The latest "Nature": Chinese scientists solve a 60-year-old puzzle and uncover the mystery of bacterial movement

▎WuXi AppTec Content Team Editor Imagine this scenario: When we swim in a swimming pool with a lot of floating obstacles, the speed is bound to be greatly reduced
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But for bacteria in the microscopic world, the opposite is true: when there are a lot of solid obstacles in the fluid and the viscosity increases, the bacteria in it may instead swim faster
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As early as 1960, scientists noticed the strangeness of bacterial swimming
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At the time, JG Shoesmith, a bacteriologist at the University of Manchester, observed that bacteria swim faster in fluids mixed with polymers than in normal fluids
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Since then, this anomalous phenomenon has attracted the attention of many scientists
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Early speculation suggested that this phenomenon was related to morphological changes in bacterial flagella
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Flagella are the helical fibers in the bacterial tail that are attached to the bacteria by hook-like structures that provide the driving force for the bacteria to move
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When these flagella turn in the same direction, the bacterial body needs to turn accordingly to balance the movement of the flagella
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However, polymers altering the shape of the flagella are conjectures, and there has long been no evidence to support this idea
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▲ Why do bacteria swim faster in complex fluids (such as the human environment)? (Image source: Cheng Xiang's research group at the University of Minnesota) Since then, scholars have successively proposed other explanations
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One hypothesis suggests that the phenomenon has something to do with the polymer's chain-like structure: As the bacteria traverse the network of chain-like molecules, the rotating flagella stretches the polymer molecules, which in turn provides the bacteria with elasticity
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Because of this process, the angle between the bacterial body and the overall direction of motion becomes smaller, so it swims faster
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However, a new study published in Science voted against this explanation
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A team led by Professor Cheng Xiang from the University of Minnesota and researcher Xu Xinliang from the Beijing Computational Science Research Center has uncovered the secrets of bacteria swimming in complex fluid environments
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The research will not only provide answers to the mystery of bacterial swimming that has persisted for more than half a century, it will also help scientists develop new treatments for those diseases caused by bacteria and design bacteria-based drug delivery systems
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"Ever since the invention of the microscope in the 17th century, people have been fascinated by the way bacteria swim
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But until now, our understanding of this process has been largely limited to simple liquids, such as water," says first author of the paper, University of Minnesota Chemistry "But how bacteria swim in real-life environments is still an open question," says Shashank Kamdar, a doctoral student in the engineering department
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In the latest paper, the research team first replaced the polymers in the fluid with small solid particles
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The kinetic properties of solid particles and polymers are very different, but the team found that instead of slowing down, the bacteria swim faster
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Therefore, it is not the properties of the chain molecules themselves that cause the bacteria to accelerate, but the need for the presence of solid "obstacles" in the fluid environment
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Under the microscope, the researchers observed how these polymers, or particles, accelerated the bacteria
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When the bacteria's body is out of alignment with the flagella, the bacteria wobbles along the spiral's trajectory, causing a loss of speed
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And when nano- or micro-scale solids are present—whether polymers or particulates—the sloshing is lessened, and the bacteria swim straighter and faster in a straight line
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▲When approaching a solid particle, the movement of bacteria is accelerated (Image source: Reference [1]) The next question is naturally: Why do bacteria move in a straight line in such a complex fluid environment? The research team's modeling studies suggest that when bacteria pass near a polymer or particle, the drag force generated causes the hooks to bend and change the direction of the flagella
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At this point, the flagella is more aligned with the bacteria's body, so as mentioned earlier, the bacteria can swim faster
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▲The existence of solid particles or polymers makes the trajectory of bacteria straighter (Image source: Reference [2]) Understanding how bacteria pass through complex viscous environments (including the human environment) can help scientists design new treatments, and even is the use of bacteria as a vehicle for drug delivery
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Professor Cheng Xiang said that understanding how bacteria swim in complex environments is very important for human health.
For example, some bacteria can cause gastric ulcers, and the gastric mucosa is a complex viscous environment
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Therefore, studying how bacteria swim in such environments is critical to understanding how diseases spread
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The discovery is also expected to inspire research on microrobots: Based on a similar design, by changing the angle between the robot's body and the synthetic "flagella," people might be able to manipulate the robot's movements
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Of course, in contrast to these potentially transformative prospects, what we can now be sure of is that these tiny individuals are indeed getting stronger and stronger when faced with obstacles
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Reference: [1] Shashank Kamdar et al, The colloidal nature of complex fluids enhances bacterial motility.
Nature (2022).
https://doi.
org/10.
1038/s41586-022-04509-3[2] Bacteria swim faster when keep them in line.
Retrieved Mar 30th, 2022 from https:// New study of how bacteria swimming could help prevent the spread of disease and improve medical obstacles treatments.
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