It is estimated that as many as 100,000 deaths occur in the United States each year as a result of gram-negative infections. World wide these organisms are a leading cause of death by infectious disease. The studies described in this grant focus on mouse infections with Salmonella typhimurium which serves as a mouse model for human typhoid. The pathogenesis of facultative intracellular parasites such as S. typhimurium is complex because the organism's growth in vivo can be affected by both extracellular and intracellular mechanisms of resistance. One approach that can aid in the delineation of mechanisms involved in the resistance to microbial infections is to identify variant mouse genes that affect pathogenesis. Understanding the function of these genes should allow us to understand much about how some mice resist infection. Although there appear to be a number of mouse salmonella resistance loci, only a few have been described because of complications that arise in genetic analyses where more than one locus is involved. Over the past couple of years, we have been developing what we hope will be an approach that should facilitate the identification of individual salmonella resistance genes. We are preparing salmonella strains isogenic for single virulence genes. By examining the difference in virulence of one such partially isogenic pair of strains in mice, we have been able to identify mice carrying the resistant and susceptible Ity alleles with more clarity than was possible in the past. This same variant salmonella strain also appears to have allowed us to identify a new salmonella resistance locus linked to the cdm locus of the mouse. We have also produced a salmonella strain that lacks a gene necessary for LT2 salmonella to cause disease in the ultrasusceptible BSVR mice. By using salmonella congenic for these virulence genes we should also be able to examine the virulence mechanisms of the salmonella that are counteracted by the specific mouse resistance genes they help to identify.