Cellular scale search and funneling drive bacterial penetration of extreme confinements.

Bacterial transport into micron-scale rigid pores plays an important role in microbial competition and community assembly. As the spatial dimensions of the pore approach the width of a bacterium, steric and hydrodynamic constraints sharply limit passive entry. Here, we demonstrate that motile Escherichia coli overcome these barriers via a two-step facilitated entry mechanism: cells scan flat surfaces for target openings, then pivot into alignment with the aid of flagellar thrust to penetrate them. This process effectively transforms the openings of narrow microfluidic channels into wide, dynamic funnels, dramatically expanding the range of permissible cell orientations and lateral offsets for successful entry. The mechanism increases motile cell influx into extreme confinement by ~30-50 times relative to co-axial entry. The accompanying dimensionality reduction in target search also confers a decisive transport advantage on motile bacteria over non-motile ones. These findings establish reduced-dimensionality target search at the cellular scale, a phenomenon previously thought to be exclusive to molecular enzymes undergoing facilitated diffusion.