Nature solves more than 60 years of problems!

The natural habitats of microbes in iNature human microbiome, marine and soil ecosystems are full of colloids and macromolecules
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This environment exhibits non-Newtonian flow properties that greatly affect the movement of microorganisms
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Although the low Reynolds number hydrodynamics of swimming flagellate bacteria in simple Newtonian fluids have been well developed, the understanding of bacterial motility in complex non-Newtonian fluids is still unclear
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Even after more than 60 years of research, fundamental questions about the nature and origin of enhanced bacterial motility in polymer solutions are still debated
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On March 30, 2022, Cheng Xiang of the University of Minnesota and Xu Xinliang of Beijing Normal University/Beijing Computational Science Research Center jointly published a research paper titled "The colloidal nature of complex fluids enhances bacterial motility" in Nature online, which showed that diluted colloidal suspension Flagellated bacteria in the solution exhibited quantitatively similar motility behaviors to those in dilute polymer solutions, in particular a general particle size-dependent motility enhancement of up to 80%, along with a strong inhibition of bacterial swing
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Due to the hard spherical nature of colloids, which vary in size and volume fraction experimentally, the findings shed light on a long-standing debate about enhanced bacterial motility in complex fluids and suggest that polymer dynamics may not be necessary to capture this phenomenon less
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A physical model incorporating the colloidal properties of complex fluids quantitatively explains bacterial oscillation dynamics and mobility enhancement in colloidal and polymeric fluids
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Taken together, the findings contribute to understanding the motility behavior of bacteria in complex fluids, which is relevant to a wide range of microbial processes and engineered bacterial swimming in complex environments
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Finally, Raphaël Jeanneret of the University of Paris, France, published a review article entitled "Bacteria swim faster when keep obstacles in line" in Nature, which systematically summarized the research progress
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Amazingly, bacteria can swim dozens of times their body length in a second
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This is equivalent to a person swimming 100 meters in less than 5 seconds
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More surprising, however, was that bacteria sometimes swam faster -- not slower -- when the fluid around them was filled with myriad obstacles that increased their viscosity
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Such fluids are called complex fluids, and they are found, for example, in the lining of our lungs and stomach
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The study found that bacteria's mysterious ability to swim faster in complex liquids is actually the result of a very simple effect: They swim straighter
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Complex fluids contain polymeric or colloidal particles that impart specific mechanical properties to the liquid between solid and simple liquids
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Although this liquid looks incredible, they are actually quite common
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It was first observed 62 years ago that mixing polymers into liquids could increase the swimming speed of bacteria
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This effect was originally attributed to changes in the shape of bacterial flagella
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Although polymers in solution can affect the swimming speed of bacteria by changing the shape of the flagella, there is no evidence to support this hypothesis
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After decades of research, two potential alternative mechanisms have emerged
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The first involves the fact that flagella can rotate hundreds of times per second, which means that their viscosity in complex fluids should be lower than that felt by the cell body
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The second mechanism is based on the fact that flagellar rotation should stretch the suspended polymer, creating an elastic recoil
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For bacteria, these extra forces should reduce the natural misalignment between the cell body and the overall direction of motion, resulting in faster swimming
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It turns out that the crux of the problem is this misalignment
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Imprecise alignment between flagellar bundles and cell bodies causes bacteria to move along helical rather than straight trajectories
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When viewed under a microscope, this 3D motion looks like a "wobble" of the cell body around a linear trajectory
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Kamdar et al.
showed that this wobble is greatly reduced when nano- or micron-sized objects are suspended in a liquid, whether they are polymers or solid particles
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The reduced wobble allows cells to move along a straighter trajectory, resulting in higher velocities along the helical axis
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Bacteria swim faster in particle-laden liquids (Image courtesy of Nature) But why do these cells swim along straighter trajectories when they are embedded in complex fluids? Kamdar and colleagues' experiments ruled out elastic stress induced by polymers previously proposed, because the team also observed the phenomenon when the fluid contained colloids rather than polymers
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Instead, the authors show that the trajectory is straighter due to the hydrodynamic phenomenon of boundary-induced torque
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With bacteria moving in complex fluids, each particle in the suspension—whether a polymer or a colloid—acts like a solid surface, creating a torque on the moving bacteria
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This torque bends the flagellar hook, reducing misalignment between the flagellar bundle and the cell body
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The result is a straighter, faster swim
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Together with their experiments, Kamdar and colleagues' mathematical models have improved our understanding of bacterial wobble
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Taken together, the findings contribute to understanding the motility behavior of bacteria in complex fluids, which is relevant to a wide range of microbial processes and engineered bacterial swimming in complex environments
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