Bacteria producing electricity
Living in extreme conditions requires a creative approach to the adaptation process. For some types of bacteria that exist under conditions of a constant lack of oxygen, this means that they need to find a way to breathe without using this element. These hardy microbes that live deep in mines, at the bottom of lakes and even in the human intestine, have developed a unique form of breathing, including the release and pumping of electrons. In other words, they can produce electricity.
Scientists and engineers are exploring how to use these microbial power plants to create new fuel cells and purify wastewater, as well as accomplish many other purposes. However, the discovery of the electrical properties of microbes has become a big problem, since these cells are much smaller than mammalian cells, and it is extremely difficult to grow them in the laboratory.
However, recently, MIT engineers have developed a microfluidic method that can quickly process small samples of bacteria and determine their specific property, closely related to their ability to produce electricity. They note that this property is polarizability. It is used to assess the electrochemical activity of bacteria. This method is safer and more effective than any other modern methods.
“The main goal is to select the strongest candidates for the desired tasks,” notes Qianru Wang, a post in the Department of Mechanical Engineering at MIT.
“A recent scientific paper suggests that there is a much wider range of bacteria with this property and capable of generating electricity. Thus, the tool that allows you to explore these organisms may be of far greater importance than we previously thought. Now we are not talking about a small handful of bacteria, ”added Kalen Bui, assistant professor of mechanical engineering at MIT.
Bui and Wang published their research today in the journal Science Advances.
Bacteria that produce electricity carry out this procedure by generating electrons inside their cells and then transferring them through their cell membranes through tiny channels formed by surface proteins, as part of a process known as extracellular electron transfer or EET.
Existing methods for studying the electrochemical activity of bacteria include the cultivation of large batches of cells and the measurement of the activity of EET proteins – this is an incredibly meticulous and lengthy process. Other methods require breaking the cell in order to extract and examine proteins. Bui sought a faster and less disruptive method to evaluate the electrical function of bacteria.
Bacteria producing electricity
Over the past 10 years, his group has been creating microfluidic chips with small channels engraved on them, through which microliter samples of bacteria pass. Each channel is clamped in the middle to form an hourglass configuration. When a voltage is applied to a channel, a compressed area, which is about 100 times smaller than the rest of the channel, will compress the electric field, making it 100 times stronger than the surrounding field. An electric field gradient creates a phenomenon known as dielectrophoresis, or the force that pushes a cell against its movement caused by an electric field. As a result, dielectrophoresis can repel a particle or stop it on tracks at different voltages, depending on the surface properties of this particle.
Researchers, including Buys, used dielectrophoresis to quickly sort bacteria by common properties, such as size and species. But this time, Bui wondered if this method could reveal the electrochemical activity of bacteria, which is a more hidden feature of microbes.
“Basically, dielectrophoresis was used to separate bacteria that differed among themselves, like, say, a frog from a bird, while we try to find differences only between frogs,” said Van.
In their new study, scientists used microfluidic plants to compare different strains of bacteria, each of which has a different electrochemical activity. The strains included a “wild type” or a natural strain of bacteria that actively generates electricity in microbial fuel cells, and several strains developed by researchers through genetic means. In general, the team sought to find out whether there is a correlation between the electrical abilities of bacteria and their behavior in a microfluidic device under the action of dielectrophoretic force.
The team missed very small microliter samples of each bacterial strain through an hourglass microfluidic channel and slowly increased the voltage on the channel: one volt per second, from 0 to 80 volts. Using a visualization technique known as particle image velocimetry, they found that the emerging electric field propelled the bacterial cells through the canal until they reached the compressed area, which had a much stronger field that pushed them through dielectrophoresis and held them in place.
Some bacteria were trapped at low voltage, and others at high voltage. Wang estimated the “capture voltage” for each cell, measured their dimensions, and then used computer simulations to calculate the polarizability of the cells — how easily the cell forms electrical dipoles in response to an external electric field.
From these calculations, Van found that more electrochemically active bacteria have higher polarizability. She observed this correlation in all types of bacteria that the group tested.
“We have the necessary evidence that there is a strong correlation between polarizability and electrochemical activity. In fact, polarizability can be used as an intermediary for the selection of microorganisms with high electrochemical activity. “
The team collaborators are currently using a method to test new bacterial strains that have recently been identified as potential electricity producers.
“If the same trend of correlation applies to these new strains, then this method will be able to get wider use, for example, in the production of clean energy, the creation of biofuels and bioremediation,” added Vaughn.