Mechanical constraints to unbound expansion of B. subtilis on semi-solid surfaces

The effects of surface mechanical constraints that may promote or preventbacterial expansion on semi-solid surfaces are largely unknown. In this work, we havemanufactured agar surfaces with different viscoelasticity, topography, and roughness.To capture the essential biophysics of the bacterial expansion we have developeda continuum model that faithfully reproduces the main patterns of the short-rangeand long-range expansion with two critical parameters: local interfacial forces andcolony viscosity. Cohesive energy of the bacterial colony that determines the extentof exploration was dependent on agar surface viscoelasticity. On soft surfaces, bacteria produce low viscoelastic colonies that allow guided population of bacteria totraverse distances that are six orders of magnitude larger than the size of the individual bacterium. Bacteria growing on stiff surfaces produce colonies with significantlyincreased viscoelasticity that prevent bacterial exploration of new territory and allowformation of a very steep cliff at the edge of the colony. Upon flooding of the roughsurfaces, we have induced aquaplaning and spreading of bacteria. A layer of waterbetween the bacterium and surface results in a loss of traction allowing bacteria tospread across the otherwise inhibitory rough surface. The results shed new light on thebacterial ability to rapidly colonize new territories.