Salmonella is a leading cause of food borne illness, causing an estimated 1.4 million cases per year in the United States. Serovar Typhimurium is responsible for about 26% of these cases (CDC, 1998). The vast majority of Salmonella infections in mammals and birds are the result of infection with S. enterica subspecies I serovars, yet very few genetic factors that are necessary for intestinal persistence in these reservoirs have been described. Intestinal persistence is critical for shedding and transmission of serovar Typhimurium in mammals and birds, yet this phenomenon and interaction of the organism with the host immune system during persistent infection is poorly understood. The long-term goal of our work is to understand the genetic basis of persistence and host range restriction of Salmonella enterica serovar Typhimurium in its mammalian hosts.