Coordinated cell movement and intercellular interactions are crucial for bacterial multicellularity and self-organization, and the mechanisms governing these processes are of active scientific interest. Individual cells interact with neighbors through various biochemical and mechanical interactions, but the role of mechanical interactions in coordination and selforganization of bacteria remains unclear. This work investigates the mechanisms underlying various multicellular patterns in Myxococcus xanthus bacteria, a model organism to study self-organization in bacteria, and the role of mechanical interactions in these self-organization behaviors using biophysical models of cell motility in an agent-based-simulation framework.

Using this framework, first I studied the mechanism of gliding cell motility in M. xanthus by discriminating motility behavior of biophysical model cells during physical cell collisions from two alternative cell motility models proposed in the literature. Comparing the model cell motility behavior with experimental cell collision behavior showed that gliding cell motility in M. xanthus requires strong cell-substrate interactions supporting one of the proposed models. New predictions from this model are independently verified in direct experimentation. Next, I investigated the mechanisms responsible for formation and alignment of M. xanthus cells in groups and their collective movement in circular and spiral patterns under starvation, by simulating intercellular interactions among a large number of model cells. Results from the simulations show that these collective cell behaviors in M. xanthus can be explained through mechanical and biochemical interactions among cells and with the substrate. Finally, I investigated the mechanism for non-monotonic colony expansion behavior observed in M. xanthus motility mutants using the agent-based-simulation framework and analyzed individual cell motility behavior from experiments under similar conditions. Results from this work provide evidence that cell-stalling, a crucial assumption made by previous models to explain non-monotonic colony expansion, does not occur due to physical interactions and is not observed in experimental M. xanthus swarms.

Results from this thesis work show that many self-organization behaviors in M. xanthus can be explained by a combination of mechanical interactions among cells, between the cells and the substrate and contact based biochemical signaling. This work improves our understanding of mechanisms governing various self-organization behaviors displayed by M. xanthus bacteria and provides a general framework to study self-organization behaviors in other surface motile bacteria.