Chemistry of Extracellular Electron Transfer

It was found that some small redox-active chemicals (termed electron shuttles) can diffuse across the cell membrane and serve as electron carriers to assist the electron transfer from the bacteria to the electrode. The electron shuttle mediated electron transfer process usually contains three steps, i.e., being reduced by cells (the electron shuttle is converted to a reductive state). 

Although the whole-cell membrane is non-conductive, there are several redox proteins anchored on/in the membrane that confer nano-scale conductivity to the membrane and directly enable electron transfer across the cell membrane. These proteins usually assemble together in the periplasm and/or on/across the outer-surface membrane and act as an electron transfer chain to relay the electron across the membrane. For example, the membrane-bound electron transfer chain of Shewanella oneidensis is a trans icosa-heme complex, MtrCAB, that can move electrons across the membrane. The MtrC is a decaheme cytochrome located on the outside of the outer cell membrane that mediates the electron transfer to the extracellular substrate [e.g., solid electrode). MtrAB is the transmembrane electron transfer module that is responsible for electron transport from the periplasm to MtrC. More interestingly, recent findings indicates that this electron conduit is capable of reverse electron transfer, ie., electron up-take from extracellular electrodes.  

One exoelectrogenic microorganism usually has more than one electron transfer route to exchange electrons between the cells and the electrode. For example, S. oneidensis MR-1 contains all three pathways for electron transfer. It can synthesize flavins and secret into the medium, which serve as 

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