Electronic nanostructures made from natural amino acids are attractive because of their relatively low cost, facile processing and absence of toxicity(1-3). However, most materials derived from natural amino acids are electronically insulating(1-6). Here, we report metallic-like conductivity in films of the bacterium Geobacter sulfurreducens(7) and also in pilin nanofilaments (known as microbial nanowires(8,9)) extracted from these bacteria. These materials have electronic conductivities of similar to 5 mS cm(-1), which are comparable to those of synthetic metallic nanostructures(2). 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.