Salmonella carriage in agricultural animals poses a major concern, as changes in the host environment can trigger rapid bacterial proliferation and shedding, thereby facilitating transmission. Prior work has demonstrated a role for Salmonella Pathogenicity Island 1 (SPI-1) in determining the outcome of such subclinical infections, and shows that SPI-1 induction is regulated by the transcriptional regulator hilD, which induces SPI-1 expression as an all-or-none event in individual bacteria. Here, we aim to elucidate the mechanism by which HilD imposes this phenotypic variance and thereby influences transmission of Salmonella. Our central hypothesis is that SPI-1 expression is a stable phenotype determined by HilD concentration, and that compounds inhibiting HilD can prevent Salmonella pathogenicity. To address this hypothesis, the objectives of this project are to: (1) determine whether SPI-1 induction is controlled by a threshold requirement for HilD; (2) determine whether individual bacteria are committed to a stable expression phenotype upon SPI-1 induction; and (3) evaluate the effects of compounds repressive for SPI-1 expression on the HilD threshold. The proposed studies address the AFRI Challenge Area of “Food Safety,” and additionally apply to AFRI Foundational Area of “Animal Health and Production and Animal Products” by characterizing the determinants of Salmonella carriage and transmission in agricultural hosts. This project is relevant to the goals of the Fellowship Program as it will provide a training experience to the applicant that will facilitate success in future research endeavors.