Scientists first discovered electricity-producing bacteria in the late 1980s. These unique microbes thrive in extreme environments, often at the bottoms of lake beds. Such an unusual function seems suited to harsh environments, but recently, scientists were surprised to find bacteria with the potential to produce electricity reside even in the depths of the human gut.
Scientists are uncovering novel bacteria with the potential to produce electricity and developing exciting new ways to harness this unique biological power. Bacteria that make microscopic spider-like wires might one day power smartphones, smartwatches, and other electronic devices.
Bacteria are not trying to produce electricity. “They are just doing this normal aspect of their physiology, which is to respire or to make energy,” said Sam Light, a microbiologist at the University of Chicago.
When humans eat, their bodies convert sugars from food into electrons. These electrons transfer to oxygen to produce energy that powers the metabolic processes of the cell. Humans take in oxygen every time they breathe, and because oxygen is soluble, this process readily occurs within the cell. But because some bacteria live in extreme environments that often lack oxygen, they direct their electrons to insoluble molecules, such as iron, that are available in the environment.
When Derek Lovley, a microbiologist now at the University of Massachusetts Amherst, scooped up mud from the Potomac River in 1987, he was only interested in exploring the ability of the bacterium Geobacter metalloproteins to make electric connections with environmental iron.1 Years later, he realized that Geobacter transferred electrons that were destined for iron molecules to electrodes placed in the mud.2 By redirecting these electrons to an electrode, he and his team harnessed electricity from bacteria.
There still isn’t another microbe known to produce electricity better than Geobacter after all this time,” said Lovley.
In an unusual place
Although Geobacter is the most frequently used microbe for electricity production, it is not the only microbe with this ability. In 2018, researchers made the surprising discovery that bacteria with electricity-producing capabilities also exist in the human gut.3
“We sort of just stumbled upon it; it wasn’t our initial intent,” said Light, first author of the study done during his time as a postdoctoral researcher at the University of California, Berkley. While investigating the common food-borne pathogen Listeria monocytogenes, Light discovered that it shared eight common genes with microbes that transfer electrons across their cell membranes. He turned to his advisor, Daniel Portnoy, to plan their approach for probing the implications of this observation.
“Because neither one of us were experts, we were advised that the gold standard in the field was to show that the electrons can transfer into an electrode,” said Portnoy, senior author of the study.
After placing Listeria in an electrochemical chamber and confirming that it produced electricity, Light and Portnoy surveyed the bacteria in the human gut and discovered that a portion of gut bacteria had the same electron-transferring genes. They placed isolated gut bacteria, one species at a time, in electrochemical chambers, where they also generated an electrical current.
Like Geobacter, gut bacteria transfer electrons out of the cell. But without a nearby electrode, it is unlikely that they produce electricity within the human body. Light is unsure where exactly in the gut the electrons transfer, but he suspects that the bacteria use iron or small molecules from the host diet.
“There were hints in the literature that [gut microbes] had this activity, but no one had put genes and proteins into the picture yet. That’s what made this so nice. We hit the genes in this genetic screen that predicted proteins that all had interesting homologies,” said Portnoy. “Different pieces of the puzzle came together nicely.”
Light and Portnoy’s work showed that bacterial ability to produce electricity may be more widespread than once thought. However, these other bacteria have weaker electrical activity than Geobacter. “In the case of specialist bacteria, they are really capable of directing all of their metabolism to the electrode or to iron outside the cell,” said Light.
The secret to Geobacter’s electrical power is that it produces thin electrically conductive filaments called nanowires when it transfers electrons to iron minerals. These nanowires stack on top of each other to produce a biofilm, creating a large electrical network, even with cells that do not directly contact the electrode.
Researchers use this electrical biofilm in waste water treatment. As Geobacter consumes sewage, it releases electrons, which are captured via an electrode and harnessed as an electrical current. “It’s not a huge source of energy. It’s just enough to offset the energy that the waste treatment plant would normally consume,” said Light.
