Bacteria are single celled organisms that can do amazing things in multicellular groups, with complex coordinated behaviors emerging from the interaction of genetic networks, chemical environments, and the physics of cell growth. Last year I wrote about the work of Tim Rudge and Fernan Federici and their incredible images of bacterial growth patterns. Their paper, with colleagues from the Haseloff Lab at the University of Cambridge, was recently published in ACS Synthetic Biology, showing how complex fractal patterns in colonies of E. coli emerge simply from the physical interactions of rod shaped cells.
In this experiment, E. coli cells are labelled with two colors of fluorescent protein (they are otherwise genetically identical) and seeded at low density onto a surface. As they grow and divide, the rod shaped cells begin to bump into each other, creating jagged boundaries between the two fluorescent populations. These jagged lines are fractal, self-similar at many scales. Using their CellModeller program, the team found that they could accurately model this fractal behavior by including only physical parameters like viscous drag, cell shape, and growth rate, rather than biological properties like cell-cell communication or chemotaxis. Indeed, when they used E. coli mutants that were spherical instead of rod-shaped, the fractal pattern disappeared.