  The ability of bacteria to colonize and grow on different surfaces is an
essential process for biofilm development and depends on complex biomechanical
interactions between the biofilm and the underlying substrate. Changes in the
physical properties of the underlying substrate are known to alter biofilm
expansion, but the mechanisms by which biofilms sense and respond to physical
features of their environment are still poorly understood. Here, we report the
use of synthetic polyacrylamide hydrogels with tunable stiffness and
controllable pore size to assess physical effects of the substrate on biofilm
development. Using time lapse microscopy to track the growth of expanding
Serratia marcescens colonies, we find that biofilm colony growth can increase
with increasing substrate stiffness on purely elastic substrates, unlike what
is found on traditional agar substrates. Using traction force microscopy, we
find that biofilms exert transient stresses correlated over length scales much
larger than a single bacterium. Our results are consistent with a model of
biofilm development in which the interplay between osmotic pressure arising
from the biofilm and the poroelastic response of the underlying substrate
controls biofilm growth and morphology.
