June 9, 2013 | 2
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.
It’s fascinating to see how such complex biological patterns can emerge from very simple physical interactions. There is a huge diversity of microbial patterns and multicellular behaviors that arise from differently shaped cells interacting and communicating in different environments with different cell logics. One interesting example cited in the paper is a study of the biophysics of wrinkly biofilms.
Like the fractal E. coli, the Bacillus subtilis cells in the biofilm are subject to physical forces that create patterns as the cells divide and the biofilm expands. The researchers found that the where there were regions of dead cells, those lateral forces would cause the biofilm to buckle vertically, creating 3D wrinkles. They could generate “synthetic wrinkle patterns” by painting denser regions of cells that would be more likely to experience cell death as the biofilm grew.
In synthetic biology, bacterial cell-cell communication has been used to genetically encode simple pattern formation and to synchronize oscillations, among many others. These papers show that even in the simplest multicellular systems, the interplay of biological, chemical, and mechanical forces can create beautiful, complex patterns.
EDIT: Check out Lab Rat’s great post about this paper too!