Bacteria can clean up toxins, oil spills and nuclear waste, essentially by eating the stuff. But until now nobody was quite sure how they did it. Gemma Reguera and her team at Michigan State University found that the key is a structure called the pilus, a hair-like appendage that acts like a wire.
The bacteria, called Geobacter sulfurreducens, transfer electrons via the pilus to the metals that they feed off of. Transferring the electrons gives the bacteria energy. It also changes the ionization state of the metal, changing it to a form that precipitates out of water. A colony of Geobacter living near a pile of nuclear waste would extract the uranium, making it easier to handle and remove.
That was all very well, and the electron transfer has even been proposed as a way to build biological batteries. But an outstanding question was how Geobacter kept the uranium (and other chemicals) in nuclear waste away from their cellular walls, where the waste could cause damage. To see what role they played it was necessary to get a bunch of bacteria to grow lots of them in a lab. Reguera's team did that by exposing the bacteria to much harsher conditions than they were used to, stimulating them to grow more pili.
What they found was that the pili act as a buffer between the bacteria and the metallic compounds. The pili are quite long relative to the bacteria, and form a conductive barrier. That barrier is also a big part of the reason Geobacter can live in environments that would kill many other organisms.
The team published their work in the Proceedings of the National Academy of Sciences. In the paper they note that knowing how Geobacter species work makes it easier to come up with strategies to clean up toxic spills. It might even allow researchers to design tiny robots to do the job, or come up with ways to grow better toxin-eating bacteria.
Photo: Geobacter cell (in orange) with the nanowires (in yellow) interspersed through the uranium (black material). Credit: Dena Cologgi and Gemma Reguera (Michigan State University).