Deep beneath the seabed, teensy bacteria “exhale” electricity through long, skinny snorkels and now, scientists discovered how to switch these microbes’ electric breath on & off.
These bizarre bacteria have two proteins, which confederate during a single hair-like structure, called a pilus, researchers reported in a new study, published Wednesday (Sept. 1) in the journal Nature. Many of those pili lie just beneath the bacterial membrane and help push the snorkels out of the cell and into the encompassing environment, thus allowing microbe to breathe.
This discovery not only reveals something unexpected about the bacteria biology but also pave the way for new technologies, from powerful microbe-powered batteries to new medical treatments for bacterial infections, senior author Nikhil Malvankar, professor of molecular biophysics & biochemistry at Yale University’s Microbial Sciences Institute.
The bacteria belong-to the genus Geobacter and can be found all across the world, growing deep under-ground in soils that are completely devoid of oxygen. Humans depend on oxygen to convert food into usable energy and to sop-up electrons that are left over from this metabolic process. If the left-over electrons accumulated, they would quickly become toxic to the body, Malvankar said.
Just like humans, Geobacter microbes generate waste electrons during metabolism, but they do not have access to oxygen like we do. So, to get rid-of their excess electrons, bacteria coat themselves in thin, conductive filaments called nanowires, which shuttle electrons out of the microbes and to other bacteria or minerals in the environment, like iron oxide.
These thin nanowires are 100,000 times smaller than the width of a person’s hair and transport electrons over huge distances, hundreds to thousands of times the first microbe’s body length.
“I cannot breathe oxygen which is like 100 meters [328 feets] away from me,” Malvankar said. “And somehow, these bacteria are using these nanowires like a snorkel, which is 100 times their size, in order that they will keep breathing over such long distances.” This impressive feat generates an electrical current as electrons continuously flow through the lengthy nanowires
But scientists discovered these nanowires in the early 2000s, Malvankar & his colleagues only recently discovered, what the cellular snorkels are literally made from. Initially, scientists assumed that nanowires were pili. This notion appeared to be supported by the fact that, if you delete the genes needed for pili construction from Geobacter bacteria, nanowires not appear on their surfaces, Malvankar said.
But there was a problem: Pili proteins do not contain any metals, like iron, that conduct electricity. Malvankar & his team investigated this conundrum in a 2019 study, published in the journal Cell, during which they examined Geobacter bacteria using cryo-electron microscopy (cryo-EM), a technique that involves shining beam of electrons through a substance to take a snapshot of its component molecules.
“That’s once we realized that there are no pili on the bacterial surface at all,” Malvankar said. “That was a big surprise.” Instead, team found that nanowires were made from proteins called cytochromes, which readily transfer electrons down their lengths, and thus make far better nanowires than pili. In a 2020 study, published in the journal Nature Chemical Biology, team reported that these cytochrome-based nanowires come in various “flavors”, which conduct electricity with different levels of efficiency.
But even after the team revealed that the chemical make-up of the nanowires, pili proteins still cropped-up in their biochemical assessments of the Geobacter bacteria. If the pili were not conducting electricity, “the real big question was, you know, what do these pili really do? Where are they?” Malvankar said.
In their new Nature study, team looked more closely at the structure of those pili by first deleting the genes for nanowires in lab-grown Geobacter sulfurreducens. The pili usually blocked in by the nanowires, so without those structures in the way, the hair-like projections sprouted from the surface of the cells. This gave team an opportunity to examine at the pili with cryo-EM, which revealed the 2 distinct proteins — PilA-N & PilA-C — within each hair.
The team also ran tests to see how well pili conducted electricity and found that “they move electrons 20,000 times slower than OmcZ,” cytochrome protein that forms the most-highly conductive Geobacter nanowires, Malvankar said; “they are just not really made to move electrons.”
That said, pili seemed like they could serve a different function, team noticed. In other bacterial species, some pili sit beneath the cell membrane and move like tiny pistons; this motion lets them push proteins through the membrane and up & out of the cell. For instance, bacterium Vibrio cholerae, which causes the diarrheal illness cholera, uses such pili to secrete cholera toxin, consistent with a 2010 report in the journal Nature Structural & Molecular Biology. In a series of experiments, team determined that the pili in Geobacter fulfill a similar role, therein they help shove nanowires through the microbial membrane.
“We found that the cytochromes are stuck inside the bacteria when the piston protein is not there,” Malvankar said. “And when we put the gene back, the cytochromes are ready to get out of the bacteria.” This then was the bacteria’s on & off switch, team concluded. Malvankar said. “And when we put the gene back, the cytochromes are ready to get out of the bacteria.” This then was the bacteria’s on & off switch, team concluded.
Looking forward, researchers decide to investigate how many other sorts of bacteria build nanowires and use them to breathe electricity. They are also curious about exploring practical applications for the research.
Researchers used Geobacter colonies to power small electronics for quite a decade, but as of yet, these bacterial batteries can produce only small amounts of power. In past research, Malvankar & his team found that the colonies can-be made more conductive under the influence of an electric field, which could help boost the power of those devices, now, new research could provide scientists another degree of control, by allowing them to switch electricity on/off.
This research could even have applications in medicine and especially, in treatments for bacterial infections, Malvankar said. For instance, Salmonella manages to outgrow beneficial bacteria in the gut because it can switch from fermentation, which produces energy slowly with no oxygen required, to respiration, which produces energy quickly & usually requires oxygen. In the low oxygen environment of the intestines, Salmonella uses a compound, called tetrathionate as a substitute for oxygen, thus outcompeting beneficial bacteria in the body.
But what-if those helpful bacteria could get a leg up? In theory, if you equipped bacteria with nanowires & introduced them into the gut, as a sort of probiotic treatment, they could potentially outcompete harmful pathogens like Salmonella, Malvankar said. Malvankar & his colleagues are studying this potential course of treatment, but the work remains in its early stages.