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Salmonella enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative enteric pathogen that causes self-limiting gastroenteritis in healthy individuals and can cause systemic infections in those who are immunocompromised. During its natural lifecycle, S. Typhimurium encounters a wide variety of stresses it must sense and respond to in a dynamic and

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a Gram-negative enteric pathogen that causes self-limiting gastroenteritis in healthy individuals and can cause systemic infections in those who are immunocompromised. During its natural lifecycle, S. Typhimurium encounters a wide variety of stresses it must sense and respond to in a dynamic and coordinated fashion to induce resistance and ensure survival. Salmonella is subjected to a series of stresses that include temperature shifts, pH variability, detergent-like bile salts, oxidative environments and changes in fluid shear levels. Previously, our lab showed that cultures of S. Typhimurium grown under physiological low fluid shear (LFS) conditions similar to those encountered in the intestinal tract during infection uniquely regulates the virulence, gene expression and pathogenesis-related stress responses of this pathogen during log phase. Interestingly, the log phase Salmonella mechanosensitive responses to LFS were independent of the master stress response sigma factor, RpoS, departing from our conventional understanding of RpoS regulation. Since RpoS is a growth phase dependent regulator with increased stability in stationary phase, the current study investigated the role of RpoS in mediating pathogenesis-related stress responses in stationary phase S. Typhimurium grown under LFS and control conditions. Specifically, stationary phase responses to acid, thermal, bile and oxidative stress were assayed. To our knowledge the results from the current study demonstrate the first report that the mechanical force of LFS globally alters the S. Typhimurium χ3339 stationary phase stress response independently of RpoS to acid and bile stressors but dependently on RpoS to oxidative and thermal stress. This indicates that fluid shear-dependent differences in acid and bile stress responses are regulated by alternative pathway(s) in S. Typhimurium, were the oxidative and thermal stress responses are regulated through RpoS in LFS conditions. Results from this study further highlight how bacterial mechanosensation may be important in promoting niche recognition and adaptation in the mammalian host during infection, and may lead to characterization of previously unidentified pathogenesis strategies.
ContributorsCrenshaw, Keith (Author) / Nickerson, Cheryl A. (Thesis advisor) / Barrila, Jennifer (Thesis advisor) / Ott, C. (Committee member) / Stout, Valerie (Committee member) / Arizona State University (Publisher)
Created2016
Description
The discovery that mechanical forces regulate microbial virulence, stress responses and gene expression was made using log phase cultures of Salmonella Typhimurium (S. Typhimurium) grown under low fluid shear (LFS) conditions relevant to those encountered in the intestine. However, there has been limited characterization of LFS on other growth phases.

The discovery that mechanical forces regulate microbial virulence, stress responses and gene expression was made using log phase cultures of Salmonella Typhimurium (S. Typhimurium) grown under low fluid shear (LFS) conditions relevant to those encountered in the intestine. However, there has been limited characterization of LFS on other growth phases. To advance the growth-phase dependent understanding of the effect of LFS on S. Typhimurium pathogenicity, this dissertation characterized the effect of LFS on the transcriptomic and phenotypic responses in both stationary and lag phase cultures. In response to LFS, stationary phase cultures exhibited alterations in gene expression associated with metabolism, transport, secretion and stress responses (acid, bile salts, oxidative, and thermal stressors), motility, and colonization of intestinal epithelium (adherence, invasion and intracellular survival). Many of these characteristics are known to be regulated by the stationary phase general stress response regulator, RNA polymerase sigma factor S (RpoS), when S. Typhimurium is grown under conventional conditions. Surprisingly, the stationary phase phenotypic LFS stress response to acid and bile salts, colonization of human intestinal epithelial cells, and swimming motility was not dependent on RpoS. Lag phase cultures exhibited intriguing differences in their LFS regulated transcriptomic and phenotypic profiles as compared to stationary phase cultures, including LFS-dependent regulation of gene expression, adherence to intestinal epithelial cells, and high thermal stress. Furthermore, the addition of cell-free conditioned supernatants derived from either stationary phase LFS or Control cultures modulated the gene expression of lag phase cultures in a manner that differed from either growth phase, however, these supernatants did not modulate the phenotypic responses of lag phase cultures. Collectively, these results demonstrated that S. Typhimurium can sense and respond to LFS as early as lag phase, albeit in a limited fashion, and that the lag phase transcriptomic and phenotypic responses differ from those in stationary phase, which hold important implications for the lifecycle of this pathogen during the infection process.
ContributorsFranco, Karla Paola (Author) / Nikerson, Cheryl A (Thesis advisor) / Bean, Heather D (Committee member) / Stout, Valerie (Committee member) / Ott, C Mark (Committee member) / Barrila, Jennifer (Committee member) / Arizona State University (Publisher)
Created2020