Model shows synchronized movement of bacteria result of fluid mechanics not behavior.

VIDEO Click on the image to watch modeled bacteria go into their dance. Credit: Michael Graham, University of Wisconsin-Madison

After long hours peering into a microscope, even the most level-headed researcher might rub her eyes and think she’s seeing strange things. Are those really pink elephants in lab coats hiding in the corner?  Are those bacteria actually ... dancing?

There are many scholarly papers documenting the collective motion of bacteria in suspension. Even more people have seen swarms of bacteria beneath a cover-slip or in a droplet of fluid moving together like nose-plugged synchronized-swimmers creating whirling patterns. But, what choreographs this complex dance?

Michael Graham and colleagues from the University of Wisconsin-Madison reported in the November 11 issue of Physical Review Letters that coordinated swimming by bacteria need not be the result of communication between individuals. Fluid mechanics, rather than behavior, can create such patterns.

In Graham’s model, a particle pushed by a flagellum represents a bacterium. The particle, like the bacterium it emulates, tends to continue moving in a straight line unless it interacts with something in its path.

“We made a very simple mathematical model of a swimming bacterium,” Graham said. “Then put a whole bunch of them into our system and let them go.” 

In Graham’s model system, the fluid is motionless until the model bacteria begin to move. “You can neglect inertia because these things are small and they’re in a viscous fluid,”  Graham said. “So the force that the flagellum pushes backward on the fluid is the same as the force that the body pushes forward on the fluid.”

A bacteria swimming in a droplet of water has a very low Reynolds number —ratio of inertial forces to viscous forces— therefore inertia does not affect its motion through a fluid; the viscosity is the main motive force. Imagine you are swimming in molasses.  What happens when you stop the motion of your arms and legs? You stop, without coasting at all. As physicist E. M. Purcell explained in his seminal speech at Harvard in 1976, “If you are at very low Reynolds number, what you are doing at the moment is entirely determined by the forces that are exerted on you at that moment, and by nothing in the past.”

The bacteria in the model do not communicate with one another. Each just moves in and is moved by the fluid. 

“If you make a suspension of them,” Graham said, “each of them drives a fluid motion that affects the motion of all the other ones.”

The model reproduces coherent large-scale patterns of motion similar to those sometimes observed under the microscope. Graham and his colleagues also found that at low concentrations, the behavior of individual model organisms mimicked that of organisms in nature. For instance, at low concentrations, model bacteria propelled from behind aggregated at the boundaries of the model system, just as spermatazoa (also propelled from behind) move toward solid surfaces.

The whirling pattern produced when the model was input with high concentrations of bacteria suggests that in nature, bacteria are capable of “stirring” the fluid around them, meaning a bacterial community’s coherent movement could facilitate the rapid distribution of food or oxygen through its local environment. In the future, the motion of bacteria might also be used to “stir” fluids in the lab. 

“There is a whole emerging technology of microfluidics,” Graham said, referring to the mixing of liquids in very small devices, on the scale of 10 to 100 microns. “Those are the kind of scales that you might expect bacteria to be able to mix things on.”

Originally published December 21, 2005

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