Differential active adhesion and a capillary instability drive global eversion and dissemination of bacterial colonies

Attractive forces between cells determine the shape and sorting behaviour of bacterial colonies. During colony development, chemical gradients form within the colony but it is unclear how they affect cohesion. Here, we discover that such gradients trigger global eversion of proliferating colonies formed by the human pathogen Neisseria gonorrhoeae. Like a jet, the inner core flows towards the periphery where it is partially dispersed and partially spreads around the core of the colony. Living dispersed cells reform colonies leading to rapid dissemination. The eversion depends on local oxygen depletion, which reduces the activity of the molecular motors that govern self-attraction: before the eversion the colony consists of a weakly cohesive spherical core surrounded by a strongly cohesive shell. An idealized computational model reveals that in this configuration a non-linear instability occurs, akin to a capillary instability, when the thickness of the strongly interacting shell falls below a critical value. We show that the probability for eversion strongly depends on the motor-generated attractive force between bacteria. Overall, our results indicate that chemical gradients induce spontaneous symmetry breaking in a mechanically active colony, thereby triggering a non-linear shape instability. This instability generates large-scale cellular fluxes that redistribute bacteria toward environments more favourable for their growth.