The discovery that a wide variety of bacteria can be persuaded to produce wire-like appendages that conduct electricity could prove vital to the development of more efficient biological fuel cells.
Bacteria that use sugars and sewage as fuel are being investigated as a pollution-free source of electricity. They feed by plucking electrons from atoms in their fuel and dumping them onto the oxygen or metal atoms in the mixture. The transfer of the electrons creates a current, and connecting the bacteria to an electrode in a microbial fuel cell will generate electricity, although not necessarily very efficiently.
A species of bacterium called Geobacter sulfurreducens, which dumps electrons onto metal, has previously been persuaded to grow nanowires to make contact with distant atoms. A deficit of metal atoms in the close vicinity of the bacteria can cause a bottleneck, so the proliferation of nanowires allows the bacteria to consume more fuel, potentially boosting the current produced by a microbial fuel cell.
Now a study by Yuri Gorby of Pacific Northwest National Laboratories in Washington State, US, and colleagues reveals that several other kinds of bacteria produce similar nanowires.
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 and powerful biological fuel cells, Gorby says. For example, they could ensure that the chemical conditions surrounding bacteria encourage it to grow as many nanowires as possible, increasing conductivity.
Bruce Rittmann of Arizona State University in the US agrees this is an important direction: “The big problem with microbial fuel cells is the rates are very low,” he says. “They need to pick up the pace a lot.”