Swarming motility is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces,^[1] and is an example of bacterial multicellularity and swarm behaviour. Swarming motility was first reported by Jorgen Henrichsen^[2] and has been mostly studied in genus Serratia,^[3][4] Salmonella,^[5] Aeromonas,^[6] Bacillus,^[7] Yersinia,^[8] Pseudomonas,^{[9][10][11][12][13]} Proteus,^[14] Vibrio^[15][16] and Escherichia.^[17][18]

This multicellular behavior has been mostly observed in controlled laboratory conditions and relies on two critical elements: 1) the nutrient composition and 2) viscosity of culture medium (i.e. % agar).^[5] One particular feature of this type of motility is the formation of dendritic fractal-like patterns formed by migrating swarms moving away from an initial location. Although the majority of species can produce tendrils when swarming, some species like Proteus mirabilis do form concentric circles motif instead of dendritic patterns.^[19]

In some species, swarming motility requires the self-production of biosurfactant to occur.^[5][20] Biosurfactant synthesis is usually under the control of an intercellular communication system called quorum sensing. Biosurfactant molecules are thought to act by lowering surface tension, thus permitting bacteria to move across a surface.

Swarming bacteria undergo morphological differentiation that distinguish them from their planktonic state. Cells localized at migration front are typically hyperelongated, hyperflagellated and grouped in multicellular raft structures.^{[11][12][21][22]}

The fundamental role of swarming motility remains unknown. However, it has been observed that active swarming bacteria of Salmonella typhimurium shows an elevated resistance to certain antibiotics compared to undifferentiated cells.^[23]

• Bacterial motility