Bacterial Motility Patterns Vary Smoothly with Spatial Confinement and Disorder

In unconfined environments, bacterial motility patterns directly reflect the internal states of the cell. Bacteria operating a run-and-tumble behavioral program swim forward when in a "run" state, and they are stalled in place when in a reorienting "tumble" state. However, in natural environments, motility dynamics are a convolution of bacterial behavior and physical constraints. Recent investigations showed that swimming through highly confined porous media exhibit extended periods of "trapping" punctuated by forward "hops," suggesting a potential shift in motility strategy. We introduce a microfluidic device to systematically explore bacterial movement in a range of spatially structured environments, bridging the extremes of unconfined and highly confined conditions. We show that run-and-tumble and hop-and-trap are not distinct locomotive modes, but end points of a continuous spectrum of motility. We present the first unifying framework, "swim-and-stall", to characterize this continuum of observed motility patterns. We demonstrate that a single control program underlies motility across all environments tested—that is, physical structure alone shapes changes in observed output. Our results establish a quantitative link between behavioral rules and environmental context, and show that can navigate dynamic, complex habitats without reprogramming their motility strategy. This robustness may explain the evolutionary persistence of run-and-tumble behavior in a diverse range of peritrichously flagellated bacteria and inform broader models of active transport in structured media.