Submitted to: Molecular Microbiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 12/31/2012
Publication Date: 3/25/2013
Citation: Hung, C., Garner, C.D., Slauch, J.M., Dwyer, Z.W., Lawhon, S.D., Frye, J.G., Ahmer, B.M., Altier, C. 2013. The intestinal fatty acid propionate inhibits Salmonella invasion through the post-translational control of HilD. Molecular Microbiology. 87(5):1045-1060. Interpretive Summary: Infection with Salmonella can cause gastroenteritis which can be severe in young, old, or immune compromised people. The infection begins when Salmonella invades the cells lining the intestinal wall. To do this, Salmonella makes invasion proteins which are produced by the expression of genes located in Salmonella Pathogenicity Island 1 (SPI1). SPI1 is a cluster of genes grouped together at a specific location in bacteria and are required to cause disease. The short chain fatty acid propionate can be abundant in the intestine of animals and has been shown to stop the production of invasion proteins. We found that propionate was the only short chain fatty acid that stops invasion; therefore it could be used to control infection if we can find ways to regulate it’s production. This control required a specific gene, hilD, located in SPI1. It was also determined that blocking the conversion of propionate to a high energy metabolic intermediate compound called propyionyl-CoenzymeA was responsible for this effect. Understanding how Salmonella bacteria cause disease is important for human and animal infections and is critical for scientists as they develop new ways to block infection.
Technical Abstract: For Salmonella to cause disease, it must first invade the intestinal epithelium using genes encoded within Salmonella Pathogenicity Island 1 (SPI1). Previous work has shown that propionate, a short chain fatty acid abundant in the intestine of animal hosts, negatively regulates SPI1 in vitro. Here we investigated how propionate represses invasion. Repression was observed at mildly acidic, physiologically relevant pH, but not under alkaline conditions, suggesting that propionate must enter the bacterial cytoplasm to exert its effects. Transcriptome analyses revealed that propionate predominantly affected SPI1, and that the closely related fatty acid acetate did not share this effect, indicating a specific mechanism for the control of virulence. Using concentrations comparable to those present in the normal mouse cecum, propionate reduced expression of the SPI1 transcriptional regulators hilD, hilA, and invF, consequently decreasing the expression and secretion of effector proteins and reducing bacterial penetration of cultured epithelial cells. This control centered on hilD, which occupies the apex of the regulatory cascade within SPI1, as the loss of only this gene among those of the regulon prevented repression by propionate. Genetic studies indicated that regulation through hilD, however, was not achieved through the control of either transcription or translation, indicating a post-translational mechanism. This was achieved, at least in part, through the altered stability of HilD, as the half-life of this protein was significantly reduced in bacteria grown in the presence of propionic acid. Further, repression by propionate on SPI1 was significantly lessened in a mutant unable to produce the metabolic intermediate propionyl-CoA by any known route, an effect specific to this fatty acid, while further metabolism of propionyl-CoA appeared not to be required. These results thus suggest a mechanism by which HilD is post-transcriptionally modified using the high energy intermediate propionyl-CoA.