Geobacter is one of the genera in the phylum of Proteobacteria and the family of Geobacteracea. Geobacter species are found in soils and aquatic sediment and have thus no requirement for air in order to survive. In other words, they are anaerobic.

What is interesting about some of the species in this family (such as G. sulfurreducens) have the ability to oxidize organic compounds, metals, radioactive metals and petroleum (wikipedia). During oxidation, the transfer of electrons from one material to the next occurs through direct contact between the metal of interest and protrusions on the bacterium, called pili.

The function of pili is diverse, ranging from aiding in the transfer of DNA (i.e. sexual reproduction), to motility, to colonization of a certain space and formation of a biofilm (in the latter case, pili are more commonly referred to as fimbriae).

It turns out that two species of the Geobacter genus (G. sulfurreducens and G. metalloreducens), use its pili for an entirely different purpose, namely for the transfer of electrons to extracellular electron acceptors or for conduction of electric current through biofilms (1, 2). Even though electron transfer is well-known in biological processes, where it usually involves mechanisms such as electron tunneling or hopping between molecules (2), electron transfer in G. sulfurreducens and G. metalloreducens, resemble more that of a wire. This can be tested relatively easily by, amongst other techniques, dropping the temperature. When electron transfer depends on the proximity of proteins, then this process slows down when the temperature decreases. In contrast, conductivity in wires usually increases with decreasing temperature. It is exactly the latter observation that was made with the pili of these two bacterial species.

But how do those pili conduct electric current?

In order to do that, the proteins that form the pili are stacked up in an array in such a way that inorganic anions, which receive the electrons, fill up the gaps that appear in between the stacked up proteins. It is this special sort of stacking of proteins and inorganic anions that may attribute metallic like conductivity to the pilli (2). Because of the way electricity is conducted and because of its resemblance to metals, such compounds can also be considered as quasi-one-dimensional organic metals (2, 3).

Great, you say, time to find a pot and some merky fluid (somehow the word ‘cylon’ comes to mind ;)) to grow bacteria, which I can use to recharge my phone..

Well, we’re not there just yet..

Unfortunately, the amount of current that flows through these nanowires is not enough to be useful to power electronic devices.

In order to make that happen, a team in the naval research institute in the US has taken a first crucial step: It modified the genome of G. sulfurreducens (I suppose, couldn’t find confirmation on the species) in such a way that two amino acids, which help build the nanowires were replaced by tryptophan (4). This ‘simple’ replacement increased the nanowire’s conductivity about 2000 times.

Aside from the huge increase in conductivity, the replacement of two amino acids also strengthened the ‘nanowire’ and making it smaller (diameter 1.5 nm) at the same time  (4).

Research is now underway to see how these new nanowires could potentially be used in electronic and computational devices (e.g. use in medical sensor technology). Very sadly, the advancement of military operations is mostly on the agenda of this institute, so any technology supporting that cause will very likely receive priority in terms of development..