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.
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