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Synchronized swimming: Collections of microorganisms make their own waves

Some microorganisms prefer the breaststroke while swimming. Others move along by essentially twisting their tail.

How populations of bacteria and other microorganisms swim is more than just a matter of style, according to Mike Graham, University of Wisconsin-Madison Harvey D. Spangler Professor of Chemical and Biological Engineering.

Studies of how microorganisms move through and diffuse liquid could enable scientists to develop artificial swimmers, enhance fluid flow in microfluidic devices, or better understand how microorganisms sample their environment.

Graham and fellow researchers Patrick Underhill and Juan Hernandez-Ortiz created a computer model that analyzes how populations of up to 100,000 model bacteria swim. Their results, published in the June 20 issue of Physical Review Letters, indicate that the particular style of swimming leads to different large-scale fluid motions and mixing.

In nature, there is great diversity in modes of propulsion—some swimming cells have one or a few large flagella, some have dozens. Others are covered with short hairs called cilia that beat back and forth to propel the cell.

Michael D. Graham

Michael D. Graham (large image)

“How did all this diversity evolve? Did the population dynamics of swimming have anything to do with it?” Graham asks.

The team’s research builds on the observation that microorganisms swim in one of two basic ways, either as “pullers” or “pushers.”

Puller microorganisms, such as algae, perform a breaststroke-like motion to “pull” liquid from the front and force it to the sides of the microorganism.

A pusher microorganism, such as E. coli, propels itself by forcing liquid backward from a tail-like flagellum or collection of flagella.

The distinction is important because pushers and pullers interact differently with fluctuations in the liquid. Pushers magnify fluctuations, while pullers dampen them. Accordingly, as the team made the simulation container for the pusher bacteria bigger, the diffusion caused by the bacteria became more pronounced.

“There is faster and faster transport the bigger your ‘box’ is,” says Graham.

This occurs despite the fact that each model bacterium is only aware of the movement of fluid around it—it has no idea that another bacterium is in the center of the whirl of fluid next to it or that the neighboring whirl of fluid has altered its own direction. Real bacteria can sense each other via chemical signals, but the researchers have not yet given their model bacteria this ability.

From here, the team plans to introduce nutrients to the simulation to determine how the movements of bacterial populations change in response to food. Additionally, the team will be comparing the model to live experiments in the laboratory of UW-Madison Biochemistry Assistant Professor Doug Weibel.

Studies of the collective dynamics of small organisms have large implications. For example, studies of krill show that these tiny animals affect ocean mixing and turbulence. Similarly, swimming microorganism populations can affect how nutrients or toxins are dispersed in water.

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6/30/2008