  In their search for metabolic resources microbes swim through viscous
environments that present physical anisotropies, including steric obstacles
across a wide range of sizes. Hydrodynamic forces are known to significantly
alter swimmer trajectories near flat and low-curvature surfaces. In this work,
we imaged hundreds-of-thousands of high-curvature scattering interactions
between swimming bacteria and micro-fabricated pillars with radii from ~1 to
~10 cell lengths. As a function of impact parameter, cell-pillar interactions
produced distinct chiral distributions for scattering angle -- including
unexpected 'counter-rotator' trajectories -- well-described by a sterics-only
model. Our data and model suggest that alteration of swimmer trajectories is
subject to distinct mechanisms when interacting with objects of different size;
primarily steric for objects below ~10 cell lengths and requiring incorporation
of hydrodynamics at larger scales. These alterations in trajectory impact swim
dynamics and may affect microbial populations in ways that depend on the shape
and placement of obstacles within an environment.
