Mono-to-multilayer transition in growing bacterial micro-colonies

Mono-to-multilayer transition is ubiquitous in cellular systems, such as the delamination of epithelia, and the formation of fruiting bodies in myxo-bacteria upon starvation. Using experiments, numerical and analytical modelling, we study the mono-to-multilayer transition in growing bacterial micro-colonies. We demonstrate that such a transition is governed by two competing effects: compression from neighbouring cells, which tends to extrude the cell from the mono-layer, and adhesion to the substrate, which tends to maintain the cells in contact with the agarose. In quasi-one-dimensional colonies (i.e. a single row of bacteria prevented from lateral motion), the mono-to-multilayer transition is completely deterministic and can be analytically described by means of a simple bead model. In two-dimensional colonies, on the other hand, the transition is a stochastic process and must be described in probabilistic terms. The dependency of the extrusion probability on various local quantities are obtained from both experiments and computer simulations, with a good agreement between the two methods.