Conductivity of an individual proteinaceous filaments, called pili or microbial nanowires, produced by wild type Geobacter sulfurreducens was measured using a low-noise electrical transport technique. It was shown that the conductivity is diminished if aromatic amino acid residues are removed from the pili. It was demonstrated that the conductivity of G. sulfurreducens pili is similar to that of synthetic organic polymers and is related to the respiration rate of the bacteria. Conductivity measurements were performed on microbial nanowires produced by other species of Geobacter with a different aromatic amino acid distribution along the wire. Also, measurements on a mutated pili with a higher aromatic residue content is presented. All these studies consistently demonstrated that the aromatic amino acid residues are essential for making conduction along microbial nanowires possible. Optical studies involving UV-Vis spectroscopy and X-ray diffraction of G. sulfurreducens and aromatic residue rich mutant pili are also presented. Denaturation vi of cytochromes was observed by lowering pH of the pili solution, implying that it is the pili themselves and not cytochromes attached to them, which are essential for conduction. The optical band gap of wild type pili with different doping level is discussed. Band gap narrowing with increasing doping level was observed. Finally, conductance tuning of wild type pili with field gating is also presented and possible transport mechanism, which can be inferred based on well established free-electron system, is discussed.