A field guide to bacterial swarming motility

Key Points Swarming motility is operationally defined as multicellular, flagella-mediated surface migration of bacteria. Swarming requires intercellular interactions, surfactant secretion and an increase in flagellar numbers. Swarming motility has often been genetically bred out of laboratory strains and is best observed in natural isolates. In the laboratory, one must take care to standardize swarming conditions. Although the specific conditions that promote swarming are species dependent, swarming generally occurs on nutrient-rich media solidified by agar concentrations of greater than 0.3%. A period of non-motility, or a swarm lag, will manifest when cells are transferred from liquid to a solid medium. The lag is thought to indicate a physiological change in cells to become swarming proficient. Some bacteria become elongated during swarming. It is not clear whether cell elongation is required for or simply co-regulated with swarming in these species. The mechanistic connection between swarming motility and cell elongation is unknown, and many swarming bacteria do not become elongated. Swarming often requires the chemotaxis sensory transduction system for functions that are unrelated to chemotaxis, or directed movement, per se . The mechanism of surface sensing (the bacterial 'sense of touch') is unknown, but swarming motility provides a strong model system for its study. Models have been proposed to explain the bacterial response to surface contact, including sensing resistance to flagellar rotation when impeded by surface contact and sensing perturbations in the Gram-negative outer membrane. The ecology of swarming is unknown, but swarming is often associated with pathogenesis. Swarming bacteria also enjoy enhanced resistance to antibiotics and eukaryotic engulfment as well as gaining enhanced nutrition and a competitive advantage from secreted surfactants. 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 surfac