Flagellar Geometry and Its Effects on Run and Tumble - A Numerical and Experimental Study

Flagellated bacteria navigate the environment using the method "run-and-tumble". Specifically, the helical flagella bundle together when spinning counterclockwise and drive the bacteria to move in a near-straight line (run); the flagella unbundle when spinning clockwise and cause the bacteria to change their direction (tumble) randomly. In this work, we study how the different helical geometries of the flagella affect the bundle/unbundle rate via their attractive/repulsive velocities from each other. We explored a wide range of helical geometries pertinent to bacterial locomotion numerically using the method of regularized stokeslet and experimentally with a benchtop setup motivated by Zang et al., 2025. Our results show that flagellar geometry has significant effects on bacterial run-and-tumble.