A Mechanical Logic for Bacterial Navigation

Abstract Bacterial motility in unconfined liquids is well understood, but many species inhabit crowded, three-dimensional environments where physical constraints shape navigation. Here, we show that the aquatic bacterium Caulobacter crescentus employs backward swimming to overcome confinement, using a force-sensitive mechanism to control directional switching. Using colloidal “µ-Traps” to confine single cells within spherical volumes accessible through a narrow aperture, we reveal that cells engage backward motility to enter pores, explore boundaries, and escape confinement. This directional switching is gated by mechanical load: moderate forces on the flagellum promote backward motion, while excessive load suppresses it, preventing futile effort. This tunable, load-sensitive control offers direct evidence for a mechanical logic that governs motility: backward movement is activated when advantageous and disengaged when not. These findings highlight how physical forces influence microbial navigation and adaptation in complex environments. Competing Interest Statement The authors have declared no competing interest.