The discovery that a wide variety of bacteria can be persuaded to produce conducting, wire-like appendages could prove vital to the development of more efficient biological fuel cells.
Bacteria that munch on sugars and sewage are being investigated as a pollution-free source of electricity. They feed by plucking electrons from atoms in their fuel and dumping them onto oxygen or metal atoms mixed in with the fuel. The transfer of the electrons creates a current, and connecting the bacteria to an electrode in a fuel cell will generate electricity, although not necessarily efficiently.
A species of bacterium that dumps electrons onto metal, called Geobacter sulfurreducens, has previously been persuaded to grow nanowires to make contact with distant metal atoms when there is a deficit round the bacterium (91av, 25 June 2005, p 21). A deficit can cause a metabolic bottleneck, so the proliferation of nanowires counters this and allows the bacteria to consume more fuel, potentially boosting the power of the fuel cell.
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A study by Yuri Gorby of Pacific Northwest National Laboratories in Washington state and colleagues reveals that several other kinds of bacteria produce similar nanowires (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0604517103).
“The bacteria grew wires that were tens of micrometres long”
Gorby’s team coaxed Shewanella oneidensis – another bacterium that dumps electrons onto metal – into producing nanowires by growing it in vats that carefully control the amount of oxygen available. This forced the bacteria to extend nanowires to make contact with more metal atoms. The nanowires were between 50 and 150 nanometres wide and extended tens of micrometres, allowing the bacteria to reach atoms hundreds of times as far away as the length of their cells.
The researchers used similar methods to encourage nanowires to sprout from other micro-organisms, including a photosynthesising bacterium called Synechocysitis PCC6803. A clearer understanding of the way bacterial nanowires form should allow engineers to make more efficient biological fuel cells, Gorby says. For example, they could ensure that the chemical conditions surrounding bacteria encourage them to grow as many nanowires as possible, increasing conductivity.
Bruce Rittmann of Arizona State University says this could improve power generation in future fuel cells. “The big problem with microbial fuel cells is the rates are very low,” he says. “They need to pick up the pace a lot.”
