Electrical phenomena are important in biology, as Galvani discovered long ago when he learnt to make dead frogs twitch. But in biology electrical currents are generally carried by currents of ions rather than electrons. The transport of electrons is important in processes like photosynthesis, but the distances over which the electrons are transported are very small – the nanometer or two that defines the thickness of a lipid membrane. So the discovery of what look like electrically conducting nanowires in a soil bacterium is rather surprising. The discovery, from a group at UMASS Amherst (press release here), was reported in Nature (subscription required for full article) a few weeks ago.
The bacteria in question are soil bacteria that make their living by metabolising iron; to do this they seem to have evolved electrically conducting filaments called pili that allow them to do electrochemistry at a distance on a particle of iron oxide. Pili are common in many types of bacteria; they’re used by pathogenic bacteria to inject toxins into host cells, and for transfer of DNA between bacteria. They’re composed of protein molecules which self-assemble into long filaments, which are anchored into the bacterial cell wall by a large protein complex.
This report still leaves some unanswered questions in my mind. The conductivity of the pili was measured using atomic force microscope based conductance mapping of a graphite surface decorated with pili that had been broken off bacterial surfaces; it would be more convincing (though much more difficult) to quantify the conductivity along the length of the filament, rather than across the thickness. More importantly, perhaps, it doesn’t yet seem to be clear what is the structural feature of the pilus-making protein in this particular bacteria that leads to its electrical conductivity (as opposed to pili from other types of bacteria, which are shown in the paper to be non-conductive). It’s still a remarkable and suggestive result, though.
Thanks to Jim Moore for a comment drawing my attention to this press release.