As part of their metabolism, the bacteria have developed a way to transfer electrons from the interior of the cell across their outer membrane to a receiving surface in the outside world.
Associate professor Moh El-Naggar of USC Dornsife explains that the process is akin to the way humans use oxygen to breathe. The body takes electrons from food and transfers those electrons to oxygen inhaled by the lungs.
The organism was discovered nearly 30 years ago and interest in learning exactly how this group of microbes essentially ‘breathes’ rocks has followed.
Associate prof. El-Naggar and a collaborative team from USC and Caltech think they have the answer, and have published research suggesting the bacteria uses nanowires to accomplish this electronic feat.
According to associate prof. Ek-Naggar harnessing energy from living, organic sources holds potential for new sustainable technology. A microbial fuel cell, for example, could generate electricity by capturing electrons from the bacteria on electrodes instead of the rocks that these organisms evolved to breathe.
"Microbes are highly evolved machines," associate prof. El-Naggar says, "and what we have here is a class that is really good at converting energy and interacting with the abiotic world."
The potential for wastewater treatment is also being explored at USC. Microbes feed on the waste, oxidising the organic substances and producing a small amount of electricity. These organisms, the team explains, could exemplify the kinds of life that exist in environments where little or no oxygen exists.
Depositing electrons outside the cell is how they survive, continues associate prof. 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. The bacteria would basically suffocate."
Under the microscope, scientists say they can see what appears to be filaments projecting from these cells. For years, the prevailing hypothesis was that these were a form of tiny hairs called ‘pili’, similar to those found on other types of bacteria.
But in 2013, a research scientist in associate prof. El-Naggar's laboratory, Sahand Pirbadian, discovered that these projections, referred to as ‘nanowires’, were actually extensions of the cell membrane covered in cytochromes. These are proteins containing iron that facilitate electron transport. Pirbadian explains the nanowires allow the bacteria to connect with surfaces much further away than one would expect.
Through light microscopy imaging, the team had an idea of the nanowires' basic composition. But they were curious whether the cytochromes were close enough together to transport electrons along the wire. If the density were high enough, they thought a bridge could form along the membrane that would allow an electron to cross onto external surfaces.
For the current study, associate prof. El-Naggar and Pirbadian collaborated with Grant Jensen and Poorna Subramanian at Caltech. Using electron cryotomography (ECT), the team explain they are able to instantly freeze cells, preserving them in a form that is close to their natural state, and then image them with nanoscale resolution in three dimensions.
Subramanian and Pirbadian say they were able to capture life-like images of the bacteria and their nanowires.
"These are not simple tubes," El-Naggar says. "They turned out to be more like a chain of membrane pearls, strung together."
With the images produced by ECT, the team claim to be the first to see how electron transport proteins are distributed in the membrane to form the nanowires. Accoridng to the researchers, while some were touching each other, many were further apart - up to 30nm - a range too far for an electron to jump. With this new information, the team proposes that the proteins float within the membrane, creating just enough collisions to allow electrons to exchange from one to the next, until they reach the end of the nanowire and transfer to the rock or metal surface.
The scientists’ next step is to confirm these collisions are, in fact, happening.
Associate prof. El-Naggar concludes: "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. We are trying build the foundations of a new generation of living electronics."