A stirring tale of bacteria

Poetry in motion are not words usually applied to bacteria. Yet, when researchers at the University of Arizona, Tucson, looked into a petri dish, that is what they saw. Groups of bacteria streamed through the fluid, creating an ever-changing pattern of swirls and blips visible to the naked eye. In a bacterial ballet, the tiny organisms seemed to be moving through the fluid of the dish in coordinated fashion, almost like flocking in birds or schooling in fish.

"We all looked at this and said, 'Oh my goodness, why is this happening?' We were all surprised. We are still surprised," marvels Raymond Goldstein, professor of physics and applied mathematics. Although there had been theoretical suggestions that such "flocking" behavior might not be limited to birds or fish, this could be the first time it has been observed in bacteria.

Bacillus subtilis swim by rotating a series of corkscrew-like appendages, called flagella, that are about five times the body length of one of the rod-shaped bacteria. In a culture, when a bacterium uses up the dissolved oxygen nearby, it swims toward the oxygen-rich surface. So do all its "companions." At the same time, though, gravity acts to pull the bacteria back down. The swimming-up and sinking-down sets up a convective current, much as does cold air sinking toward the floor of a room.

The currents created by one swimming bacterium affect the others. Once a critical concentration of bacteria is reached, the motions of thousands of flagella set up additional large currents in the fluid, creating the organized jets and vortices. "This is like a large group of people who, by the very act of swimming in a pool, get carried around by the mutually reinforcing turbulence their swimming created," explains Goldstein.

Bacteria sometimes act in concert once a certain number of them are massed in one place. They may detect each other's presence by chemicals each secretes into the medium, a phenomenon called "quorum sensing." Bioluminescent bacteria, for example, use quorum sensing to know when to turn on their lights. Quorum sensing also is used by the bacteria that cause gum disease.

Goldstein says the swirls and jets the researchers observed effectively stir the fluid, making it well-mixed. That could help the bacteria detect one another.

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