Researchers were initially excited about the possibility of using bacteria like Geobacter as an alternative source of energy, but over the years, research in that area did not pan out as planned. “It works great at a laboratory scale, but making large-scale systems has proven to be difficult,” said Lovley who has worked on Geobacter applications for over a decade.
Now, Lovley has pivoted to a more exciting pursuit. “We sheer the nanowires off the microbe, so we’re not dealing with a bacterium anymore, and make electronic devices with the wires,” said Lovley.
At present, many electronic devices are powered through silicon-based wires. There is a limited supply of silicon, and it is especially difficult to mine. “I liked it because, as we go toward the ‘internet of things,’ we’re making more and more electronics that are regarded as disposable, and we have this mounting electronic waste problem,” said Lovley. Having an organic, degradable, highly conductive wire, without the need for toxic chemicals or labor-intensive energy, is important for environmental cleanup and sustainability.
Together with his collaborator Jun Yao, an electrical engineer at the University of Massachusetts Amherst, Lovley is developing biosensors. “We all want to potentially have a smartwatch that can tell us how we're doing and look for early indicators of dementia or other diseases,” said Lovley.
Using microbe-produced nanowires, Lovley and Yao developed an ammonia biosensor with higher sensitivity than previously-described sensors. The technology may one day sense kidney disease or other metabolic disorders by measuring ammonia in a patient’s breath.4
One advantage of bacterial-based nanowires is the ability to genetically engineer the microbes so that the filaments they create recognize different compounds of interest. Lovley continues to make biosensors that can detect different targets, but he has made some other surprising discoveries along the way.
Approximately two years ago, Xiaomeng Liu, a graduate student in Yun Yao’s laboratory, placed a thin film of Lovley’s microbe-produced nanowire on an electrode and discovered that the system produced electricity before he even applied an electrical potential. The nanowires extracted energy from humid air as water vapor condensed and evaporated on the surface.5 “It's free electricity for doing nothing,” said Lovley. The discovery offers the potential for an easy, clean, self-sustaining continuous energy-harvesting strategy—if it can be scaled up sufficiently.
Using bacterial nanowires, Lovley and Yao also developed a neuromorphic memory device with a voltage comparable to that of human neurons.6 “It works like the memory in the brain using the nanowires as a kind of neuron scaffold,” said Lovley. Unlike a computer memory, these neuromorphic devices can directly process, learn, and respond to signals.
The possible applications for electrically conductive bacterial nanowires continue to unfold. Geobacter grows slowly and can be difficult to culture, but Lovley will not fall prey to the problem of scalability again. He genetically engineered a strain of Escherichia coli, a common lab bacterium to produce the nanowires. “I wanted other people to be able to make them, because if you look at material science, big growth in the field occurs when the material becomes available,” said Lovley.
Of all the applications for electrically conductive bacteria, Lovley believes that nanowires hold the greatest potential for improving society. He and his team continue to uncover new possibilities as they prospect through the microbial world. “We were very geographically focused initially, but there are many microbes throughout the microbial world that have similar filaments that are electrically conductive,” said Lovley. “We have just started to scratch the surface here.”
1. D. Lovley et. al., “Anaerobic production of magnetite by a dissimilatory iron-reducing microorganism,” Nature, 330: 252-254, 1987.
2. D.R. Bond et. al., “Electrode-reducing microorganisms that harvest energy from marine sediments,” Science, 295(5554): 483-485, 2002.
3. S.H. Light et. al., “A flavin-based extracellular electron transfer mechanism in diverse Gram-positive bacteria,” Nature, 562: 140-144, 2018.
4. A.F. Smith et. al., “Bioelectronic protein nanowire sensors for ammonia detection,” NanoResearch, 13: 1479-1484, 2020.
5. X. Liu et. al., “Power generation from ambient humidity using protein nanowires,” Nature, 578: 550-554, 2020.
6. T. Fu et. al., “Bioinspired bio-voltage memristors,” Nature Communications, 11(1861), 2020.