Intercellular Interactions Promote Collective Behavior in Bacterial Colonies and Developing Epithelia

Collective behavior has been observed in many animal groups including schools of fish, flocks of birds, as well as in colonies of swarming bacteria, and even in group of cells in epithelial tissues of Drosophila fruit fly during development. The self-organizing behavior and adjustment, which results from local interactions, leads to group behavior at time and space scales that are larger than the scale of interactions between individual organisms and cells. In this thesis, we study the impact of physical properties of individual cells and intercellular interactions on the swarming behavior in bacterial colonies and self-organization of epithelial cells during development. In particular, by using combination of computational modeling and experimentation, we explore the role that pili interactions play in expanding swarms of the pathogenic bacterium Pseudomonas aeruginosa under different environmental conditions. We find that pili-pili interactions in a colony of wild-type P. aeruginosa slow down the swarm expansion as well as enable bacteria to alter their movement to avoid antibiotics. We also demonstrated that periodic reversals of motion direction with periods within the experimentally observed range, high flexibility of bacterial cells and a moderate level of strength of cell-cell adhesion are crucial for a colony of self-propelled rod-shaped Myxococcus xanthus to spread protein within the population efficiently. This contact-mediated protein exchange mechanism is necessary for bacterial self-organization during predation, fruiting body formation and genetic repair of damaged cells. Finally, we study role of cellular mitotic rounding during growth of epithelial tissues. Our results show that increase of cortical stiffness and reduction in cell-cell adhesion are the main factors in increasing cell roundness, and change in the cytoplasmic pressure is the primary factor that controls the cell size.