The latest "Nature": Chinese scientists have solved 60 years of problems and unraveled the mystery of the movement of bacteria

▎ WuXi AppTec content team editor

Imagine a scenario where when we swim in a pool with a lot of floating obstacles, the speed is bound to be greatly reduced. 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 swim faster.

As early as 1960, scientists noticed the strangeness of bacteria swimming. At the time, bacteriologist J. Thompson of the University of Manchester G. Shoesmith observed that bacteria swim faster in polymer-mixed fluids than in ordinary fluids. Since then, this anomalous phenomenon has attracted the attention of many scientists.

Early conjectures suggested that this phenomenon was related to morphological changes in the bacterial flagella. The flagella are spiral-like fibers in the tail of the bacteria that are connected to the bacteria through a hook-like body structure, providing a boost to the movement of the bacteria. When these flagella are rotated in the same direction, the bacterial body needs to rotate accordingly to balance the movement of the flagella. However, it is only conjecture that polymers will change the morphology of the flagella, and there has been no evidence to support this view for a long time.

Why do bacteria swim faster in complex fluids, such as the human environment? (Image source: Cheng Xiang Research Group, University of Minnesota)

Since then, scholars have successively proposed other explanations. One hypothesis suggests that this phenomenon is related to the chain structure of the polymer: as the bacteria pass through the network formed by the chain molecules, the turning flagella stretches the polymer molecules, which in turn provides the bacteria with elasticity. Because of this process, the angle between the bacterial body and the overall direction of motion becomes smaller, so it swims faster.

However, a new study published in Science voted against this explanation. The team, led by Professor Cheng Xiang of the University of Minnesota and Researcher Xu Xinliang of the Beijing Center for Computational Sciences, unveiled the secrets of bacteria swimming in complex fluid environments. Not only will the study provide answers to the mystery of bacterial swimming that has lasted for more than half a century, but it will also help scientists develop new therapies for those diseases caused by bacteria and design bacteria-based drug delivery systems.

"Ever since the invention of the microscope in the 17th century, people have been attracted to the way bacteria swim. But until now, our understanding of this process has been largely limited to simple liquids, such as water," said shashank Kamdar, lead author of the paper and a phD student in the Department of Chemical Engineering at the University of Minnesota. ”

In the latest paper, the research team first replaced the polymer in the fluid with solid small particles. The kinetic properties of solid particles and polymers are very different, but the team found that the bacteria swimming in them not only did not slow down, but actually faster. Therefore, what causes the bacteria to accelerate is not the nature of the chain molecules themselves, but the need for solid "obstacles" in the fluid environment.

Under the microscope, the researchers observed how these polymers or particles accelerated the bacteria. When the bacterial body is not in the same direction as the flagella, the bacteria will sway along the trajectory of the spiral, causing a loss of speed. And when nanoscale or micron-scale solids are present—whether polymers or particulate matter—such shaking weakens, and bacteria swim straighter and faster in a straight line.

▲When approaching solid particles, the movement of bacteria accelerates (Image source: Reference[1])

The question that follows, of course, is: Why do bacteria move in a straight line in such a complex fluid environment? Through modelling, the research team proposed that when bacteria pass near polymers or particles, the resulting drag force causes the hook-like body to bend and change the direction of the flagella. At this point, the flagellar is more aligned with the direction of the bacterial body, so as mentioned earlier, the bacteria can swim faster.

The presence of solid particles or polymers makes the trajectory of the bacteria straighter (Image source: Reference[2])

Understanding how bacteria travel through complex, sticky environments, including the human environment, can help scientists design new therapies and even use bacteria as vehicles for drug delivery. Professor Cheng Xiang said that understanding how bacteria swim in complex environments is very important for human health, for example, some bacteria can cause stomach ulcers, and the gastric mucosa is a complex viscous environment. Therefore, studying how bacteria swim in such an environment is crucial to understanding the way disease spreads.

The findings are also expected to inspire research into miniature robots: based on a similar design, by changing the angle between the robot body and the synthetic "flagella", people may be able to manipulate the robot's movements. Of course, compared to these potentially transformative prospects, what we can now be sure of is that these tiny individuals have indeed become more and more courageous in the face of obstacles.

Resources:

[3] New study of how bacteria swim could help prevent the spread of disease and improve medical treatments. Retrieved Mar 30th, 2022 from https://www.eurekalert.org/news-releases/948099

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