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Turning failure into function: Marine bacteria reorient via a flagellar buckling instability

Kwangmin Son (MIT), Jeffrey Guasto (MIT), Roman Stocker (MIT)

Hydrodynamics of Swimming Microorganisms

Tue 9:00 - 10:30

Barus-Holley 191

Many marine bacteria swim using a single helical flagellum connected to a rotary motor via a 100 nm long structure called ‘hook’. While these bacteria apparently have just one degree of freedom, allowing them to swim only back and forth, they in fact exhibit large angular reorientations mediated by an off-axis motion or ‘flick’ of their flagellum, recently discovered by X.L. Wu’s group. Yet, the mechanism underlying this seemingly impossible behavior has remained elusive. Using high-speed video microscopy, we discovered that cells swim forward for ten milliseconds immediately prior to the flick, indicating that the flick occurs when the hook is under compression. Direct measurements of the hook’s mechanical properties, combined with mechanical stability theory, revealed that the mechanism underpinning this unique turning behavior is the buckling of the hook. We verified this hypothesis by reducing the load on the hook through modulating cell swimming speed: the frequency of flicks diminishes sharply when the load on the hook falls below a threshold, consistent with the critical nature of buckling. This elegant, under-actuated turning mechanism is widespread among marine bacteria and may provide a novel design concept in micro-robotics.