Collective behaviours in bacterial populations can result in dramatic organisation across a large scale. The mechanisms that lead to emergence of these patterns are poorly understood. We have been examining how the process of active expansion across semi-solid media by Pseudomonas aeruginosa biofilms manifests as an intricate network of interconnected trails. We have developed sophisticated computer vision to track and analyse individual cell movements during this process and have used atomic force microscopy to examine the topography of the media beneath the biofilms. Our studies have revealed that biofilm expansion by P. aeruginosa is a stigmergic phenomenon that involves the forging of interconnected furrows that guide the movements of bacterial cells as they move through the furrow network toward the leading edge. Furthermore we have determined that extracellular DNA facilitates this process by co-ordinating individual cell movements to enable efficient expansion of the biofilm.