Abstract

In this talk I will present  the genesis of patterns occurring in bacterial colonies:  in biofilms and  in fluid suspensions. The first case concerns the growth of a simple drop containing bacteria with moderate adhesion to a rigid substrate. The initial circular geometry is lost during the growth expansion, contour undulations and buckling appear, ultimately a rather regular periodic focusing of folds repartition emerges. Predictions of these morphological instabilities, according simple rules,  will be presented as bifurcations of solutions in nonlinear elasticity, characterized by typical driving parameters. The substrate plays a critical role limiting the geometry of the possible modes of instabilities and anisotropic growth, adhesion and toughness compete to eventually give rise to wrinkling, buckling or both.  In the second part, I will present a continuous model for the self-organization of expanding bacterial colonies under chemotaxis, proliferation and eventually active-reaction which either give cohesion or on the contrary dispersion to the colony. Taking into account the diffusion and capture of morphogens complicates the model since it induces a bacterial density gradient coupled to bacterial density fluctuations and dynamics. Nevertheless under some specific conditions, it is possible to investigate the pattern formation as a usual viscous fingering instability. This explains the similarity and differences of patterns according to the physical bacterial suspension properties and explain the factors which favor compactness or branching. 

Joined work with Min Wu