Biofilm spatial structure: Active layer dynamics and transitions in surface roughness

Characterising the spatial structure of biofilms is important because it controls diverse characteristics such the extent of pathogen adhesion, genetic mixing and hence potential for cooperation, antibiotic penetration and the chances of fixation of antibiotic resistant mutants. We have used the iDynoMiCs agent-based biofilm modelling software to investigate in detail the role of the layer of metabolically active cells on biofilm surface roughness. We find that the active layer dynamics drive a phenomenon known as pinning, in which gaps in the active layer lead to parts of the biofilm interface becoming stationary and being left behind the moving front. Based on this behaviour, we distinguish three key phases: an unpinned phase in which no pinning occurs, a depinned phase in which pinning sites appear but can be overcome, and a pinned phase in which pinning sites appear and remain. We argue that this is similar to a self-organised pinning-depinning transition known in statistical physics. We also present preliminary results on how these different phases affect the genetic diversity of the biofilm.