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.
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.
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I am grateful to R. Belas, H. Berg, E. Déziel, R. Harshey, D. Kysela, L. McCarter, G. O'Toole, P. Rather, R. Rudner, J. Shrout and D. Weibel for thoughtful discussions about swarming motility and critical reading of the manuscript. I also thank P.R., G.O'T. and R.R. for donation of the bacterial strains used in figure 7b–d. Work in my laboratory is supported by the US National Institutes of Health (grant GM093030).
The author declares no competing financial interests.
Of bacteria: growing as dispersed cells in a liquid environment.
A complex molecular machine, assembled from over 40 different proteins, that is the motor for swimming and swarming motility. Rotation of a membrane-anchored basal body rotates a long, extracellular, corkscrew-shaped filament that acts like a propeller to generate force.
- Type IV pilus
A proteinaceous pilus that extends from one pole of the cell, attaches to a surface and retracts, thus acting as the motor for twitching motility. Retraction causes the cell body to move towards the anchor point of the pilus.
- Focal-adhesion complex
A putative cell surface-associated complex that anchors a bacterium to a substrate and might act as a motor for gliding motility. When coupled to an internal motor, the cell body moves relative to the focal-adhesion complex.
A secreted molecule that associates with a surface and acts like a lubricant to reduce surface tension.
Of a bacterium: with an increased number of flagella on the cell surface.
- Quorum sensing
A strategy by which bacteria regulate gene expression in a manner that is dependent on high population density.
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Kearns, D. A field guide to bacterial swarming motility. Nat Rev Microbiol 8, 634–644 (2010). https://doi.org/10.1038/nrmicro2405