Bacterial Surface Motility is Modulated by An Abiotic Jamming Transition and is Independent of Chemotaxis and Individual Motility

The need to travel over surfaces is ubiquitous in the microbial world, where bacterial groups move at speeds of ~30 um/s despite their low Reynolds number environment. Bacillus subtilis is a model organism for the study of directed, collective motion over surfaces with groups exhibiting motility on length scales three orders of magnitude larger than themselves in a few doubling times. While genetic and chemical studies clearly show that surface tension gradients and water availability are required for this ‘ultrafast’ group motility, the relative importance of chemosensing, exogenous nutrient gradients, and individual motility are largely unknown from an experimental viewpoint. We demonstrate that contrary to simulations of bacterial growth on surfaces, B. subtilis does not rely on chemotaxis for direction, that the rate of dendritic expansion of the colony is faster when bacteria are motile but that the same type of group motility is possible even with non-motile cells, and that water availability is likely a control parameter modulating an abiotic jamming transition that determines whether the group remains fluidized and therefore motile.