The colonisation of a soft passive material by motile cells such as bacteria is common in biology. The resulting colonies of the invading cells are often observed to exhibit intricate patterns whose morphology and dynamics can depend on a number of factors, particularly the mechanical properties of the substrate and the motility of the individual cells. We use simulations of a minimal 2D model of self-propelled rods moving through a passive compliant medium consisting of particles that offer elastic resistance before being plastically displaced from their equilibrium positions. It is observed that the clustering of active (self-propelled) particles is crucial for understanding the morphodynamics of colonisation. Clustering enables motile colonies to spread faster than they would have as isolated particles. The colonisation rate depends non-monotonically on substrate stiffness with a distinct maximum at a non-zero value of substrate stiffness. This is observed to be due to a change in the morphology of clusters. Furrow networks created by the active particles have a fractal-like structure whose dimension varies systematically with substrate stiffness but is less sensitive to particle activity. The power-law growth exponent of the furrowed area is smaller than unity, suggesting that, to sustain such extensive furrow networks, colonies must regulate their overall growth rate.

“Active fluids”, such as suspensions or colonies of motile bacterial cells, are known to self-organise to create complex patterns. It is of considerable interest to understand how such patterns may be modified by mechanical interactions of an active fluid with its environment. It has been observed that motile bacteria form furrows when confined to move as a monolayer across a soft semi-solid substrate [3]. We propose a particle-based model of active rods moving in a soft, elastically bound substrate. The substrate becomes fluid like after being displaced beyond a critical distance. Simulations show the emergence of permanent furrow networks with a distinctive fractal-like structure. We explain the physical origins of this behaviour through a coarse-grained model of stigmergy – the phenomenon of self-organisation through interactions between agents mediated by their environment.