Bacteria Build “Power Cables” to Make More Energy

Compared to human cells with their multiple mitochondria powerhouses, bacterial cells have a power production problem. Most bacteria generate energy using cellular components that are packed into their membrane.

This causes a tricky problem. The more energy a bacterium has to make, the more room it needs for the proteins, lipids, and other molecules involved in cellular respiration. With a limited membrane surface to work with, simply adding more of these components can become a space problem.

Now, in a new study published in Nature Microbiology, researchers have found that the bacterium Bacillus subtilis can seemingly get around this limit by building microscopic protein-and-lipid tubes that extend its energy-making system beyond the cell membrane.¹

“It’s almost like plugging an extension cable into a single wall socket,” said Ashleigh Kropp, a study coauthor and postdoctoral researcher at the University of Melbourne.

This discovery reveals a novel way for bacteria to increase their respiratory capacity without simply building more membrane. “If it's true, it's like a completely new mechanism,” said Nikhil Malvankar, a molecular biophysicist at Yale University who was not involved in the study.

A Kitchen With Only One Socket

Mitochondria are like spacious kitchens with plenty of counter space and electrical outlets. “More space and more sockets mean mitochondria can ‘cook’ faster and more efficiently than bacteria working with a cramped, single-socket kitchen. So how do bacteria get around this lack of space and sockets? That's the question we set out to answer,” Kropp said.

In a prior study, Kropp and her team discovered that some bacteria use a quinone-transporting protein to help perform cellular respiration.² Quinones normally shuttle electrons within the cell membrane during cellular respiration, and a quinone-transporting protein can help connect this electron-transfer process to the cell's energy-making machinery.

“After discovering the first [quinone-transporting protein], we went looking in bacterial genomes to see if any similar ones exist,” Kropp said.

They came across a similar structure in the soil bacterium, B. subtilis. It was made of two proteins, nicotinamide adenine dinucleotide dehydrogenase (Ndh) and Ndh-coupling protein (Ncp). Four copies of each protein assemble into a complex, with Ncp forming a hollow tube. Multiple complexes can then connect end-to-end, creating a longer filament. This means that the bacterium can extend its energy-making system beyond the cell membrane instead of having to fit all of it into the membrane itself.

According to the researchers, the hollow interior of this filament is particularly important because it creates a hydrophobic pathway that could accommodate quinones. So instead of relying entirely on molecules diffusing through the membrane, the structure could potentially provide a confined route for quinones to move between respiratory components.

“The quinone-transporting filaments are like the multi-board. They plug into the membrane with one access point but can funnel the electricity to multiple appliances, expanding the energy production capacity of the bacteria,” Kropp said.

Surprisingly Short Power Cables

Kropp and her team also wanted to know whether these filaments could actually form inside B. subtilis, rather than appearing only when the team produced the proteins in the laboratory. Using single-particle cryo-electron microscopy, they rapidly froze material taken from B. subtilis cells and examined the individual protein complexes, finding structures that looked like the ones they had produced in the lab. This suggested that the Ndh–Ncp complexes can naturally assemble into quinone-transporting filaments in the bacterium.

However, Malvankar, who studies bacterial structures that transport electrons, wanted to know whether quinones actually move through the hollow center of the structure.

“Computer simulations suggest that quinones could move through the tube, but direct measurements are still needed to show that they do so and at what rate,” he said.

The length of the filaments also surprised Malvankar. He explained that other bacterial filament structures, such as pili and flagella, can be around 10 times longer than the bacteria that produce them. Electron-conducting bacterial nanowires studied previously can also extend far beyond the organisms that produce them.³

In contrast, the filaments in this study are roughly 40 nanometers long, while a typical B. subtilis cell is about 4 micrometers across, making them around 100 times shorter than the bacterium itself.

The enormous size difference makes the proposed function of the filaments intriguing to Malvankar. “Think of a human, right? For example, you and me are one meter long. So, if you have a wire 100 times smaller than your size, that is one centimeter. Can I really use that as a real wire?” he said.

He would like to see these structures working inside cells. Kropp agreed that understanding the filaments in their natural setting is an important next step toward validating their findings.

“All scientific studies have limitations. In this case, what we still would like to do is investigate the quinone-transporting filaments in the actual cells. These experiments are important because seeing processes occurring in cells is the most accurate image we can get of what is truly happening,” Kropp said.

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