Flagellar coordination in a swimming multicellular bacterium

Multicellularity has independently evolved at least 25 times in Earth’s history, primarily among the eukaryotes. In each of these cases, the constituent cells must develop robust mechanisms to coordinate their collective behaviors, for example growth and locomotion. To expand our understanding of the strategies by which natural selection has solved this generic control problem, we study the motility of the only known obligatory multicellular bacterium. Cells of the genus Magnetoglobus live exclusively in spherical communities called consortia, which are composed of a monolayer of tens of cells. Hundreds of flagella project outwards from the surface of each consortium, enabling it to move at speeds up to 190 µm/s. We use micro particle image velocimetry to measure the average flow field around swimming consortia. Inverting this flow field for the spatial distribution of flagellar forces reveals striking coordination: flagella on the forward-facing hemisphere exert pulling forces that are precisely balanced by pushing forces from the rear. A comparison of the flow fields produced by consortia with different swimming speeds gives insight into the mechanisms by which the cells coordinate their flagella.