  The enteric bacterium Proteus mirabilis, which is a pathogen that forms
biofilms in vivo, can swarm over hard surfaces and form concentric ring
patterns in colonies. Colony formation involves two distinct cell types:
swarmer cells that dominate near the surface and the leading edge, and swimmer
cells that prefer a less viscous medium, but the mechanisms underlying pattern
formation are not understood. New experimental investigations reported here
show that swimmer cells in the center of the colony stream inward toward the
inoculation site and in the process form many complex patterns, including
radial and spiral streams, in addition to concentric rings. These new
observations suggest that swimmers are motile and that indirect interactions
between them are essential in the pattern formation. To explain these
observations we develop a hybrid cell-based model that incorporates a
chemotactic response of swimmers to a chemical they produce. The model predicts
that formation of radial streams can be explained as the modulation of the
local attractant concentration by the cells, and that the chirality of the
spiral streams can be predicted by incorporating a swimming bias of the cells
near the surface of the substrate. The spatial patterns generated from the
model are in qualitative agreement with the experimental observations.
