Spreading rates of bacteria colonies depend on substrate stiffness and permeability

Many bacterial species develop surface-dwelling multi-cellular colonies known as biofilms. Biofilm growth is widely regarded to depend on physical properties of the underlying substrate, such as substrate stiffness and porosity. Biofilm studies are however largely restricted to agar substrates, which have complex mechanical properties and in which stiffness and porosity cannot be independently tuned. Here, we report the use of synthetic polyacrylamide hydrogels with tunable stiffness and controllable pore size to assess the effects of substrate mechanics on biofilm development. We use time lapse microscopy to track the growth and form of expanding Serratia marcescens colonies and traction force microscopy to measure forces the bacteria exert on the surface. We find that biofilm colony growth increases on purely elastic substrates with increasing substrate stiffness, unlike what is found on traditional agar substrates. We also find that bacteria-generated traction forces increase with greater substrate stiffness. Our results suggest that the transport and spread of bacteria can be independently modified and controlled by substrate stiffness and new models of biofilm growth based on the contribution of substrate mechanics are needed.