A stirring tale of bacteria.

• Position: Organisms

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...