Geobacter sulfurreducens pili support ohmic electronic conduction in aqueous solution

The bacterium Geobacter sulfurreducens is a model biological catalyst in microbial electrochemical devices. G. sulfurreducens forms electrically conductive, electrode-associated biofilms, but the biological structures mediating electrical conduction from cells to the electrodes are a matter of debate. Bacteria in these communities produce a network of fiber-like Type IV pili, which have been proposed to act either as inherent, protein-based electronic conductors, or as electronically inert scaffolds for cytochromes mediating long-range charge transport. Previous studies have examined pilus conduction mechanisms under vacuum and in dry conditions, but their conduction mechanism under physiologically relevant conditions has yet to be characterized. In this work, we isolate G. sulfurreducens pili, and compare the electronic conduction mechanism of both live biofilms and purified pili networks under dry and aqueous conditions. Solid-state I–V characteristics indicate that electronic transport in films of purified pili is representative of conduction in a fiber percolation network. Electrochemical gating measurements in a bipotentiostat device configuration confirm previous results suggesting redox currents dominate live biofilm conduction. Purified pili films, however, exhibit non-redox electronic conduction under aqueous, buffered conditions, and their conductivity increases with decreasing temperature. These findings show that isolated pili possess inherent, non-redox-mediated conductivity consistent with a metallic-like model of charge carrier transport. The results demonstrate an experimental platform for studying electronic transport in biomaterials and suggest that pili serve as an exemplary model for designing bioelectronic interfaces.