Abstract: Colonization resistance is one of the primary benefits the gut microbiota provides for the host animal. How the gut bacterial community remains stable and resists invasion has been a subject of much research over many decades. We are using Salmonella Typhimurium as a model pathogen in mice, in hopes of discovering new facets to this well-studied interaction. In vitro and in vivo results point to amino acids as a potentially important nutrient for Salmonella. Key protective anaerobic bacteria also ferment amino acids, thus giving support for a nutrient competition basis for colonization resistance.