Cracking of a solid-like bacteria ring leads to fingerlike protrusions in Pseudomonas aeruginosa swarms

Many species of bacteria can rapidly colonize a nutrient-rich surface through a collective motility known as swarming. Previous studies model swarming as a fluid-like process facilitated by factors such as biosurfactants and Marangoni flow. We have identified a solid-like ring of bacteria that forms on the outer edge of Pseudomonas aeruginosa colonies. As the swarm radially expands, this ring stretches and breaks apart. This initiates a morphological change in the colony’s growth from a circular mat to fingerlike protrusions. We have developed a minimal model based on cracking mechanics in order to predict the number of cracks formed and how that varies with droplet size, growth rate, time of their occurrence, etc. Our work clearly highlights that solid-like behavior can play a dominant role in certain swarm patterns, which are relevant to biofilm growth and spread of infection.