ORIGINAL

Abstract

Microbial ecosystems are ubiquitous around the world and are an essential part of many processes fundamental to our own lives. However, because of their complexity and variety, a complete picture of how they function is still missing. Studies trying to uncover parts of this conundrum have focused on many different scales and dealt with different levels of detail. From large-scale studies describing the composition of microbial ecosystems, to small-scale ones reaching the limits between the physical and biological descriptions. In the latter end of the spectrum are studies about the spatio-temporal organization of microbial colonies. Usually describing the growth of a single species, these studies give a thorough description of the mechanisms of pattern formation in simple colonies. In this thesis, we complement them by looking at the spatio-temporal dynamics of a predator-prey system. Our predator, the amoebae Dictyostelium discodideum, is a motile organism who feeds on bacteria. We characterize this system by looking at the macro and micro scales, connecting the development of the predator colony to the behaviors of single amoebae. At the macro-scale, we record the growth of D. discoideum colonies, showing that their radii expand at a constant speed. At the micro scale, we track single cells showing how they invade a bacterial biofilm in an organised, oriented invading front. Then, we look at the other side of the system: the prey. Using Nystatin, a bacterial toxin, we show that bacterial chemical defenses can decelerate the advance of predation waves. In the end, we use an adaptation of the Keller-Segel model with a logistic growth term to describe the movement of the amoebae. Our model can explain the oriented movement of amoebae at the boundary of the colony and the colony’s constant expansion speed. We find approximate analytical formulas for the speed of the expanding fronts and check our results with numerical simulations. Altogether, we analyze the role of growth, movement and chemotaxis on the formation of predation patterns during the invasion of a bacterial biofilm.