Biological evolution and the physics of growing microbial colonies
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In this thesis I investigate the role of spatial structure, cell-cell interactions and horizontal gene transfer on the genetic composition of growing microbial colonies. In the first part I study how the roughness of the growing layer of the colony depends on the shape of colony-forming cells. To study its impact I develop an off-lattice Eden-like model in which cells are represented as spherocylinders with a variable aspect ratio. I show that the roughness of the expansion front is not significantly affected by the shape of cells and that the dynamic scaling of growing front belongs to the KPZ universality class. Roughness is an important and easy to measure feature which affects the probability of fixation of genetic lineages in the colony. Another feature contributing to the genetic composition of a microbial community is horizontal gene transfer, which is investigated in the second part of this thesis. I develop an agent-based computational model of bacterial cells which grow, divide, and interact mechanically. I focus on plasmid conjugation, in which donors transfer a plasmid (a small, circular DNA molecule) to plasmid-free recipients. I show that bacteria in the expanding colony segregate into sectors of donors and acceptors. Donor sectors grow at the expense of acceptor sectors and that effect can be effectively described by coalescing random walkers that perform biased random walk on the colony expansion front. I use numerical and analytical methods to show that the plasmid eventually spreads to the whole colony given enough time, and I also show that this time is unrealistically long for experimentally determined conjugation rates and therefore real colonies are expected to have both acceptor and donor sectors. Furthermore, my simulations show that segregative plasmid loss at the moment of cell division can counteract the effect of conjugation and can lead to fixation of plasmid free cells. I also show that changes in nutrient concentration and the resultant change in roughness of the expansion front affect the rate of plasmid spread into population. Quantitative and qualitative results obtained in this section may serve as a tool to extract plasmid invasion rates from experimental data. In the last part of this thesis I investigate how the physical factors, such as finite strength of conjugative junctions, affect the conjugation process. I develop a computational model of plasmid transfer in which conjugative junctions are explicitly modelled as short, spring-like tubes that connect conjugating cells. My results show that factors such as junction creation rate and its strength can significantly affect the conjugation performance. I study different situations corresponding to different experimental scenarios (well-mixed colony on a filter paper, colliding colonies) and show that shear forces acting between cells can significantly lower the rate of plasmid transfer. My results can explain why conjugation occurs very rarely in some of these scenarios investigates in laboratory assays.