From Polar Clusters to Active Nematics: Experimental Signatures of Swarming Dynamics in Bacterial Monolayers

Bacterial swarms provide a tractable natural model of active matter, where their dynamics illuminate the principles of collective behavior and self-organization phenomena. In particular, the mechanistic and dynamical features of monolayer swarming are critical in driving the transition to multilayer structures at the onset of biofilm formation. Here, we investigate monolayer swarms of Serratia marcescens across varying cell body aspect ratios and area fractions. The results show that at intermediate-to-low densities, bacteria form local dynamic clusters, with the distribution of cluster sizes determined by aspect ratio and area fraction. At higher densities, elongated bacteria align into active nematic states with half-integer topological defects, which point to a potential nucleation mechanism for multilayer formation. These findings provide new physical insights into how cellular morphology and density govern bacterial swarming dynamics and drive the early transition from monolayer swarming to multilayered biofilm development.