Abstract

Bacteria migrate on surfaces using diverse mechanisms. Many flagellated species move on agar using a form of collective motility termed swarming. Swarming has been thought to require flagellar propulsion. Here, we report the rapid migration of Escherichia coli and Salmonella enterica on agar surfaces in the absence of active propulsion by flagella. While immotile in liquid, filament-less and rotation-defective mutants of Salmonella LT2 and E. coli migrate on surfaces at rates comparable to wild-type. In contrast to the “sliding” motility reported in many species, the surface migration here is not primarily growth-driven and is inhibited by surfactant. It is thus a novel form of surface movement that we call “surfing.” Surfing cells acidify the plates, and surfing is found to depend on the presence of fermentable sugar. We propose a model in which fermentation at the colony front produces osmolytes that draw water from the agar to generate a moving bulge at the margin of the expanding colony.