Collective Motion and Nonequilibrium Cluster Formation in Colonies of Gliding Bacteria

TLDR

The study characterizes cell motion in gliding bacterial colonies and shows that collective motion emerges via the organization of cells into larger moving clusters. Collective motion arises at a critical packing fraction of about 17%, exhibiting a scale‑free cluster‑size distribution (exponent 0.88 ± 0.07) and giant number fluctuations, matching simulations of self‑propelled rods and indicating that self‑propulsion and rod shape alone can drive the transition.

Abstract

We characterize cell motion in experiments and show that the transition to collective motion in colonies of gliding bacterial cells confined to a monolayer appears through the organization of cells into larger moving clusters. Collective motion by nonequilibrium cluster formation is detected for a critical cell packing fraction around 17%. This transition is characterized by a scale-free power-law cluster-size distribution, with an exponent $0.88\ifmmode\pm\else\textpm\fi{}0.07$, and the appearance of giant number fluctuations. Our findings are in quantitative agreement with simulations of self-propelled rods. This suggests that the interplay of self-propulsion and the rod shape of bacteria is sufficient to induce collective motion.