Bacterial swarming: a re-examination of cell-movement patterns.

Many bacteria simultaneously grow and spread rapidly over a surface that supplies them with nutrient. Called 'swarming', this pattern of movement directs new cells to the edge of the colony. Swarming reduces competition between cells for nutrients, speeding growth. Behind the swarm edge, where the cell density is higher, growth is limited by transport of nutrient from the subsurface to the overlying cells. Despite years of study, the choreography of swarm cell movement, the bacterial equivalent of dancing toward an exit in a very dense crowd of moving bodies, remains a mystery. Swarming can be propelled by rotating flagella, and either by pulling with type IV pili or by pushing with the secretion of slime. By identifying patterns of movement that are common to swarms making use of different engines, a model of swarm choreography can be proposed.