Fracture patterns in Pseudomonas aeruginosa swarms

Bacterial swarming is commonly modeled as a fluid-like process, but experiments on Pseudomonas aeruginosa have revealed the formation of cracks at the edge of the growing colony. Motivated by this observation, we have developed a growing and moving solid model for the bacterial colony. Our theory is formulated within a continuum mechanics framework: it applies standard momentum balance laws and models growth, as is commonly done, by representing the final configuration as a composition of a growth map and an elastic deformation map. Under certain simplifications—reasonable in the context of our experiments and supported by full three-dimensional finite element simulations—we obtained closed-form solutions for the deformation, stresses, and energies in our solid model. Using variational fracture mechanics principles, we identified the physical mechanisms governing the onset of fracture in the colony. Our results reveal how solid-like behavior can dominate at the colony edge and manifest through the formation of wrinkles and cracks. We further show that these solid-mechanical features act as precursors to the morphological transition from circular fronts to finger-like protrusions at the swarm edge.