Abstract Salmonella enterica serovar Typhimurium is the causative agent of salmonellosis, leading to ~150 million cases of gastroenteritis annually around the world, making it one of the most common foodborne diseases. Salmonella enterica serovars are especially reliant on secreted protein effectors for virulence. These effectors can mimic and reprogram host cellular functions to create a beneficial environment for the invading bacteria, such as formation of the intracellular salmonella containing vacuole (SCV) and antagonization of the immune response. My lab previously found that increased intracellular replication of S. Typhimurium and production of anti-inflammatory cytokine interleukin-10 (IL-10) by host cells is associated with the secreted protein effector SarA. SarA acts through host STAT3 (signal transducer and activator of transcription) signaling by mimicking the function of host cytokine receptor gp130. However, despite homology between SarA and gp130, I have shown that SarA leads to greater STAT3 phosphorylation over a longer period of time than gp130. Previous research in the field of STAT3 signaling suggests that the kinetics of STAT3 activation have a dramatic effect on whether the downstream transcriptional targets are pro- or anti-inflammatory. I hypothesize that S. Typhimurium effector SarA evolved molecular characteristics to hijack and prolong host STAT3 signaling to promote important anti-inflammatory responses during acute infection in the gut. I propose mutagenizing SarA and measuring how these manipulations alter 1) SarA binding to STAT3 and negative regulators, 2) SarA-directed phosphorylation of STAT3, 3) expression of downstream transcriptional targets and 4) SarA-associated burden and IL-10 phenotypes in cells and mice. My lab has previously shown that SarA leads to increased STAT3 phosphorylation and fitness in systemic sites (spleen, liver) during intraperitoneal (I.P.) and chronic murine infection models. However, neither of these models represent the natural oral infection route of S. Typhimurium in humans. I have shown that wild-type and complemented Salmonella Typhimurium have ~100x greater burden in the small intestine compared to ∆sarA during infection in an oral murine model. I will further elucidate the mechanism by which SarA signaling acts on the mucosal immune responses to benefit S. Typhimurium during oral infection. This proposal aims to understand the molecular basis for robust STAT3 activation by SarA, and how S. Typhimurium hijacking of the host STAT3 signaling pathway impacts infection outcome during intestinal infection in vivo.