Moh El-Naggar, scientist and associate professor of physics, biological sciences and chemistry at USC Dornsife, believes that bacteria are nature’s microscopic power plant. El-Naggar and his team focused their work on the Shewanella oneidensis species of bacteria. This type of bacteria basically breathes rocks. These organisms are examples of life where little to no oxygen exists, like a space planet or under water.
The bacteria have developed their metabolism to transfer electrons from the interior of the cell, across to the outer membrane to a receiving surface in the outside world. This is similar to the way that humans take in oxygen to breathe.
Shewanella was discovered 30 years ago by Kenneth Nealson, Wrigley Chair in Environmental Studies and professor of earth sciences and biological sciences at USC Dornsife. More recently, the bacteria has been of interest because researchers want to know exactly how it pulls off this “breathing” trick. El-Naggar and his team believe that they know the answer.
Taking energy from living organic sources could potentially be huge for sustainable technology. A microbial fuel cell could potentially generate electricity through capturing electrons from bacteria on electrodes, rather than the rocks that Shewanella uses to breathe.
"Microbes are highly evolved machines," El-Naggar said. "And what we have here is a class that is really good at converting energy and interacting with the abiotic world."
The electric bacteria are currently being explored at in a wastewater treatment at USC. Microbes use waste for energy, therefore oxidizing organic substances and producing a small amount of electricity.
The bacterium survives through depositing electrons outside the cell, according to El-Naggar.
“If one were to shut down the ability to transfer the electron out of their system, they would not be able to make energy," he added. "The bacteria would basically suffocate."
The team used light microscopy imaging to figure out the basic composition of the nanowires. The team wanted to investigate further to figure out if the cytochromes are close enough together that they can transport electrons along the wire. The researchers believed that an electron can cross onto external surfaces if the density was high enough to create a bridge on the membrane.
El-Naggar teamed up with Grant Jensen and Poorna Subramanian at Caltech, who are experts in the use of electron cryotomography (ECT). With ECT, cells are instantly frozen and preserved in a form that is close to the natural state and then images them. With this method, the collaborative team was able to capture images of bacteria and their nanowires and they made an interesting discovery.
"These are not simple tubes," El-Naggar said. "They turned out to be more like a chain of membrane pearls, strung together."
By using ECT, the team could see how electron transport proteins are distributed in the membrane. Because of this, the researchers believe that the proteins float within the membrane, creating just enough collisions that allow electrons to exchange from one to the next until they reach the end of the nanowire.
Next, the team needs to confirm that the collisions are happening. There is still a lot to be learned about the membrane, but there is so much potential in the research.
"My lab is driven by the idea that we could develop new machines, where living cells are functioning as part of a hybrid biotic-abiotic system," said El-Naggar. "We are trying build the foundations of a new generation of living electronics."
The paper on this research was published in the Proceedings of the National Academy of Sciences.