Bacteroides are one of the numerically dominant genera of bacteria of the human intestinal microbiota. In this ecosystem, these species are mutualistic symbionts, providing beneficial properties to humans. Outside of the natural colonic niche, Bacteroides fragilis is an opportunistic pathogen and is the leading cause of anaerobic bacteremia and intraabdominal abscesses. B. fragilis has unique characteristics that are instrumental in the ability of this organism to provide both symbiotic benefits in the intestine and to cause disease if the bacterium gains access to extra-intestinal sites. The objective of my research is to understand the molecular mechanisms governing the complex regulation of these characteristics.