Single Nanoparticle Tracking Reveals Efficient Long-Distance Undercurrent Transport in Upper Fluid of Bacterial Swarms

Flagellated bacteria move collectively in a swirling pattern on agar surfaces immersed in a thin layer of viscous “swarm fluid,” but the role of this fluid in mediating the cooperation of the bacterial population is not well understood. Herein, we use gold nanorods (AuNRs) as single particle tracers to explore the spatiotemporal structure of the swarm fluid. Individual AuNRs are moving in a plane of ∼2 μm above swarms, traveling for long distances in high speed without interferences from bacterial movements. The particles are lifted and transported by collective mixing of small vortices around bacteria during localized clustering and de-clustering of motile cells. Their motions fit the Lévy walk model, revealing efficient fluidic flows above the swarms. These flows provide obstacle-free highways for long-range material transportations, allow swarming bacteria to perform population-level communications, and imply the essential role of the fluid phase on the emergence of large-scale synergy.