Collective Vortex Dynamics in a Monolayer Swarm of Enterobacter sp. SM3

Swarming bacteria migrating across moist agar surfaces move collectively and often form dynamic vortex patterns. Using a controlled dilution protocol, we established an Enterobacter sp. SM3 monolayer amenable to reliable flow mapping by particle image velocimetry (PIV). The resulting PIV analysis shows that dense SM3 swarms display stable, persistent vortices, whereas more dilute monolayers exhibit only transient short-range flows. Analysis of the velocity field reveals systematic differences in mean speed and spatiotemporal velocity correlations. Both the velocity and vorticity fields show enhanced coherence and increasingly fluid-like dynamics at higher densities. These results demonstrate how local cell density governs the organization of collective motion in bacterial swarms.