Abstract
Chemotactic bacteria form emergent spatial patterns of variable cell density within cultures that are initially spatially uniform. These patterns are the result of chemical gradients that are created from the directed movement and metabolic activity of billions of cells. A recent study on pattern formation in wild bacterial isolates has revealed unique collective behaviors of the bacteria Enterobacter cloacae. As in other bacterial species, Enterobacter cloacae form macroscopic aggregates. Once formed, these bacterial clusters can migrate several millimeters, sometimes resulting in the merging of two or more clusters. To better understand these phenomena, we examine the formation and dynamics of thousands of bacterial clusters that form within a 22 cm square culture dish filled with soft agar over two days. At the macroscale, the aggregates display spatial order at short length scales, and the migration of cell clusters is superdiffusive, with a merging acceleration that is correlated with aggregate size. At the microscale, aggregates are composed of immotile cells surrounded by low density regions of motile cells. The collective movement of the aggregates is the result of an asymmetric flux of bacteria at the boundary. An agent-based model is developed to examine how these phenomena are the result of both chemotactic movement and a change in motility at high cell density. These results identify and characterize a new mechanism for collective bacterial motility driven by a transient, density-dependent change in motility.
Highlights
In populations of chemotactic bacteria, coupling of the directed movement of individual cells in response to nutrients or chemical stimuli gives rise to spatio-temporal collective phenomena, including swarm bands and aggregates [1,2,3]
We examine the patterns formed by the bacteria Enterobacter cloacae
An unusual behavior of these bacteria is the collective movement of cells after the initial aggregation into a tiny spot
Summary
In populations of chemotactic bacteria, coupling of the directed movement of individual cells in response to nutrients or chemical stimuli gives rise to spatio-temporal collective phenomena, including swarm bands and aggregates [1,2,3]. These macroscopic structures are the result of an emergent pattern of bacterial cell density that forms due to the coordinated movement and metabolic activity of billions of bacterial cells in an initially uniform environment. It is unclear whether the collective properties within these three species encompass the full range of collective behaviors observed in all chemotactic bacteria, or if our understanding of these behaviors extends to other species of chemotactic bacteria
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