Metal-like microbial nanowires

Metal-like microbial nanowires. Although most bacteria are electrical insulators, some do support electric currents. Debates remain, however, over the nature of that transport (see PHYSICS TODAY, December 2010, page 18 ). New work by microbiologist Derek Lovley, physicist Mark Tuominen, and colleagues at the University of Massachusetts Amherst shows that the anaerobic bacterium Geobacter sulfurreducens can conduct along filaments attached to the cell membrane. Composed of the protein pilin, the filaments—only 3–5 nm wide and up to tens of microns long—have metal-like properties akin to those of synthetic organic conductors. Furthermore, the filaments form extended networks, like the one seen here in dark blue surrounding a bacterium 0.5 µm wide, that can conduct over centimeter distances. The researchers studied the nanowire networks by growing films of living G. sulfurreducens on top of two electrodes separated by a 50-µm gap. The biofilms had a conductivity of 5 mS cm⁻¹, comparable to that of manmade polymer nanostructures, and exhibited many hallmarks of metallic behavior. In a transistor configuration, a nearby gate electrode could modulate the film’s conductivity by a factor of 100. At moderate temperatures, the conductivity showed an exponential temperature dependence reminiscent of quasi-one-dimensional organic metals. Disorder effects began to dominate at low temperatures, which suggests that appropriate processing to remove network defects could improve the conductivity for potential applications. Microbial nanowires, say the researchers, could open the door to a range of conducting nanomaterials that are naturally occurring, nontoxic, and inexpensive to produce, for such uses as environmental cleanup, biosensors, and power applications. (N. S. Malvankar et al., Nat. Nano., in press, doi:http://dx.doi.org/10.103/nnano.2011.119 .)