Right under our feet exist little creatures that could become a new source of renewable energy. They are microbes living in mud and soil, known as “electrogenic bacteria”. These bacteria live their lives producing electrons as they break down organic matter. These electrons can be used as electricity, but making that practically feasible requires bacteria with considerably strong electrogenic abilities.

Since 2021, Professor Kazuya Watanabe, from Tokyo University of Pharmacy and Life Sciences, and Miraikan have been working together to find such bacteria, that we call “super-electrogenic bacteria”. These five years, we have hosted the project “Let’s all hunt for super-electrogenic bacteria!” In this project, middle and high school students first think of hypotheses on where super-electrogenic bacteria might live. Then they collect mud and/or soil near their homes or schools and compare those by creating “mud batteries” (microbial fuel cells). A mud battery can be made by adding water to the mud or soil as necessary, inserting electrodes, and then connecting them with wires and a resistor. The project provides a standardized research kit for setting up and testing these mud batteries.

From the first to the fifth rounds of the “Let’s all hunt for super-electrogenic bacteria!” project, a total of 120 teams participated. Together, they made 475 mud batteries, and determined the electricity-generating capacity (maximum output) of 306 of them. If a mud battery generates a lot of electricity (relatively speaking), it may contain super-electrogenic bacteria. Therefore, such mud batteries are sent to Professor Watanabe’s lab for further investigation.

But how can super-electrogenic bacteria be isolated from mud batteries? And in what kind of setting can they be used? That is what I asked Manami Hagiwara, a student from Professor Watanabe’s lab who is working on isolating super-electrogenic bacteria from mud batteries.

How are you trying to find super-electrogenic bacteria?

Hagiwara: ‘First of all, I connect the mud batteries that the project’s participants sent me to a data logger to collect accurate data on their electricity-generating capacity. Participants measure the voltage once a day and the maximum output once a week, but with the data logger we can monitor the voltage continuously. We also use a device that automatically analyzes the maximum output.’

‘Next, we isolate bacteria that are good at generating electricity from the mud batteries – meaning we separate them from other microbes and mud – through either what is known as the “direct method” or using the “electrochemical cell method”. And we also do microbiome analysis: finding out what microbes there are through DNA analysis.’

“The direct method”

Hagiwara: ‘When using the direct method, we cut a mud battery’s anode – which has a lot of electrogenic bacteria stuck to it – in tiny pieces. Then we mix them with a liquid and smear it onto a culture medium. A culture medium is a jelly-like substance that contains food for microbes. In this case, we use a culture medium that was developed to cultivate Geobacter, a well-known type of electrogenic bacteria.’

‘After a while, the bacteria form lumps (colonies) that are so big they can be seen with the naked eye. At first, most colonies consist of multiple types of bacteria, so we take the ones that are dark red – the color of Geobacter colonies – smear them onto a new culture medium and wait again until colonies appear. Then we take only the dark red colonies again, smear them onto yet another new culture medium, etc. We repeat this process until only one species of bacteria is left. At that point, we have isolated our electrogenic bacteria (separated it from all other microbes).’

“The electrochemical cell method”

Hagiwara: ‘When using the electrochemical cell method, we do not directly apply (fragments of) the anode to a culture medium. Instead, we first grow more electrogenic bacteria using an “electrochemical cell”. This electrochemical cell is a device containing multiple electrodes and a culture solution (a solution containing food for microbes) for growing Geobacter electrogenic bacteria. We cut off a part of a mud battery’s anode and use it as the so-called “working electrode” of the electrochemical cell. In addition, we make it easier for electrogenic bacteria to stick to the working electrode by applying some voltage (electric potential) to it. And then the Geobacter bacteria will grow on the working electrode while feeding on the culture solution.’

‘Once we have enough Geobacter bacteria, we take out the electrochemical cell’s working electrode and cut it into tiny pieces. From there on out, we isolate the electrogenic bacteria in the same way as we did with the direct method. The difference between both methods is that with the direct method we start out with an anode that may still have a lot of non-electrogenic bacteria stuck to it, but with the electrochemical cell method we first increase the number and share of electrogenic bacteria using the working electrode.’

