A field guide to bacterial swarming motility

The mechanisms that allow bacteria to swim through liquid environments are well understood, but much less is known about how bacteria migrate across solid surfaces, a process known as swarming. In this Review, Daniel Kearns describes the requirements and phenotypes associated with swarming motility. How bacteria regulate, assemble and rotate flagella to swim in liquid media is reasonably well understood. Much less is known about how some bacteria use flagella to move over the tops of solid surfaces in a form of movement called swarming. The focus of bacteriology is changing from planktonic to surface environments, and so interest in swarming motility is on the rise. Here, I review the requirements that define swarming motility in diverse bacterial model systems, including an increase in the number of flagella per cell, the secretion of a surfactant to reduce surface tension and allow spreading, and movement in multicellular groups rather than as individuals.