Directed bacterial movement that is controlled in this way is known as chemotaxis, and has been observed in individual microbes as well as in colonies that organize into biofilms or other structures. Because M. xanthus uses chemotaxis-like pathways to move over its prey, the researchers call this behavior predataxis. (A video is at nytimes.com/science.)

The swarming changed over time. When a lot of E. coli was available, the bacteria moved back and forth tightly; Dr. Kirby likened it to a vacuum cleaner moving repeatedly over a dirty spot. As the prey was consumed, the pattern lengthened and dissipated. Presumably the bacteria needed to release a lot of digestive enzymes at first, but less and less as time went on, he said.

Further study of this kind of coordinated behavior may help in understanding certain diseases that involve motile bacteria, Dr. Kirby said, and in developing methods to clean up environmental contaminants using microbes.