||Salmonella enterica serovar typhimurium ( S. typhimurium ) is a Gram negative bacterium that is responsible for the disease gastroenteritis in humans. A series of recent studies have shown that S. typhimurium cultures grown in spaceflight or analogous ground-based conditions of low fluid shear stress (<1 dyne/cm 2 ) exhibit increased virulence and genome-wide changes in gene expression. An analysis of the S. typhimurium genes differentially regulated in response to low fluid shear conditions (the low fluid shear stimulon) led to the identification of several previously uncharacterized genes predicted to encode transcriptional regulatory proteins. These genes may have unexplored roles in regulating bacterial physiology. One such gene is ydcI, located in a single gene operon between two oppositely-oriented genes in the S. typhimurium genome. The YdcI protein sequence is highly conserved in many genera of Gram negative bacteria including many genera containing pathogens. I demonstrate that the ydcI gene is expressed in a range of other Gram negative bacteria; this is the first investigation into the expression of ydcI in other species. Utilizing lacZ fusion constructs, I analyze ydcI expression in S. typhimurium and demonstrate that the gene is negatively auto-regulated under the conditions tested. I use a gel shift assay to show that YdcI is able to specifically bind a DNA probe from the ydcI promoter region. I also examine the role of the ydcI gene in the survival of S. typhimurium in extreme environmental stress including acidic, thermal, oxidative, and osmotic stress conditions. I demonstrate that the ydcI gene is required for full resistance to acid stress, but is dispensable for survival under thermal and osmotic stresses. The results indicate that the ydcI gene encodes a conserved DNA binding protein that is involved in aspects of bacterial physiology related to acid stress resistance.