Electron current is inherent to microbial metabolism. Microorganisms transfer electrons from an electron donor (lower potential species) to an electron acceptor (higher potential species). If the electron acceptor is an external ion or molecule, the process is called respiration. If the process is internal, electron transfer is called fermentation. The microorganism attempts to maximize their energy gain by selecting the electron acceptor with the highest potential available. In nature, mainly minerals containing iron or manganese oxides are reduced. Often soluble electron acceptors are depleted in the microbial environment. The microorganism can also maximize their energy by selecting a good electron donor that can be easily metabolized. These processes are done by extracellular electron transfer (EET). The theoretical free energy change (ΔG) for microorganisms relates directly to the potential difference between the electron acceptor and the donor. However, inefficiencies like internal resistance will decrease this free energy change. The advantage of these devices is their high selectivity in high speed processes limited by kinetic factors.
The most commonly studied species are Shewanella oneidensis and Geobacter sulfurreducens. However, more species have been studied in recent years.
On March 25, 2013, scientists at the University of East Anglia were able to transfer electrical charge by allowing bacteria to touch a metal or mineral surface. The research shows that it is possible to tether bacteria directly to electrodes.