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While bacteria producing electricity have been found in exotic environments such as mines and lake bottoms, scientists have missed a source closer to home: the human gut.
Listeria bacteria carry electrons through their cell wall in the environment as tiny currents, assisted by ubiquitous flavin molecules (yellow dots). (Amy Cao graphic, copyright UC Berkeley)
UC Berkeley scientists have discovered that a bacterium commonly causing diarrhea, Listeria monocytogenes, produces electricity using an entirely different technique from known electrogenic bacteria, and that hundreds of other bacterial species use this same process.
Many of these sparkling bacteria are part of the human intestinal microbiome, and many, like the virus responsible for listeriosis, a foodborne illness, which can also cause miscarriages, are pathogenic. The bacteria that cause gangrene (Clostridium perfringens) and nosocomial infections (Enterococcus faecalis) and some pathogenic streptococcal bacteria also produce electricity. Other electrogenic bacteria, like lactobacilli, are important in the fermentation of yogurt, and many are probiotics.
"The fact that so many bugs that interact with humans, whether as pathogens or probiotics or in our microbiota or involved in the fermentation of human products, are electrogenic -" said Dan Portnoy, professor of molecular medicine at UC Berkeley . and cell biology and plant and microbial biology. "That could tell us a lot about how these bacteria infect us or help us have a healthy gut."
The discovery will be good news for those currently trying to create live batteries from microbes. Such "green" bioenergy technologies could, for example, generate electricity from bacteria in waste treatment plants.
The research will be published online on September 12th before the print publication of October 4th. Nature.
Breathable metal
Bacteria generate electricity for the same reason that we breathe oxygen: to eliminate the electrons produced during metabolism and support the production of energy. While animals and plants transfer their electrons to oxygen inside mitochondria from every cell, bacteria in oxygen-free environments – including our guts, but also alcoholic and cheese fermentation tanks and the acid mines – have to find another electron acceptor. In geological environments, it was often a mineral – iron or manganese, for example – outside the cell. In a sense, these bacteria "breathe" iron or manganese.
A microbial battery made with newly discovered electrogenic bacteria. The electrodes (CE, WE) are placed in pots full of bacteria, producing up to half a millivolt of electricity. Ajo-Franklin photo.
The transfer of electrons out of the cell to a mineral requires a cascade of special chemical reactions, the extracellular electron transfer chain, which transports the electrons in the form of a tiny electrical current. Some scientists have used this chain to make a battery: stick an electrode in a bottle containing these bacteria and you will be able to generate electricity.
The recently discovered extracellular electron transfer system is actually simpler than the already known transfer chain and appears to be used by bacteria only when necessary, perhaps when oxygen levels are low. . Until now, this simplest electron transfer chain has been found in single cell walled bacteria – microbes classified as Gram-positive bacteria – that live in a flavin-rich environment, derived from the vitamin B2.
"It seems that the cell structure of these bacteria and the vitamin-rich ecological niche they occupy make it much easier and more profitable to transfer electrons from the cell," said lead author Sam Light, a postdoctoral fellow. "Thus, we believe that the classically studied mineral-breathing bacteria use extracellular electron transfer because they are crucial for survival, whereas these newly identified bacteria use it because they are" easy "."
To see how robust this system is, Light has teamed with Caroline Ajo-Franklin of the Lawrence Berkeley National Laboratory, which explores interactions between living microbes and inorganic materials for potential applications in carbon capture and sequestration and production. bio-solar energy.
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It's a shock for the system to consider that microbes could live very busy lives in our gut.
– Nature News & Views Commentary by Laty Cahoon and Nancy Freitag
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She used an electrode to measure the electrical current flowing from the bacteria – up to 500 microamperes – confirming that it is actually electrogenic. In fact, they produce about as much electricity – about 100,000 electrons per second per cell – as the known electrogenic bacteria.
Light is particularly intrigued by the presence of this system in Lactobacillus, essential bacteria for cheese production, yoghurt and sauerkraut. Perhaps, he suggests, the transport of electrons plays a role in the taste of cheese and sauerkraut.
"It's a big part of the physiology of bacteria that people do not realize and that could be manipulated," he said.
Light and Portnoy have many more questions about how and why these bacteria have developed such a unique system. Simplicity – it is easier to transfer electrons through a cell wall than in two – and the possibility – taking advantage of the ubiquitous flavin molecules to get rid of the electrons – seems to have allowed these bacteria to survive both rich in oxygen and oxygen – poor environments
Other co-authors are Lin Su and Jose A. Cornejo of Berkeley Lab and Rafael Rivera-Lugo, Alexander Louie and Anthony T. Iavarone of UC Berkeley. The research was funded by the National Institute of Allergy and Infectious Diseases of the National Institutes of Health and the Office of Naval Research.
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