A Shocking Discovery: Electrogenic Bacteria in Human Gut

We all know it. We’ve all heard its name. It’s all around us and inside of us, all of the time. Bacteria. And it can produce electricity.

Bacteria are those pesky microbes, typically only 0.2–5.0µm in diameter, that have a tendency to make us ill, such as food poisoning (Salmonella enteric) and pneumonia (Streptococcus pneumoniae). But they are also essential to everyday life on our little planet and that includes us humans, from movie stars to struggling journalists. Recently, the benefits of caring for our own gut microbiome has become the topic of much discussion in the public in regards to digestion and our ability to fight infections. Certain bacteria aid in the digestion of materials us humans aren’t so good at, such as plant material, and bacteria on our skin aids in preventing harmful bacteria from successfully colonising.

Listeria monocytogenes

On 12 September 2018, scientists from the University of California, Berkeley, published a paper which exposed one such bacterium that can produce a small amount of electricity and can be found in the human gut. However, this is not one of those friendly bacteria we rely on. Listeria monocytogenes is typically ingested via contaminated food and is the most common bacterium responsible for listeriosis, a disease which can range from muscle aches and fever to septicaemia or meningitis, and 20 to 30 percent of all listeriosis cases are fatal. In short, you wouldn’t want to find this bacterium in your probiotic yoghurt.

Electrogenic bacteria, that being bacteria which produce electricity, are not unknown to scientists but were for a time mostly relegated to bacteria which live in the most extreme of conditions, known as extremophiles. However, more recent studies have begun to find more and more species of bacteria which possess the ability to generate a small electrical current, such as L. monocytogenes. This is achieved by moving electrons from inside to outside of their cell wall.

External Electron Transfer

In humans, energy is produced via aerobic respiration. This is where our cells use glucose and oxygen to create ATP (adenosine triphosphate), which are the cell’s primary energy storage molecules. The final phase of this process is the electron transport chain and is responsible for the vast majority of the ATP created. In this phase, which takes place in the mitochondria of the cell, oxygen is essential for the transport of electrons so as to actively pump hydrogen ions (protons) out of the mitochondrial matrix, through the inner membrane, into the intermembrane space. This results in the protons diffusing back into the mitochondrial matrix via ATP Synthase, a specialised protein complex which utilises the energy of these diffusing protons to produce the ATP molecules. In anaerobic conditions (no oxygen) this phase is not achievable and the cell produces very few ATP molecules.

Electrogenic bacteria have evolved to still achieve an equivalent of the electron transport chain in anaerobic conditions, known as external electron transfer (EET). In this process, electrogenic bacteria use metal-containing minerals as electron acceptors to fill the role of oxygen in transporting electrons out of the cytosol to the exterior environment of the bacterial cell. Thereby continuing to create ATP unhindered by the absence of oxygen and producing a small electrical current in their external environment in the process. It has been speculated that this small current could also be used for cell-to-cell communication.

The scientists at the University of California, Berkeley, discovered that L. monocytogenes specifically utilises flavins, a diverse family of yellow-coloured compounds which include vitamin B12, as electron acceptors in oxygen-deficient conditions. Further experimentation discerned a locus (an area of a chromosome) of eight genes which are responsible for EET. Relatives of these genes, called orthologues, are found in hundreds of species of bacteria, including some pathogens and intestinal microbiota. This study’s findings suggest that EET is much more prevalent throughout bacteria than previously thought, including some used in alcohol, yoghurt and cheese production.

EET is almost exclusive to Gram-positive bacteria; that being bacteria with only one cell membrane. Gram-negative bacteria have a second cell membrane which appears to make EET incompatible with their structure.

Why is all This Important?

This is likely the most common question scientists hear regarding any new discovery of this kind.

In our continued pursuit for more environmentally friendly electricity production, bacteria which can produce electricity naturally are a truly sustainable resource we cannot afford to ignore. Throughout the 20th century, microbial fuel cells were created using bacteria as electricity producers. Now, in the 21st century, such devices are being tested in wastewater treatment plants to generate electrical current.

This is just the beginning of the use of bacteria as a sustainable resource and microbial fuel cells are just the beginning of harnessing this resource for a more environmentally friendly future. Furthermore, the more we understand these bacteria, especially those which can harm us, the better prepared we can be to protect ourselves from them.

Further Reading

Brenner, M., Lobel, L., Borovok, I., Sigal, N. and Herskovits, A. (2018). Controlled branched-chain amino acids auxotrophy in Listeria monocytogenes allows isoleucine to serve as a host signal and virulence effector. PLOS Genetics, 14(3), e1007283.

Edwards A.M. (2014) Structure and General Properties of Flavins. In: Weber S., Schleicher E. (eds) Flavins and Flavoproteins. Methods in Molecular Biology (Methods and Protocols), vol 1146. Humana Press, New York, NY.

Light, S., Su, L., Rivera-Lugo, R., Cornejo, J., Louie, A., Iavarone, A., Ajo-Franklin, C. and Portnoy, D. (2018). A flavin-based extracellular electron transfer mechanism in diverse Gram-positive bacteria. Nature.

Logan, B. (2009). Exoelectrogenic bacteria that power microbial fuel cells. Nature Reviews Microbiology, 7(5), 375–381.

Sacco, N., Bonetto, M. and Cortón, E. (2017). Isolation and Characterization of a Novel Electrogenic Bacterium, Dietzia sp. RNV-4. PLOS ONE, 12(2), e0169955.

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