6 research outputs found
Evolution on the microbial battlefield
No full text<p>Microbes live in dense communities where strains and species compete for space and nutrients. Cells within these communities produce a large array of secretions, such as toxins or scavenging molecules, that kill and inhibit other strains. While competition is common and important for microbes, its impacts on their communities remains poorly understood. In this thesis, I study the impacts of competition on (1) the evolution of secretions and (2) horizontal gene transfer. Using a wide range of approaches, including game theory, population genetics, differential equations and individual-based models, I investigate the evolution and ecology of diverse microbial communities. First, I study the production of iron- scavenging siderophores–a trait often assumed to be cooperative–and show how they can function as a competitive trait that harms other strains by starving them of iron. My competitive model predicts that siderophores should be upregulated in response to competitors and I show this fits with both published and new experimental data (Chapter 2). I next study the logic of bacterial warfare proper: the evolution of strategies to produce antibiotics and bacteriocins to kill other strains. I show that the most robust strategy for using antibiotics is to attack only when detecting the attack of others, a negative form of tit-for-tat (Chapter 3). The second half of my thesis studies how competition influences another major bacterial trait, horizontal gene transfer. Firstly, I show how competition and migration combine to enable single genes to sweep horizontally through microbial communities, thus providing an explanation for a large body of data that shows that these sweeps are both common and important (Chapter 4). Finally, I ask how genetic transfer can evolve in microbial communities, despite the potential for high individual costs from taking up foreign DNA. As for the evolution of sex in eukaryotes, I show that epistasis for fitness can promote horizontal transfer under certain conditions. However, by studying regulated gene transfer, I argue that key to horizontal transfer evolution in bacteria is their ability to actively upregulate transfer in response to stress (Chapter 5). My thesis helps to map out the rules of interaction in microbial communities, and how these rules affect major phenotypes, including siderophores, antibiotics and horizontal genetic transfer. To close, I discuss how the effects of clinical antibiotics mirrors many of the effects of natural competition between microbial strains. I argue that only by understanding natural competition can we understand how the use, and overuse, of clinical antibiotics affects microbes.</p
