Original Article: El-Naggar, M., Wanger, G., Leung, K., Yuzvinsky, T., Southam, G., Yang, J., Gorby, Y. (2010). Electrical transport along bacterial nanowires from Shewanella oneidensis MR-1. Proceedings of the National Academy of Sciences, DOI 18127-18131.
By: Myesha Jahin, Kathy Shaw, and Sarah Fixon-Owoo
May the odds be ever in your favor
By: Myesha Jahin, Kathy Shaw, and Sarah Fixon-Owoo
May the odds be ever in your favor
Before we dive into this dense stuff, take a deep breath.
No, really-- take a deep breath and feel the oxygen fill up your lungs. These
molecules of oxygen you’re taking into your system are the reason we live.
They’re the reason many organisms live. (In case you need a quick refresher:
oxygen is the terminal electron acceptor in the oxidation reactions of aerobic
cellular respiration, and oxidation reactions are how our cells extract energy
from the foods we eat!) Now, imagine that you’ve suddenly found yourself in an
environment that is oxygen-deficient and rich in poisonous metals. What are the
odds of surviving? It turns out, if you’re a dissimilatory metal-reducing
bacterium with nanowires, they’re pretty good.
The nanowire solution in S. oneidensis
First discovered in the Geobacter species, bacterial nanowires are conductive extracellular extensions that scientists suspect can transfer electrons. Nanowires have been found in many diverse microorganisms, ranging from photosynthetic cyanobacteria to dissimilatory metal-reducing bacteria such as our star microbe, Shewanella oneidensis. S. oneidensis typically inhabits deep-sea sediments and soil environments. As you can imagine, deep-sea sediments are relatively oxygen-deficient, and DMRB dwelling in these oxygen-deficient conditions don’t have a wide variety of dissolved electron acceptors to choose from. In other words, DMRB really can’t afford to be picky; they must utilize more readily available chemicals like solid metals as their final electron acceptors. This is different from many other prokaryotes that typically use dissolved electron acceptors like oxygen or sulfate. And, not to mention, this is completely unheard of and unfathomable to creatures like us-- can you imagine having to breathe or eat metal? Herein lies the unique challenge that dissimilatory metal-reducing bacteria face: they have no choice but to work with insoluble metals like Fe (III) and Mn (IV) as terminal electron acceptors. Luckily, scientists have found, these neat little bugs have developed a strategy that involves direct transfer of electrons from their insoluble metal substrates using nanowires.
First discovered in the Geobacter species, bacterial nanowires are conductive extracellular extensions that scientists suspect can transfer electrons. Nanowires have been found in many diverse microorganisms, ranging from photosynthetic cyanobacteria to dissimilatory metal-reducing bacteria such as our star microbe, Shewanella oneidensis. S. oneidensis typically inhabits deep-sea sediments and soil environments. As you can imagine, deep-sea sediments are relatively oxygen-deficient, and DMRB dwelling in these oxygen-deficient conditions don’t have a wide variety of dissolved electron acceptors to choose from. In other words, DMRB really can’t afford to be picky; they must utilize more readily available chemicals like solid metals as their final electron acceptors. This is different from many other prokaryotes that typically use dissolved electron acceptors like oxygen or sulfate. And, not to mention, this is completely unheard of and unfathomable to creatures like us-- can you imagine having to breathe or eat metal? Herein lies the unique challenge that dissimilatory metal-reducing bacteria face: they have no choice but to work with insoluble metals like Fe (III) and Mn (IV) as terminal electron acceptors. Luckily, scientists have found, these neat little bugs have developed a strategy that involves direct transfer of electrons from their insoluble metal substrates using nanowires.
(Photo credit: http://mfc-muri.usc.edu/public/about.htm) The
genome of S. oneidensis was sequenced in 2002 by The Institute for Genomic
Research (TIGR) in Rockville, Maryland-- Kathy’s hometown!
Hard evidence for hardwired nanowire
conductivity
Up until a study by Mohammed Y. El-Naggar and colleagues,
the newly discovered bacterial nanowires in Shewanella
and Geobacter had merely been
suggested as a mechanism for transporting electrons. A few studies (Reguera et
al., 2005; Gorby et al., 2006; El-Naggar et al., 2008) had demonstrated local
conductivity of nanowires, but none had yet investigated or obtained
measurements of electron transport along
the length of these fascinating
bacterial filaments. In 2010, El-Naggar et al. came out with new findings
demonstrating electrical conductivity along bacterial nanowires as an actual,
viable strategy of extracellular electron transport in S. oneidensis MR-1. Using two primary methods to measure electron
transport rates along and resistivity of bacterial nanowires, they examined
individual nanowires using 1) Direct Transport Measurement by nanofabricated
electrodes and 2) Conducting Force Atomic Probe microscopy to report electron
transport measurements along individual nanowire filaments of S. oneidensis MR-1 cultures in the
absence of an electron acceptor source.
