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ABSTRACTSalmonella entericaserovar Typhimurium uses the Salmonellapathogenicity island 1 (SPI1) type III secretion system (T3SS) to induce inflammatory diarrhea and bacterial uptake into intestinal epithelial cells. The expression of hilA, encoding the transcriptional activator of the T3SS structural genes, is directly controlled by three AraC-like regulators, HilD, HilC, and RtsA, each of which can activate hilD, hilC, rtsA, and hilAgenes, forming a complex feed-forward regulatory loop. Expression of the SPI1 genes is tightly controlled by numerous regulatory inputs to ensure proper timing in production of the T3SS apparatus. Loss of FadD, an acyl coenzyme A (acyl-CoA) synthetase required for degradation of long-chain fatty acids (LCFAs), was known to decrease hilAexpression. We show that free external LCFAs repress expression of hilAindependently of FadD and the LCFA degradation pathway. Genetic and biochemical evidence suggests that LCFAs act directly to block primarily HilD activity. Further analyses show that in the absence of FadD, hilAexpression is downregulated due to endogenous production of free LCFAs, which are excreted into the culture medium via TolC and then transported back into the bacterial cell via FadL. A fadLmutant is more virulent than the wild-type strain in mouse oral competition assays independently of LCFA degradation, showing that, in the host, dietary LCFAs serve as a signal for proper regulation of SPI1 expression, rather than an energy source.IMPORTANCETo cause disease, Salmonellamust respond to diverse environmental cues to express its invasion machinery at the appropriate location in the host intestine. We show that host intestinal free long-chain fatty acids (LCFAs) affect Salmonellainvasion by reducing expression of the SPI1 type III secretion system, acting primarily via the AraC-like activator HilD. Degradation of LCFAs is not required for this regulation, showing that free LCFAs serve as a cue to proper intestinal localization to invade host epithelial cells and not as a nutrient source.