While it might not appear so at first, the bacterium Geobacter sulfurreducens and the Hoover Dam share a common trait: they both produce electricity. Of course, there is a huge difference in the scale of the energy produced; nonetheless, bioenergy and bioelectronics researchers are quite interested in these little microbes. If you could understand their conductivity, you could improve the design of bioreactors and bioelectronics. Unfortunately, the molecular mechanisms behind G. sulfurreducens’ conductance have remained unclear.
To survive, living cells need energy, and to get that energy they rely on oxidation. While most organisms complete oxidation with a transfer of electrons and reduction of oxygen within their cells, G. sulfurreducens lives underground in an environment deprived of oxygen. Instead, it oxidizes organic matter, namely iron (III) oxide.
But iron oxide is insoluble, so the bacterium evolved a mechanism to get around this: finger-like extensions called pili that form conductive nanowires. These nanowires shuttle electrons out of the cell to be transferred to the iron particles.
The conductivity of these natural nanowires was first described in 2011 paper by Lovley’s group (2). They found that the microbial nanowire formed by the pili conducts electricity with delocalized electrons, like a metal wire.
That finding drew some criticism because the pili were comprised of protein filaments, but amino acids, the building blocks of proteins, are generally insulators. Also, in other known systems, electrons are passed along a chain, moving discretely from one molecule to another. “It’s a new paradigm for biological electron transport,” said Lovley.
In his new paper, Lovley set out to describe the mechanism of pili conductivity in G. sulfurreducens. His inspiration was synthetic organic conducting polymers, which owe their conductance to fortuitously spaced aromatic rings. He reasoned that pili proteins might work similarly, and a sequence analysis suggested five aromatic amino acids whose positions might confer conductance.
So the team engineered a version of the pili protein with non-aromatic side chains at each of the five positions. The resulting protein looked normal by transmission electron microscopy and retained all of the physical properties of the native proteins, except one: the conductance plummeted.
Now, the team is now looking more closely at the structure of the wild-type protein to determine the location of the aromatic proteins involved in conductance, which Lovley suspects may be near the outer surface. In addition, while the conductance dropped drastically when the protein was mutated, it didn’t disappear altogether. Something is facilitating that low level of conductance, but Lovley’s team doesn’t know what that is yet.
It also remains to be seen if other microbes have similar conductance systems. “It seems unlikely that only one organism would have figured this out, but I really don’t know how widespread it is,” said Lovley.