- Some unusual bacteria conduct electricity through a structure never observed before.
- Researchers use cryo-electron microscopy to identify the shape and composition of that structure in bacteria called Geobacter sulfurreducens.
- The findings can be used to miniaturize electronic, build pacemakers without wires, and develop tiny yet powerful batteries.
Some bacteria have the ability to conduct levels of electricity, and they haven’t been extensively studied yet. Determining their internal structure and functional roles could help scientists understand how they actually create an electric current and whether it’s possible to develop an effective and long-lasting microbial fuel cell on a large scale.
Now, researchers at the University of Virginia Health System have made an astonishing discovery about how unusual bacteria, such as Geobacter sulfurreducens, living in soil and sediment can conduct electricity.
The rod-shaped microbe, Geobacter sulfurreducens, has a gram-negative cell wall. Their cells can conduct electricity and change electrons into electricity for long periods of time. Since they have a fully sequenced genome and well-developed genetic system, Geobacter sulfurreducens serve as a model organism for the broader phenomenon of extracellular electron transfer.
A Biological Structure Never Before Seen In Nature
Previously, it was believed that Geobacter sulfurreducens uses hair-like appendages called pili, to conduct electricity. There were several reasons behind this belief –
- Electron-transferring cells showed high levels of messenger RNA for PilA protein.
- Geobacter sulfurreducens’ amino acid sequence is similar to the sequence of other pili-producing bacteria
- Genomic organization of sulfurreducens pilus biosynthesis genes was also similar to other pili-producing bacteria.
- A PilA deletion mutant strand lacked filaments and couldn’t transfer electrons extracellularly.
Despite this information, there has never been any direct sign that conductive Geobacter sulfurreducens filaments are composed of PilA.
In this study, researchers directly identified the structure and composition of extracellular appendages of Geobacter sulfurreducens using cryoelectron microscopy. They showed that Geobacter sulfurreducens transmit electricity through flawlessly ordered fibers made of a completely different protein.
The foreground shows an atomic model of microbial nanowires while background contains an electron micrograph of two bacteria surrounded by the nanowires | Courtesy of researchers
Just like how an electric cord contains metal wires, these proteins surround a core of metal-comprising molecules. These Geobacter “nanowires” are modified pili that are used to transmit electrons to extracellular electron acceptors. They are 100,000 times thinner than a human hair.
Mass spectrometry, gel electrophoresis, atomic force microscopy, and conductivity measurements show that these nanowires are the same filaments that were previously assumed to be type IV pili. Overall, the findings establish a previously unknown group of protein-based nanowires based on cytochrome polymerization.
How Is It Useful?
These tiny structures can be used in a wide range of applications, from developing biological sensors to cleaning up pollution. They could serve as a bridge between living cells and electronics.
The technology (cryo-electron microscopy) to understand such bacteria did not exist until 5 years ago. Now we can actually analyze the structure of these filaments at the atomic levels.
In the near future, scientists can use such structures to miniaturize electronics, build pacemakers without wires, tiny yet powerful batteries, and develop plenty of other medical advances.