Ursell, TristanRhodeland, Benjamin2020-02-272020-02-272020-02-27https://hdl.handle.net/1794/25272Microbes routinely face the challenge of acquiring territory and resources on wet surfaces. Cells move in large groups inside thin, surface bound water layers, often achieving speeds of 30 µm/s within this environment, where viscous forces dominate over inertial forces (low Reynolds number). The canonical Gram-positive bacterium Bacillus subtilis is a model organism for the study of directed, collective migration over surfaces with groups exhibiting motility on length scales three orders of magnitude larger than themselves within a few doubling times. Genetic and chemical studies clearly show that the secretion of endogenous surfactants and availability of free surface water are required for this ‘ultrafast’ group motility. However, the relative importance of individual motility, chemosensing, and the presence of exogenous nutrient gradients in precipitating group surface motility are largely unknown. Here I use novel experiments to strengthen the case that (i) B. subtilis does not rely on chemotaxis to determine group motility direction, to establish that (ii) the rate of dendritic expansion has only a weak dependence on motility and that rapid dendritic group motility is possible even with non-motile cells, and demonstrate for the first time that (iii) water availability is likely a sensitive control parameter modulating an abiotic jamming transition that determines whether the group remains fluidized and therefore collectively motile. These data suggest that rapid surface motility does not result from individual motility and chemotaxis properties of the bacteria, but rather that a combination of biologically generated surface tension gradients and abiotic granular jamming regulate this ubiquitous ecological process. This dissertation includes previously published co-authored material.en-USAll Rights Reserved.bacteriajammingsurface tensionElectronic Thesis or Dissertation