Electronic nanostructures made from natural amino acids are attractive because of their relatively low cost, facile processing and absence of toxicity^,,. However, most materials derived from natural amino acids are electronically insulating^,,,,,. Here, we report metallic-like conductivity in films of the bacterium Geobacter sulfurreducens and also in pilin nanofilaments (known as microbial nanowires^,) extracted from these bacteria. These materials have electronic conductivities of ∼5 mS cm⁻¹, which are comparable to those of synthetic metallic nanostructures. They can also conduct over distances on the centimetre scale, which is thousands of times the size of a bacterium. Moreover, the conductivity of the biofilm can be tuned by regulating gene expression, and also by varying the gate voltage in a transistor configuration. The conductivity of the nanofilaments has a temperature dependence similar to that of a disordered metal, and the conductivity could be increased by processing.