Friday, November 16, 2018

Meet the Scientists Using Swarms of "Remote Control" Bacteria to Study Collective Behavior

“It's unbelievable to be able to move a joystick and watch an organism that is 10x smaller than the width of my hair move across a screen,” says Christopher Pierce, a doctoral student at The Ohio State University (OSU) working with Dr. Ratnasingham Sooryakumar.

(Click image to enlarge)
After being exposed to a magnetic field for 3 seconds, certain species of magnetic bacteria start to form intricate patterns.
Image Credit: C. J. Pierce, H. Wijesinghe, E. Mumper, B. H. Lower, S. K. Lower, and R. Sooryakumar
This isn’t a video game—the tiny organism is real and alive, swimming in a pool whose magnified image is projected onto Pierce’s screen. Its name, Magnetotacticum magneticum AMB-1, hints at how Pierce takes control: The little guys are innately magnetic. They thrive in the sediment of shallow water, where oxygen is present but at lower levels than in atmosphere, ingesting iron and processing it into magnetic crystals.

Those crystals act like permanent magnets, orienting the bacteria relative to the Earth’s magnetic field, presumably as a means of navigation. AMB-1 are swimmers, propelled by a long tail-like flagellum on each end of their bodies. When suspended in a fluid, they start to swim in response to the introduction of a magnetic field, and change the direction they're moving when the orientation of the field changes. To take control of their motion, Pierce just takes control of the magnetic field in their environment.

In research recently published in the American Physical Society’s journal Physical Review Letters, Pierce, Sooryakumar, and their colleagues from OSU describe the physical processes by which large numbers of these bacteria self-organize into visually stunning arrangements. The research provides a foundation for exploring how other types of organisms self-organize into complicated structures, like the hypnotic shifting clouds formed by large flocks of birds.

But like a lot of great science, it started partly by accident. While studying an unrelated aspect of AMB-1, some fluid containing the bacteria leaked out of where it was supposed to be. Pierce explains, “I noticed that when I applied a magnetic field that pointed the cells toward [a] surface, they started clumping up.” This was surprising, so after finishing the project he was in the middle of, Pierce and his colleagues decided to take a closer look.

(Click image to enlarge)
This series of images shows AMB-1 before the magnetic field is turned on, after 3 seconds, and again after 50 seconds. You can see show they go from being uniformly distributed to forming networks of filaments and then isolated islands.
Image Credit: C. J. Pierce, H. Wijesinghe, E. Mumper, B. H. Lower, S. K. Lower, and R. Sooryakumar.
The researchers noticed that when there were lots of bacteria in the fluid swimming toward a surface, they self-organized into fascinating patterns—rotating clusters and networks that reminded the team of large-scale structures in the galaxy. To find out why, the team isolated two of the little guys and did some experiments.

The team observed that if you use a magnetic field to make two AMB-1 cells approach each other, they attract and rotate around one another. This behavior, the researchers found, is the result of the flagella's motion. Just as the wake of a boat influences the motion of water in a lake, propelling flagella influence the motion of the fluid in which they are swimming. When two bacteria come together, their interaction is driven by this motion—interestingly, this is similar to behavior observed in algae and another species of non-magnetic bacteria that clusters at surfaces.

When a lot of bacteria swim toward a solid surface, the wake produced by each one attracts nearby bacteria, which in turn attract other nearby bacteria—and so on. The result is a dense, swirling pattern of rapidly growing clusters. The growth doesn’t last forever though; at a certain point, repulsive magnetic forces start to push the clusters apart—remember, the bacteria are innately magnetic. It’s the balance of these two effects, the motion of the fluid and the magnetic forces, that lead to relatively stable patterns, says Pierce. To check that assertion, the team developed a simple mathematical model of this situation, and its predictions matched the experimental results.

Under a magnetic field strength of 100 G, that of a typical iron magnet, the team watched M. magneticum AMB-1 progress from uniform density to filament-like networks and then high-density, relatively stable islands.
Image Credit: Physical Review Letters, DOI:https://doi.org/10.1103/PhysRevLett.121.188001, and C. J. Pierce, et al.

Taking things even further, the team repeated the experiment with different densities of bacteria and different magnetic field strengths. For each iteration, they took pictures of the patterns as they developed and stabilized. Then, using a computer algorithm, the team compared the amount of order in each system, pinpointing when and under what conditions the bacteria started to self-organize.

Here’s why that matters.

Biological systems are complicated. The behavior of living things usually depends on a huge number of variables, which makes it difficult to uncover the fundamental principles behind something like self-organization. “As is often the case in physics, we want to find the simplest system that has all the properties we are interested in understanding as a starting point,” says Pierce. This is that system for the type of self-organization we see in groups of organisms or cells.

“Because we have so much control, we can do experiments that can't be done, say, with E. coli, or other species that form similar clusters,” explains Pierce. “Even something as simple as turning them all on end at the same time, from an initially random starting position, gives huge insight into the physics of how the process happens,” he says.

Team members (left-right) Brian Lower, Christopher Pierce, Hiran Wijesinghe, Eric Mumper, and Ratnasingham Sooryakmar.
Image Credit: Evan Jasper.
Kendra Redmond

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