Hundreds of electricity producing bacteria, including pathogenic, probiotic and fermenting bacteria

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.

Scientists at the University of California, Berkeley, have discovered that a bacterium that causes diarrhea, Listeria monocytogenes, produces electricity using a technique that is totally different from that of 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 bacteria, such as lactobacilli, play an important role in the fermentation of yoghurt, and many of them 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. These "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 of every cell, bacteria present in oxygen-free environments – including our guts, but also alcoholic fermentation tanks and cheese and acid mines – must find another acceptor of electrons. 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.

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 simpler 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 cellular structure of these bacteria and the vitamin-rich ecological niche they occupy make transferring the electrons from the cell much easier and more cost effective," said lead author Sam Light, a postdoctoral fellow. "Thus, we believe that the classically studied mineral-breathing bacteria use extracellular electron transfer because it is 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.

She used an electrode to measure the electrical current that circulates 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, a bacterium essential for the production of cheese, 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's easier to transfer electrons through a cell wall rather than 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 rich environments and poor in oxygen.

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More information:
Mechanism of extracellular transfer of flavin-based electrons in various Gram-positive bacteria, Nature (2018). DOI: 10.1038 / s41586-018-0498-z, https://www.nature.com/articles/s41586-018-0498-z