Collective Dynamics of Multicellular Magnetotactic Bacteria

Multicellularity has evolved in at least twenty-five lineages. Among the bacteria, only one a group of δ-proteobacteria called ``Multicellular Magnetotactic Bacteria'' (MMB) are obligately multicellular. These bacteria live in aggregates composed of 10--60 individual cells, which quick die if removed from the aggregate. Each cell in the aggregate has about 30 flagella, normal to the outer surface of the aggregate. Although each cell pushes the aggregate in a different direction, the cells exert a total force that is parallel to the aggregate's net magnetic moment, typically to within a few degrees. Amazingly, cells are able to coordinate their motility even as they grow and divide, new magnetic crystals are formed, and the aggregate elongates and divides. Here we examine how physical interactions between cells in a proto-multicellular aggregate allow them to coordinate their growth and motion before the evolution of shared chemical signaling pathways. We first present experimental observations of the motion of a single aggregate and discuss how cells in the aggregate coordinate their motility. We then examine the collective dynamics of a swarm of these multicellular bacteria.