  Understanding how bacteria move in porous media is critical to applications
in healthcare, agriculture, environmental remediation, and chemical sensing.
Recent work has demonstrated that E. coli, which moves by run-and-tumble
dynamics in a homogeneous medium, exhibits a new form of motility when confined
in a disordered porous medium: hopping-and-trapping motility, in which cells
perform rapid, directed hops punctuated by intervals of slow, undirected
trapping. Here, we use direct visualization to shed light on how these
processes depend on pore-scale confinement and cellular activity. We find that
hopping is determined by pore-scale confinement, and is independent of cellular
activity; by contrast, trapping is determined by the competition between
pore-scale confinement and cellular activity, as predicted by an entropic
trapping model. These results thus help to elucidate the factors that regulate
bacterial motion in porous media, and could help aid the development of new
models of motility in heterogeneous environments.
