报告摘要：
　　When establishing bio­lms, many bacterial species are able to move across surfaces in multicellular groups, a process known as swarming. Flagellated bacteria swarm within a thin layer of ‑uid. This micro‑uidic environment is critical to the physiology of swarming cells. We aimed to understand the interaction between swarming cells and the ‑uid layer. Using micron-sized bubbles as novel ‑ow tracers, we discovered that the ‑agellar action of cells drives an extensive stream (or river) ‑owing along the colony edge, which may provide an avenue for long-range communication between cells. We also revealed a complex pattern of ‑uid drifts within the colony, and suggested a simple mechanism of swarm spreading.