Bacteria can colonize various surfaces, including tissues and materials, forming surface-attached biofilms. In this study, Débarre, Lecuyer and colleagues show that substrate stiffness has an impact on early microcolony formation. The authors report that in response to varying substrate stiffness, the shape of Pseudomonas aeruginosa microcolonies varies: they formed defined, dense hemispherical colonies on the softest hydrogels, whereas the bacteria were distributed in a thin layer across the surface on stiff hydrogels. Using a mutant that lacks type IV pili (T4P; which is incapable of twitching motility) abolished this effect, and the authors only observed dense hemispherical colonies on all substrates, which suggests that T4P-mediated surface motility has a role in the modulation of microcolony formation. Moreover, they report that the twitching velocity increased with substrate stiffness, and rigidity-dependent modulation of twitching also affected the mixing of different strains, as well as biofilm matrix distribution following surface colonization. The authors propose that the mechanical interaction between the elasticity of the substrate and the T4P results in the different observed phenotypes.