Propulsion without propellers: How bacteria move on surfaces without flagella

Physical forces play a crucial role in microbial behavior. In free-swimming bacteria, the hydrodynamics of flagellar propulsion drive motility. On surfaces, physical interactions are equally important, though often complex and easily overlooked. We introduce “swashing,” a novel mode of surface motility where bacteria migrate without active propulsion, driven instead by fluid flows generated by metabolic activity. Mutants of Salmonella and Escherichia coli lacking flagella migrate on surface at rates comparable to wild-type strains. Swashing is supported by fermentable sugars and inhibited by surfactants. We propose that fermentation at the colony’s edge creates osmotic gradients, drawing water from the agar and forming a fluid bulge that facilitates bacterial spreading. We conclude that flagellar propulsion is dispensable for surface motility in bacteria; instead, fermentation-driven fluid flows can enable bacterial spreading. Given the significance of surface motility in colonization, biofilm formation, and pathogenicity, this mechanism could have important biological implications.