For the first method, they used chemically fixed samples of S. oneidensis from continuous cultures
on Si/SiO2 substrates with metallic contact pads (Figure 1) and then Secondary Electron Imaging was used to locate
individual nanowires contacted by electron-beam deposition of platinum
electrodes. Figure 2A shows a single
wire extending from S. oneidensis.
The electron transport rate of Shewanella was found to be about 10^9 electrons
per second at 100 mV (2 A). The measured resistivity was found to be 1Ω cm comparable to moderately doped silicon
wires. When the nanowire was cut
(Figure 2B), there were no measurable current. The plot illustrates the
current versus voltage graph before and after cutting the wire. This shows that
the nanowires were the only means of conductivity.
Figure 1. Scanning Electron Microscope (SEM) images
showing how the individual bacterial nanowires were connected to the
nanofabricated Platinum electrodes. The zoom in shows a single wire protruding
from a S. oneidensis Cell (El-Naggar
et al. 2010)
Next, El-Nagger and coworkers used Conducting Force atomic probe microscopy to measure the resistance of nanowires as a function of its length and at different points. Conductive Atomic Force Microscopy (C-AFM) is a form of atomic force microscopy in which a conductive tip is scanned in contact with a sample surface, while a voltage is applied between the tip and the sample, producing a current image and a topographic image. They immobilized chemically fixed samples on si/sio2 substrates with lithographically patterned Au (gold) microgrids) as electrodes and observed a linear relationship between wire length and resistance which is usually observed in most cases that follows Ohm’s Law (since R, Resistance, is directly proportional to L, length and the longer the length, the greater the resistance and vice-versa). This enabled them to extrapolate the curve to zero to obtain the overall contact resistance of 58 M ohm and a bulk resistivity on the order of 1Ω cm, which was similar to earlier results that they found using nanofabricated platinum electrodes . (figure 3).
Figure 3. CP-AFM images of Shewanella Oneidensis MR-1. a) S.
oneidensis cells randomly placed in a
Si/SiO2 microgrid; B) A single wire connected to Au electrode in contact mode
AFM; c) The curve obtained from probing the nanowire 600 nm away from the gold
electrode; d) Plot of total resistance as a function of distance shows linear
relationship (El-Naggar et al. 2010)
Further research carried by
El-Naggar showed that mutants lacking MtrC, which is an outer membrane decaheme
C cytochrome essential for metal reduction and OmcA, produced pili-like
appendages, morphologically similar to S.
oneidensis but would not conduct current. The mutants were cultivated under
the same conditions and conductivity was measured using the same techniques
with nanofabricated electrodes and they were found to be non-conductive. This
observation led them to conclude that c-types cytochromes are essential
elements for electron transport in the nanowires for Shewanella. Other
bacterial species such as the Geobacter have some other mechanism because of
their long-range electron transport.
A cute illustration of a cell extracting electrons from its food source.
Bonus
Bacteria Facts
●
The name, S.
oneidensis, originates from the fact that the bacterium was first
discovered in the sediments of Lake Oneida, New York.
●
Geobacter can be used in radioactive sites to contain
spill and pollution
●
Microbes using nanowires to transfer electrons can be
analogous to using a breathing tube to aid in oxygen uptake
●
Incredible clip showing nanowires growing in real time!
https://www.youtube.com/watch?v=0YFxg8tqT9k
What Next?
Data collected from the experiment shows that electrons are
transported along the entire length of S.
oneidensis nanowires. However,
understanding the actual mechanism of electron transport and how this electric
bacterium fully works is still unknown. The reported finding has motivated
future research into understanding molecular structure and composition of
bacterial nanowires as well as the physical transport mechanism involved in
this process. Further implications of study will involve how bacteria nanowires
can be used to make better improved performance lithium batteries and improve
environmental conditions to accelerate the rate of bioremediation. The new
findings will lead to optimization of biofuel cells and electron transport
between cells and inorganic surfaces. Also the discovery of MFC (microbial fuel
cells) could help solve pollution problems and create a cheap and renewable
energy sources for both developed and developing nations. Moreover, this
revolves around how well we understand the function of nanowires.
The Bloggers
(Left to right) Myesha Jahin, Kathy Shaw,
and Sarah Fixon-Owoo
References:
El-Naggar, M., Wanger, G., Leung, K.,
Yuzvinsky, T., Southam, G., Yang, J., Gorby, Y. (2010). Electrical transport
along bacterial nanowires from Shewanella oneidensis MR-1. Proceedings of the National Academy of Sciences, DOI 18127-18131.
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