For humans, electricity began with Ben Franklin flying a kite with a key that got struck by lightning. We’ve come a long way since then, but we weren’t the first ones to use electricity - bacteria were. There are two known bacteria that produce electricity, Geobacter and Shewanella. We are only just beginning to understand them, but they may help us produce cleaner energy.
The image below is from this video, which you can watch for an overview of the genus we will be talking about today, Geobacter.
But how do we use You may have heard of microbial fuel cells; they’re systems that convert bacterial chemical energy to electric energy that we can use. This alternative to batteries and engines is significantly more efficient and has low emission levels. They are renewable and have indefinite lifetimes.
Geobacter sulfurreducens is one species that is being studied with industrial applications in mind. But what gives it the ability to produce electricity? The key is in its pili, which are the hair-like fibers extending from the outer capsule of the cell. They’re made up of protein subunits called pilins. They mostly help the bacteria either adhere to a surface or move around, but in G. sulfurreducens they also carry electrons as part of respiration, producing an electric current.
Generic bacteria drawing with pili / rod-shaped G. sulfurreducens on dirt
G. sulfurreducens is a Gram-negative bacteria with type IV pili (T4P) that work by extending, sticking to a surface, and then retracting to pull the cell forward. This form of motility is called twitching, which is what it looks like under the microscope.
Pilus extension is mediated by a protein called PilB and retraction is mediated by PilT. Both of these proteins are ATPases, meaning they are powered by ATP hydrolysis. As shown in the image below, they are found in the inner membrane of the cell. The pilus extends by polymerizing pilin protein subunits (called PilA in T4P), which is powered by PilB, and retracts by depolymerizing PilA.
Pil motor complex
In a paper published October 2016, Speers et al. identified the genes coding for PilB and PilT proteins. There was one gene coding for the PilB protein in G. sulfurreducens, but there were four different pilT genes, designated pilT1, pilT2, pilT3, and pilT4. The challenge was then to figure out which one coded for the main PilT retraction motor, the one that did the heavy lifting.
The first thing they did was compare the sequences of the four PilT proteins coded for by pilT1 through pilT4. They found that PilT3 and PilT4 were the most similar to the PilT ATPases in other species. A higher degree of similarity suggests that the sequences were conserved evolutionarily, and so those proteins are probably very important. Therefore, based on that initial sequence analysis PilT3 and PilT4 were immediately the most likely candidates for the main PilT motor.
Then they looked at where the genes were located on the bacterial chromosome. The genes pilB and pilT4 were found in the same gene cluster as the pilin-encoding gene pilA (see image below). This is significant because bacterial genomes often cluster genes in similar pathways because if one is needed the others will be, too. In other words, if pilA was transcribed but not pilB and pilT, pilin subunits would be produced but there would be no ATPases to assemble them into pili. Likewise, pilB and pilT are useless without pilA transcription. The three genes would have to be transcribed together. By this reasoning, pilT4’s proximity to pilA and pilB again suggested that it coded for the main pilT motor.
Pil gene locations on G. sulfurreducens
The group then set out to experimentally verify their prediction that PilT4 was the main motor. To do this, they used a mutant strain of bacteria that didn’t have a pilT gene at all, meaning that it couldn’t produce its own PilT proteins. The species used was Pseudomonas aeruginosa. Mutants are visually distinguishable because deleting pilT causes two phenotypic mutations: hyperpiliation (the bacteria produce more pili) and a loss of twitching motility. This causes the cells to stick together more, and they form a biofilm.
These phentotypic mutations can be fixed if a pilT gene is introduced that is functionally similar enough to the deleted gene. The test was then to express each of the four G. sulfurreducens pilT genes to see what happened. You can see their results in the image below.
The wild-type strain is pictured in the top left; PAK is the strain of bacteria. Aside from the wild-type strain, the most cell growth occurred with PilT3 and PilT4 restored. Both were able to restore twitching motility, allowing the cells to spread farther from the inoculation point and experience greater overall colony growth. However, microscopic inspection revealed that only PilT3 was able to fix hyperpiliation. You can see this in the image because the PilT4 biofilm is denser than the PilT3. Still, the fact that both restored twitching motility shows that both can function as ATPases.
The last thing they did was investigate the role of the pili in electricity conduction. They found that PilT4 was necessary for conduction and that it promoted growth on an electrode better than PilT3 (or any of the other PilT proteins). Electricity is really just the passing of electrons down a wire, or something else conductive – in this case, the pili.
So, what’s the point of producing an electric current like this? Well, when the cells undergo respiration in order to produce energy, they take electrons from a high-energy compound and use them to produce ATP. At the end, the electrons need somewhere to go. When humans undergo respiration, we use oxygen as our final electron acceptor; this is called aerobic respiration. It’s why we need to breathe! Some bacteria can also use oxygen, but others cannot. If they use anything else as a final acceptor, it’s called anaerobic respiration. G. sulfurreducens is a strict anaerobe, and it uses its conductive pili to dump the used electrons onto sulfur or iron in the soil. That’s where the species name comes from – sulfurreducens. Sulfur-reducing!
The authors explain how the antagonistic action of PilB and PilT (remember, PilB controls pilus extension and PilT controls retraction) is evolutionarily advantageous to G. sulfurreducens. The pili need to constantly move so that the bacteria can have new iron and sulfur to give electrons to in its natural environment, the soil. After dumping its electrons, the pilus can retract and then re-extend to find a new electron acceptor. It can do this repeatedly until it finds one in the soil. PilT4 controls not only cell movement in G. sulfurreducens but also respiration!
So they figured out that PilT4 was the main PilT motor in G. sulfurreducens. But what did the other three PilT proteins do then? The authors provided some educated guesses: PilT3 may help PilT4 function, or it may function in a protein secretion pathway. PilT1 also seems to be involved in protein secretion, but a different kind. The role of PilT2 remains unclear, but it is unique to G. sulfurreducens, so it must be highly specialized. Further research could, and probably will, pursue the functions of these proteins.
The more we know about G. sulfurreducens the better we will be able to use it for our purposes. After all, bacteria have a lot of talents that we don’t. They can clean up our messes better than we ever could. There is a species to metabolize every kind of waste we produce - there are bacteria that can metabolize oil spills, pollutants in the air and water, plastic, radioactive ions, sewage, acid runoff. Learning about these bacteria and being to able to use them industrially is vital to the future health the environment.
References:
First image, linked video: Seeker. Can Bacteria Live Off Electricity? YouTube.
Second image: Socratic.org
Third image: Greeniac Nation
Fifth, sixth image from main paper.
Last image: Langley Fertilizers.
Other references:
Brahic, C. (2014). Meet the electric life forms that live on pure energy. New Scientist.
Bond, D.R., and Lovley, D.R. (2003). Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69, 1548-1555.
Craig, L., Pique, M.E., and Tainer, J.A. (2004). Type IV pilus structure and bacterial pathogenicity. Nat Rev Micro 2, 363-378.
Shi, W., and Sun, H. (2002). Type IV Pilus-Dependent Motility and Its Possible Role in Bacterial Pathogenesis. Infect. Immun. 70, 1-4.
White, G.F., Edwards, M.J., Gomez-Perez, L., Richardson, D.J., Butt, J.N., and Clarke, T.A. (2016). Chapter Three - Mechanisms of Bacterial Extracellular Electron Exchange. In Advances in Microbial Physiology, Poole, Robert K. ed., Academic Press) pp. 87-138.
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