When researchers first looked at the long tendrils grown by “electric bacteria” called Shewanella, they thought it was just common bacterial hair (or pili) for sensing surfaces and connecting to other bacteria. Now, an examination of their structure reveals that they’re actually nanowires that can conduct electricity. The work was published in Proceedings of the National Academy of Sciences this week.
“The pili idea was the strongest hypothesis, but we were always cautious because the exact composition and structure were very elusive. Then we solved the experimental challenges and the hard data took us in a completely different direction. I have never been happier about being wrong,” says Moh El-Naggar at the University of Southern California in a news release. “In many ways, it turned out to be an even cleverer way for bacteria to power themselves.”
To see what the nanowires are made of, El-Naggar and colleagues used techniques from genetics and molecular biology to narrow down the proteins involved. First, during the formation of nanowires, they noticed an increase in the expression of genes for transporting electrons -- but no such increase was observed in the expression of genes for pili.
Then, the team deprived the bacteria of oxygen -- forcing them to stretch out their nanowires on command -- and stained various cell parts and specific proteins so that they would fluoresce for the camera (video below). Like us, bacteria pass electrons to oxygen for energy; in anaerobic settlings, electric bacteria can still make the energy molecule ATP by pushing electrons to rocks and minerals.
The team discovered that the nanowires are extensions of the bacteria’s outer membrane. And rather than bacterial hair proteins, the extensions are comprised of specialized proteins called cytochromes, which shuttle electrons.
"What the cell is doing is actually morphing a little bit," El-Naggar tells Popular Science. "It's extending its outer membrane in the shape of a long tube." The cytochromes allow the bacteria to transfer electricity to a variety of solid surfaces and to power themselves.
Understanding how microbes build up and break down minerals could help researchers harness them for fuel cells, batteries, and systems that turn waste into electricity.
Here’s a video of the nanowires reaching out. These few-second clips were the result of a lot of behind-the-scenes juggling: simultaneously labeling multiple features, keeping a camera focused on wriggling bacteria, and combining optical techniques with atom-scale microscopy for higher resolution.