Many mature bacterial colonies and biofilms are complex three-dimensional (3D) structures. A key step in their developmental program is a transition from a two-dimensional (2D) monolayer into a 3D architecture. Despite the importance of controlling the growth of microbial colonies and biofilms in a variety of medical and industrial settings, the underlying physical mechanisms behind single-cell dynamics, collective behaviors of densely-packed cells, and 3D complex colony expansion still remain largely unknown. In this talk, I will explore the mechanisms behind the 2D-to-3D transition of motile Pseudomonas aeruginosa colonies.
I will discuss a new motility-induced, rate-dependent buckling mechanism for their out-of-plane growth. In particular, we find that swarming of motile bacterial colonies generate sustained in-plane flows. The viscous shear stresses and dynamic pressures arising from these flows allow cells to overcome cell-substrate adhesion, leading to buckling of bacterial monolayers and growth into the third dimension. Modeling bacterial monolayers as 2D fluid films, I will discuss relationships that elucidate the competition between in-plane viscous stresses, pressure and cell-substrate adhesion. We will also see that bacterial mono-layers can exhibit crossover from swarming to kinetically-arrested, glassy-like states above an onset density, resulting in distinct 2D-to-3D transition mechanisms. Combining experimental observations of P. aeruginosa colonies at single-cell resolution, molecular dynamics simulations of active systems, and theories of glassy dynamics and 2D fluid films, one can develop a dynamical state diagram that predicts the state of the colony, and the mechanisms governing their 2D-to-3D transitions.
1. S. C. Takatori, and K. K. Mandadapu, "Motility-induced buckling and glassy dynamics regulate three-dimensional transitions of bacterial monolayers", (2020) arXiv:2003.05618.