This document discusses microbial fuel cells (MFCs) powered by the bacteria Geobacter sulfurreducens. G. sulfurreducens is able to generate electricity through metabolizing substrates and transferring electrons to an anode. It is able to transfer electrons through protein structures called c-type cytochromes and filaments called pili. The formation of biofilms by G. sulfurreducens on the anode allows the cells to transfer electrons more efficiently through direct contact and intercellular protein interactions. Research aims to engineer strains of G. sulfurreducens that can generate higher currents through increased expression of proteins involved in electron transfer pathways and biofilm formation.

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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. 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. TTHHAANNKK YYOOUU!!!!!!

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; 72:7345-7348.
• Salgado, Carlos Andres. Microbial fuel cells powered by
Geobacter sulfurreducens. MMG 445 Basic Biotech. 2009 5:1
• 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;
24:3498-3503
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