Figure 1. Observation of the collective dynamics of WT S. marcescens 274 bacteria, swirling in a drop.

(A) A snapshot of an overnight (18 h) culture-drop placed on a glass slide; image taken by an upright phase contrast microscopy. All cells are spherical and are located on the upper surface of the drop. Bacterial density (fraction of area occupied by bacteria) ρ = 0.67. Scale bar equals 10 µm. (B) A schematic side view, illustrating how cells swim from a homogeneously dense volume to the upper surface of the drop, forming a dense monolayer and an extremely sparse bulk.