(5c) Motility-Induced Buckling of Bacterial Monolayers

One key step in the development of many bacterial colonies and biofilms is a transition from a two-dimensional (2D) monolayer into a three-dimensional (3D) structure. In this talk, we explore the motility-induced mechanisms behind the 2D-to-3D transition of dense Pseudomonas aeruginosa colonies. We show that motile bacterial colonies exhibit collective swarming motion to generate in-plane flows, including simple shear flow and squeeze flow. We show that the resulting 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 [1]. The ability of these in-plane flows and the resulting fluid stresses leading to out-of-plane transitions can be understood by analyzing the following simple scenario. Let us consider a simple shear flow, i.e., Couette flow, where the spatial gradient in velocity is described by a strain rate and gives rise to compression and extension along two principal axes. For sufficiently large strain rate, we show that the forces along the compressional axis enable the bacteria to overcome cell-substrate adhesion, causing an out-of-plane deformation of the monolayer leading to “buckling”, and thus nucleating the transition into the third dimension. Combining experimental observations of P. aeruginosa colonies at single-cell resolution, molecular dynamics simulations of active particles, and theories of 2D fluid films, we provide a new motility-induced, rate-dependent buckling mechanism governing their 2D-to-3D transitions.

(See corresponding abstract by Dr. Kranthi Mandadapu, for an associated work on glassy dynamics in bacterial colonies.)

[1] S. C. Takatori, and K. K. Mandadapu, “Motility-induced buckling and glassy dynamics regulate three-dimensional transitions of bacterial monolayers”, arXiv:2003.05618 (2020).