Bacteria live complicated lives, in which they must swim through fluids of varying viscosities and interact with surfaces, sediment, and other cells. Accordingly, flagellar systems have evolved adaptive mechanisms to facilitate motility through a wide range of complex environments. At the molecular level, we characterize mechanosensitivity in the rotary nanomachine that propels bacterial swimming, the bacterial flagellar motor (BFM). Using magnetic tweezers to manipulate external torque across the functional range of the motor, we present a model for load-dependent assembly in BFM of E. coli. We also explore how bacterial locomotion at the whole-cell level adapts to structured environments that display varied levels of packing density (i.e., confinement) and packing structure (i.e., disorder).  We use a microfluidic device to systematically tune these environmental parameters and demonstrate that E. coli motility patterns smoothly transition along a continuum controlled by a few behavioral and environmental parameters. We discuss the features of these results that extend to bacterial species beyond E. coli (and those that don't!), underscoring the importance of considering flagellar motility in the context of a bacterium's environment and evolutionary history.