“Microbiome analysis”

Hagiwara: ‘Reading this far, you may have realized that with both the direct method and the electrochemical cell method, we use culture medium or solution for Geobacter bacteria. However, there are also other types of electrogenic bacteria, and they may not like the same food as Geobacter. In short, it is highly possible that we cannot isolate other bacteria than just Geobacter like this. This increases the risk of overlooking other (super)electrogenic bacteria.’

‘But if there are any bacteria that are even better at generating electricity than Geobacter, we definitely don’t want to miss them! Therefore, we also do microbiome analysis: we analyze the DNA of microbes on mud batteries’ anodes to find out which microbes live there. To differentiate between microbes that accidentally ended up on the anode and microbes that grow there because they generate electricity, we also analyze the DNA of microbes living in the mud itself. If we find a microbe that is clearly more numerous at the anode, it is likely that it has something to do with generating electricity. In the future, we want to look up how to grow these microbes and isolate them as well.’

So, did you actually find super-electrogenic bacteria?

Hagiwara: ‘Yes! We found a strain of Geobacter bacteria that is capable of generating even more electricity than the standard strain. You see, there are different groups even within one species of bacteria, that have their own distinctive genetic characteristics. Those groups are called “strains”. We isolated a strain of Geobacter bacteria from a sample collected from a river in Nasu by a team from Seibu Gakuen Bunri High School as part of “Let’s all hunt for super-electrogenic bacteria!” in 2024. This strain generated an electrical current that was 3 times stronger than the standard strain of Geobacter!’

‘When we cultivated this super-electrogenic strain of Geobacter, it formed lumps in the culture solution. The culture solution consists of food for the bacteria dissolved in water, so this strain of Geobacter probably doesn’t dissolve well in water. Microbes that don’t dissolve well in water are known to adhere well to electrodes. Because of that, they can transfer electrons to the electrode more easily, so they can generate more electricity.’

What about practical applications of electrogenic bacteria?

Hagiwara: ‘I think electrogenic bacteria will be useful for wastewater treatment. Although researchers use artificial wastewater or juices from food waste rather than real waste water, they found that adding electrogenic bacteria decreased the amount of organic matter polluting that water. In short, the electrogenic bacteria made the (artificial) wastewater cleaner by eating the organic matter it contained.’

Huh?! If you are only going to use them to clean water, then what is the point in putting so much effort into finding super-electrogenic bacteria?

Hagiwara: ‘Of course, we will use them to generate electricity too. Actually, the current method of cleaning wastewater uses microbes that need oxygen to break down organic matter. Providing oxygen to those bacteria takes a lot of energy. On the other hand, electrogenic bacteria don’t need oxygen to break down organic matter. In fact, many of them will die if they are exposed to oxygen. So, if we use electrogenic bacteria instead of the microbes that are currently used to clean wastewater, we can not only save energy but also generate electricity! I think that would lead to substantially lower costs for wastewater treatment.’

‘By the way, I personally have high hopes for an application that has not been researched very much just yet: generating hydrogen using electrogenic bacteria. When electrogenic bacteria eat organic matter and emit electrons, they actually also emit hydrogen ions. There is research that tries to use those hydrogen ions to make hydrogen. Hydrogen is receiving a lot of attention as a fuel that does not emit CO₂ when burned, so I would love to try making hydrogen with the electrogenic bacteria found through the “Let’s all hunt for super-electrogenic bacteria!” project.’

Final thoughts

In the blink of an eye, the fifth edition of “Let’s all hunt for super-electrogenic bacteria!” has come to an end already. Manami Hagiwara’s story about how it truly led to the discovery of super-electrogenic bacteria left a strong impression on me, as one of the organizers of the project. Realization of a microbial power plant is still a far way off, but when I think of the possibility of harnessing the abilities of electrogenic bacteria for wastewater treatment or making hydrogen, I get excited about the sustainable future society it may bring about!