Salmonella enterica serovar Typhimurium is a common cause of gastroenteritis. Virulence is determined by five Salmonella Pathogenicity Islands (SPI) encoded on the bacterial chromosome. SPI1 and SPI2 encode the structural components of two Type Three Secretion Sytems (TTSS) that translocate effectors into the host cell. The SPI1 effector SigD is an inositol phosphatase that induces the activation of Akt/PKB a mammalian serine threonine kinase. The role of Akt activation in pathogenesis remains unclear, however, in cultured epithelial cells it appears to be involved in preventing the onset of apoptosis in infected cells. More specifically we have found that a sigD deletion mutant induces Caspase 3 activation compared to wild type. This may be important for the survival and replication of intracellular bacteria. In macrophages activation of Akt by Salmonella is more complex since lipopolysaccharide will activate the kinase in the absence of SigD. Comparison of the two pathways has revealed important differences. In particular, SigD-mediated activation is insensitive to Wortmannin a well described inhibitor of Akt activation via phosphatidylinositol-3-kinase. Thus SigD-dependent Akt activation appears to occur via a novel pathway. We have found that sigD also has effects up to 15 hours post invasion (p.i.). This is an important finding since it indicates that SPI1 effectors are not only involved in very early events. In this study we have investigated the role of SigD in iNOS induction in infected macrophages. We found that iNOS induction is affected by Sigd presumably in an Akt dependent process. Furthermore we found that SigD protein is detectable in infected cells for up to 8 hours p.i. Real time PCR confirmed that SigD continues to be expressed for at least 4 hours p.i. The S. Typhimurium is one of the most common causes of gasteronteritis in humans. Usually the disease is self limiting, however, with the emergence of antiobiotic resistance and an expanding number of immunocompromised individuals there are valid concerns that Salmonellosis will cause more mortality in the future. Currently there is no animal model for salmonellosis. In mice S. Typhimurium causes a systemic disease that is similar to Typhoid fever. In humans Typhoid fever is caused by S. Typhi. The development of an animal model in mice would be extremely useful to the field especially now that there are such a variety of knockout mice available. We have taken several approaches to developing a model and have data suggesting that there may indeed be potential for a murine Salmonellosis model. In particular pretreament of mice with the antibiotic Streptomycin appears to make them more susceptible to infection with S. Typhimurium. Significantly these pretreated mice develop colitis which may be used to model human gasteroenteritis. These studies are now being expanded to investigate whether the SPI1 TTSS has other as yet undiscovered roles in post invasion processes.
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