Controlling the synchronization and symmetry breaking of coupled bacterial pili on active biofilm carpets

Abstract In the low Reynolds number regime, active biological systems utilize nonreciprocal cyclic activities to achieve motility, as seen in the spinning of bacterial flagella and the beating of cilia. Coupling among these active mechanical components leads to synchronization, and emergence of metachronal waves. Here, we report that biofilms of Pseudomonas nitroreducens form active carpets surfaces textured with various topological defects generating large-scale propagating waves. On these active structures, nonreciprocally coupled extension and retraction activities of bacterial pili drives the collective oscillations. Surprisingly, this collective behavior exhibits broken left-right symmetry. We discover that this symmetry-breaking process is primarily driven by an aging-related frequency gradient across the biofilm. Leveraging these insights, we further demonstrate the ability to control the collective dynamics of these waves, including symmetry breaking, transitions from spiral waves into target and propagating plane waves by manipulating the biofilm properties. Overall, our findings illuminate the fundamental role of nonreciprocally interacting active components in regulating synchronization, collective dynamics, and symmetry-breaking phenomena in biological systems.