Salmonella are facultative anaerobes that are responsible for a variety of diseases in humans ranging from self limiting gastroenteritis to enteric fever. Salmonellosis is usually initiated by oral inoculation via ingestion of contaminated food . Upon infection the bacterium employs numerous virulence factors to evade the host's immune system. In our lab we have demonstrated that one such factor is the periplasmic superoxide dismutase, SodCI. Normally, macrophages engulf and bombard foreign microbes with a battery of reactive oxygen species, which include superoxide. SodCI appears to confer protection against the phagocytic burst. Loss of SodCI confers 7-10 fold attenuation in S. typhimurium strain 14028. Despite being 60% homologous to SodCI, SodCII, another periplasmic SOD present in this strain, does not contribute to virulence. Our lab has shown that certain physical differences exist between SodCI and SodCII that account for the discrepancies in virulence phenotype. SodCI exists as a dimer in solution, is protease resistant, and is not released by osmotic shock from the periplasm. Conversely, SodCII is monomeric, is protease sensitive, and is released from the periplasm by osmotic shock. The purpose of this project is to further characterize the structural and functional features of SodCI that allow it to confer virulence. To address this we are constructing hybrid SodC proteins and assaying them for SOD activity, susceptibility to proteinase K, propensity for release by osmotic shock, and ultimately their contribution to virulence.