Tumbling and Geometric Effects in Bacterial Transport through Micropillar Arrays 

Geometric confinement and intrinsic motility jointly determine how microorganisms traverse structured environments. We directly compare the transport of wild-type Escherichia coli in micropillar arrays with that of a smooth-swimming mutant lacking the ability to tumble. While the trajectories of the smooth-swimming strain exhibit frequent adherence to pillar circumferences, the run-and-tumble cells tend to repel themselves from the pillar regions, forming distinct trapping zones for both species. By varying the pillar heights and axial confinement, we show that these distinct geometric effects are insensitive to the contributions of axial features to hydrodynamic interactions or cell motility. Our results therefore suggest that a factor more fundamental than tumbling capability demarcates these distinct transport behaviors.