ABSTRACT Our gut microbiota protects us from infection through a phenomenon called colonization resistance. Established microbes confer colonization resistance by blocking harmful invaders through consumption of critical nutrients and altering the gut environment. Among these protective microbes, certain “keystone species” play an outsized role, yet what makes them so effective remains incompletely understood. Our work focuses on E. coli Nissle 1917, a probiotic strain that shields against Salmonella infection. We hypothesize that its protective power stems from its ability to migrate into specific intestinal niches where it competes directly with Salmonella for simple sugars, overcoming inhibitory metabolites and preempting the pathogen’s growth. In Specific Aim 1 we will determine the mechanism through which consumption of simple sugars by E. coli limits Salmonella growth. In Specific Aim 2 we will determine the mechanism by which chemotaxis contributes to colonization resistance. Using a combination of animal modeling and an innovative in vitro gut habitat model, completion of these aims will define the metabolic and behavioral traits that make E. coli a keystone defender. Understanding these principles will not only reveal how probiotics work but also open new avenues for preventing infections by reshaping the gut ecosystem itself.