Swashing: a propulsion-independent form of bacterial surface migration

Physical forces play a crucial role in microbial behavior. In free-swimming bacteria, the hydrodynamics of flagellar propulsion govern motility. On surfaces, physical interactions are equally important though often complex and easily overlooked. Surface migration occurs in many flagellated bacteria and has been linked to diverse mechanisms, such as flagellar propulsion, surface tension, osmolyte production, or combinations thereof. However, the relative importance of these physical effects remains unclear. Here, we describe "swashing," a form of surface movement in which bacteria migrate without active propulsion. Mutants of Salmonella enterica and Escherichia coli lacking flagellar filaments and motility proteins exhibit surface migration comparable to wild-type strains. Swashing is supported by fermentable sugars and inhibited by surfactants. We propose a model in which fermentation at the colony edge creates an osmotic gradient that draws water from the agar, generating fluid flows that propel the colony outward. Overall, our findings show that flagellar propulsion is dispensable for surface migration; instead, fermentation-driven fluid flows may be sufficient to enable bacterial spreading.IMPORTANCEBacteria move on surfaces using a variety of mechanisms, with important implications for their growth and survival in both the clinical setting (such as on the surface of medical devices) and the wild. Surface motility in Gram negative model species S. enterica and E. coli has been extensively studied and is believed to depend on flagellar propulsion. Here, we show surface expansion in these species even in the absence of propulsion by the flagella. We suggest that this flagella-independent movement is tied to fermentation and resulting osmotic flow: As cells ferment sugars, they create local osmolarity gradients, which generate a wave of fluid that drives expansion.