipulation Engineered strains with higher expression of:
pilA, OmcZ, OmcB, OmcE, and OmcS genes.
More pili  allowing the formation of thicker biofilms and more nanowires for electrons to be transferred
More c-type cytochromes  enabling the transfer of electrons to anode surfaces
A modelling exercise predicted the effects of specific gene deletions in Geobacter sulfurreducens on the rate of respiration  Optknock strain design methodology
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14. Strains selected in anaerobic environment
Spontaneous Mutant
Produced pili much more efficiently than the wild type strain  the pilA gene was over expressed.
The expression of c-type cytochromes was equal to the control strain  genes like omcZ, omcS, omcB and omcE were not up regulated.
This strain was also motile through the action of flagella  cells could move to the anode much more quickly before biofilms were formed.
Directed most of its net electron flow to the anode rather than to cell synthesis
Strains selected in anaerobic environment
placed on a different MFC
current production methods of different MFCs were compared
One strain showed higher current yields than the initial Geobacter sulfurreducens strain
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15. Limits and Applications
At this moment, the biggest concern is trying to obtain higher current levels which could actually generate enough power to drive complex mechanisms
Up to now, current levels are around 14 mA which means they could be used to power very simple components
Because this technology is still relatively new, the actual current densities that could be generated are still unknown
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16. Conclusions
Geobacter sulfurreducens has a promising future in the field of MFCs because of the ability of this organism to transfer electrons to the anode through c-type cytochromes and pili allow it to generate relatively high levels of current.
More research is required to determine how the microorganism could be engineered to create strains which are able to generate more current in MFCs. So that it will have higher electricity outputs and will eventually be available in the market.
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17. THANK YOU!!!

18. References
Reguera, G., Nevin, K.P., Nicoll, J.S., Covalla S.F. Biofilm and Nanowire Production Leads to Increased Current in Geobacter sulfurreducens Fuel Cells. Appl Environ Microbiol 2006; 72:
Salgado, Carlos Andres. Microbial fuel cells powered by Geobacter sulfurreducens. MMG 445 Basic Biotech :1
Yi, H., Nevin, K.P., Kim, B., Franks, A.E., Klimes, A., Tender, L.M. Lovley, D.R. Selection of a variant of Geobacter sulfurreducens with enhanced capacity for current production in microbial fuel cells. Biosens Bioelectron 2009; 24:
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