| The individual components of a biological system often express simple behaviors that lead to the spontaneous emergence of order and complexity; the whole is greater than the sum of its parts. Emergent complexity may arise spontaneously or, more interestingly, in response to an external force. In this thesis, we study how various stresses---starvation, crowding, jagged topographies and cytocidal chemicals---drive the emergence of ordered collective behaviors in two different microorganisms: Escherichia coli bacteria and breast cancer cells.;We first study the ecological order that emanates from bacterial populations competing under starvation conditions. We show that the very simple rules followed by single individuals results in population-wide fitness maximization.;We then investigate how the emergence of multicellularity results from cell-cell interactions of unicellular organisms as a consequence of crowding and increased cell densities. To achieve this, we study the development of Escherichia coli biofilms and demonstrate how each individual cell's aggregation dynamics lead to the formation of complex multicellular structures. Using in situ measurements, we also investigate the physical properties of the biofilm's motion and deformation.;Then, we demonstrate how the swimming dynamics of single bacterial cells inside jagged, funnel-like geometries leads to the emergence of complex migratory patterns. In particular, we create ratchet-like barriers that redirect the motion of single E. coli cells and use them to influence their collective migratory behavior. We show that cells are able to navigate against motion-rectifying barriers though collective interactions.;Finally, we review and propose new approaches to study the evolutionary dynamics of drug resistance by studying the collective response to drug-induced stress gradient in malignant cancer tissues. We suggest that there is a fundamental mechanistic connection between the rapid evolution of drug resistance in cancer and bacteria and, given this fundamental connection, propose that studying bacterial communities can provide deeper insights into the dynamics of adaptation and the evolution of cells within tumors. |
