Salmonella enterica serovar Typhimurium (S. Typhimurium) is a common cause of enterocolitis in humans and cattle but causes a systemic, typhoid-like, disease in susceptible mice. This facultative intracellular pathogen invades non-phagocytic cells, such as those found in the intestinal epithelium, and survives within a modified phagosome known as the Salmonella-containing vacuoles (SCV). Two type III secretion systems (T3SS), which translocate distinct cohorts of bacterial effector proteins into the host cell, are required for establishment of the intracellular replicative environment. Invasion is dependent on T3SS1 whereas T3SS2 is induced intracellularly and is associated with intracellular survival/replication and biogenesis of the SCV. Recently we have been investigating the relative importance of vacuolar versus cytosolic replication in epithelial cells. In approximately 5-20% of infected cells S. Typhimurium escape from the vacuole and survive and replicate in the cytosol. We have found that these cytosolic bacteria trigger a non canonical caspase 4/11 inflammasome response that is important for antimicrobial defense in the intestinal epithelium. Currently we are completing a study showing that at later time points the T3SS1 is functionally active in the cytosolic bacteria whereas the T3SS2 is functionally active only in the vacuolar bacteria.
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