Bacterial motility patterns adapt smoothly in response to spatial confinement and disorder

Recent studies have shown that Escherichia coli in highly confined porous media exhibit extended periods of ‘trapping’ punctuated by forward ‘hops’, a significant restructuring of the classical run- and-tumble model of motility. However, bacterial species must navigate a diverse range of complex habitats, such as biological tissues, soil, and sediments. These natural environments display varying levels of both (1) packing density (i.e., confinement) and (2) packing structure (i.e., disorder). Here, we introduce a microfluidic device that enables precise tuning of these environmental parameters, allowing for a more systematic exploration of bacterial motility bridging the extremes of unconfined and highly confined conditions. We observe that motility patterns characteristic of both hop-and-trap and run-and-tumble models coexist in nearly all environments tested, with ensemble dynamics transitioning between these behaviors as both confinement and disorder increase. We demonstrate that dynamics expected from the hop-and-trap model emerge naturally from a modified run-and- tumble model under specific environmental constraints. Our results suggest that bacterial motility patterns lie along a continuum, rather than being confined to a small set of discrete locomotive modes.