Power companies channel electrons around using copper wires. As it turns out, certain bacteria appear to do something similar. In the absence of oxygen, a number of common soil bacteria species grow tiny nanowires, along which they push electrons to nearby rocks. This movement of the electrons produces energy, which the bacteria use to make ATP — the molecule all cells use to power everything they do. However, this energy production strategy is rather unusual; outside of these species, most cells, including human cells, produce their energy using internal processes, not external ones.
That said, scientists have long known about these bacteria that transfer electrons to minerals. It’s just that the details of the process were hazy. After all, it’s difficult to visualize bacterial nanowires, which can be just 10 nanometers wide. But now, one team of physicists and biologists has imaged Shewanella oneidensis soil bacteria growing nanowires live. Yes, as in “live on TV.” Along the way, the scientists discovered what the nanowires are made of. The wires are actually formed from the bacteria’s outer membranes; the species has two membranes that form their “skin.”
“What the cell is doing is actually morphing a little bit,” Mohamed El-Naggar, a physicist at the University of Southern California who led the new study, tells Popular Science. “It’s extending its outer membrane in the shape of a long tube.” Previously, scientists had thought bacterial nanowires were made of pili, hair-like appendages that are common on single-celled organisms.
“This solves a long-standing mystery about how exactly does the charge move on these structures,” El-Naggar says. Bacterial cell membranes have proteins embedded in them called cytochromes, which—ta-da!—are known to pass electrons to one another. Pili don’t have cytochromes. (Nevertheless, there’s evidence that other species may truly use pili to make their nanowires, El-Naggar says.)
El-Naggar has studied nanowire-making bacteria for several years now. In 2012, Popular Science named him one of the Brilliant 10 young scientists of the year for his work. Studies like his could one day lead to bioelectric devices that combine both silicon components and biological ones. After all, if there are bacteria that are able to transfer electrons to rocks, then they could also transfer electrons to components in a circuit. Engineers are also trying to incorporate nanowire-making bacteria into fuel cells.
There’s still plenty of work to do on both fronts. There’s no guarantee bacterial fuel cells or circuits will be more efficient than the ones being used today. But the idea is that if biological circuits do work, “you kind of end up getting the best of both worlds,” El-Naggar says. The resulting circuit could have both components that fix themselves and replicate themselves (from the bacteria) as well as super-precise components (the man-made parts).
El-Naggar and his team published their work today in the journal Proceedings of the National Academy of Sciences.