Swashing motility: A novel propulsion-independent mechanism for surface migration in and .

Bacterial motility over surfaces is crucial for colonization, biofilm formation, and pathogenicity. Surface motility in and is traditionally believed to rely on flagellar propulsion. Here, we report a novel mode of motility, termed "swashing," where these bacteria migrate on agar surfaces without functional flagella. Mutants lacking flagellar filaments and motility proteins exhibit rapid surface migration comparable to wild-type strains. Unlike previously described sliding motility, swashing is inhibited by surfactants and requires fermentable sugars. We propose that the fermentation of sugars at the colony edge produces osmolytes, creating local osmotic gradients that draw water from the agar, forming a fluid bulge that propels the colony forward. Our findings challenge the established view that flagellar propulsion is required for surface motility in and , and highlight the role of a fermentation in facilitating bacterial spreading. This discovery expands our understanding of bacterial motility, offering new insights into bacterial adaptive strategies in diverse environments.