The collective motion and merging of macroscopic bacterial aggregates

Many chemotactic bacteria generate spatial patterns of cell density. These emergent patterns are the result of the directed movement of cells following chemical gradients self-generated by bacterial metabolic activity. Here we report collective behaviors exhibited by the bacteria Enterobacter cloacae. Enterobacter cloacae spread uniformly within soft agar will eventually form millimeter-sized aggregates of cells. Surprisingly, these aggregates migrate up to several millimeters after formation, sometimes resulting in the merging of two or more clusters of cells. We examined the formation and dynamics of thousands of bacterial clusters that form within large culture dishes, analyzing both the spatial order of the aggregate pattern and the dynamics of aggregate migration. At the microscale, aggregates are composed of immotile cells surrounded by low density regions of motile cells. We understand the collective motion of these aggregates as the result of an asymmetric flux of bacteria at the aggregate boundary. These findings demonstrate how a transient, density-dependent change in motility can lead to the formation of large-scale and dynamics patterns of cell bacteria.