  Many mature bacterial colonies and biofilms are complex three-dimensional
(3D) structures. One 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 remain largely unknown. In this work, we explore
the mechanisms behind the 2D-to-3D transition of motile Pseudomonas aeruginosa
colonies; we provide a new motility-induced, rate-dependent buckling mechanism
for their out-of-plane growth. We find that swarming of motile bacterial
colonies generate sustained in-plane flows. We show that 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, we
identify universal relationships that elucidate the competition between
in-plane viscous stresses, pressure and cell-substrate adhesion. Furthermore,
we show that bacterial monolayers 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, we develop a dynamical state diagram that predicts the state of the
colony, and the mechanisms governing their 2D-to-3D transitions.
