| Summary: | <p>Abstract</p> <p>Background</p> <p>The type III secretion systems (T3SSs) encoded by <it>Salmonella </it>pathogenicity island 1 and 2 (SPI-1 and SPI-2) are important for invasion of epithelial cells during development of <it>Salmonella</it>-associated enterocolitis and for replication in macrophages during systemic infection, respectively. <it>In vitro </it>studies have previously revealed hierarchical transport of different SPI-1 factors and ordered synergistic/antagonistic relationships between these proteins during <it>Salmonella </it>entry. These results suggest that the level and timing of the expression of these proteins dictate the consequences of bacterial infection and pathogenesis. However, the expression of these proteins has not been extensively studied <it>in vivo</it>, especially during the later stages of salmonellosis when the infection is established.</p> <p>Results</p> <p>In this study, we have constructed bacterial strains that contain a FLAG epitope inserted in frame to SPI-1 genes <it>prgI</it>, <it>sipA</it>, <it>sipB</it>, <it>sopE2</it>, <it>spaO</it>, and <it>sptP</it>, and investigated the expression of the tagged proteins both <it>in vitro </it>and <it>in vivo </it>during murine salmonellosis. The tagged <it>Salmonella </it>strains were inoculated intraperitoneally or intragastrically into mice and recovered from various organs. Our results provide direct evidence that PrgI and SipB are expressed in <it>Salmonella </it>colonizing the spleen and cecum of the infected animals at early and late stages of infection. Furthermore, this study demonstrates that the SpaO protein is expressed preferably in <it>Salmonella </it>colonizing the cecum but not the spleen and that SptP is expressed preferably in <it>Salmonella </it>colonizing the spleen but not the cecum.</p> <p>Conclusion</p> <p>These results suggest that <it>Salmonella </it>may express different SPI-1 proteins when they colonize specific tissues and that differential expression of these proteins may be important for tissue-specific aspects of bacterial pathogenesis such as gastroenterititis in the cecum and systemic infection in the spleen.</p>
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