Hydrodynamic coupling drives bacterial flagellar synchrony

Synchronization is a fundamental phenomenon observed across a wide range of physical, chemical, and biological systems. Even under the low-Reynolds-number conditions that govern microbial motility, synchronization of rhythmic processes is considered essential for diverse activities ranging from single cells to populations, yet experimental evidence remains scarce. Here, we measured bacterial flagellar dynamics at the level of single motor units, revealing a synchronization phenomenon among nanoscale molecular machines. The observed intermittent in-phase synchronization was analyzed using a novel hydrodynamic model incorporating elastic deformation of the flagella, showing that stronger hydrodynamic coupling promotes more stable phase-locking. These findings suggest that synchronized rotation induced by weak and intermittently repeated fluid-mediated interactions could facilitate flagellar bundle formation and enhance swimming efficiency. This study bridges functional molecular nanomachines in living cells and nonlinear dynamics, providing a new physical perspective on synchronization in active biological systems.