In studies of the motion of tiny swimming bacteria, scientists at the U.S.
Department of Energy's Argonne National Laboratory found that the microscopic
organisms can stir fluids remarkably quickly and effectively. As a result, the
bacterial flagella could act like tiny motors to mix chemicals in biomedical
kits, among other applications.
This image shows the 3-D concentration distribution of swimming bacteria Bacillus subtilis in thin liquid film obtained by optical coherence tomography.
Igor Aronson, an Argonne materials scientist, and Andrey Sokolov, a former
graduate student from the Illinois Institute of Technology and now postdoctoral
researcher at Princeton University, piled Bacillus subtillis bacteria into thin
films to decode the physics that govern how they move. The bacteria, each equipped
with many tiny flagella, or tails, shot back and forth across the films.
About five microns long, the bacteria typically swim about 20 microns per second.
Packed together, they swim faster—up to 100 microns per second. "Scaled
to human size, it’s like the speed of a train,” Aronson said. “They
are like small torpedoes.”
Two mechanisms govern the movements of bacteria in films. Like humans, B. subtillis
needs oxygen to survive. In a crowded film, the oxygen levels become depleted,
sending bacteria swimming up to the surface for a gulp of oxygen and then back
down again: a phenomenon called bioconvection.
Bacterial motion is also dictated by a principle called hydrodynamic entrainment.
Bacteria whose paths cross tend to physically attract each other, and they pair
up to travel in tandem across the field.
Sokolov and Aronson had earlier developed a technique they called “bacteria
crowd control”, a method of concentrating bacteria that uses a mild electrical
current to force the bacteria closer together. They saw that concentrated bacteria
self-organize into large “schools” that swim in synch—swarms
of bacteria moving together, controlled by the simple laws of physics.
In the current study, the researchers wanted to see whether the vigorous swimming
would mix chemicals evenly throughout the fluid. “They swim so fast that
we found no significant gradients of any chemical except oxygen,” Aronson
said.
And some oxygen mixing did occur: the bacteria improved the diffusion by 100-fold.
A second surprise came when the researchers began to measure the viscosity
of the fluid in which the bacteria swam. Viscosity measures how easily a fluid
flows; for example, honey has a higher viscosity than water.
Adding more matter to a fluid usually makes it thicker or more viscous—for
example, adding dirt to water produces sluggish, viscous mud. So conventional
wisdom would suggest that adding lots of bacteria to the fluid would increase
its viscosity.
“But to our great surprise, the fluid actually became much less viscous—by
a factor of seven,” Aronson said. Thousands of tiny motors powering the
bacteria actually help the fluid to flow.
Aronson thinks the bacteria could be recruited as tiny micro-motors for a number
of future applications. For example, bacterial mixers could combine small amounts
of fluids for tests and samples. The tinier a sample is, the harder it becomes
to mix; attempting to combine small samples is difficult for this reason. For
small, portable on-the-go medical testing kits, which need to mix tiny amounts
of chemicals, a pinch of bacteria could be the magic bullet.
Two papers are referenced in this article. The paper “Enhanced mixing
and spatial instability in concentrated bacterial suspensions” was published
in Physical Review E and is available online. The second paper, “Reduction
of viscosity in suspension of swimming bacteria”, was published in Physical
Review Letters and is available online.
Funding for this research was provided by the Department of Energy’s
Office of Basic Science.
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