PROJECT SUMMARY The long-term goal of this proposal is to improve outcomes of delivery of small molecules, peptides, and proteins to the human gut by engineered live biotherapeutic bacteria. To accomplish this, we will use two strategies to optimize biotherapeutic growth in the gut environment. First, we will match prebiotic compounds (complex carbohydrates not digested by the body) with heterologous expression of enzymes that allow cells to utilize these privileged carbon sources. Second, we will create genome-scale libraries in our candidate biotherapeutics, E. coli Nissle 1917 and Lactococcus lactis, to identify gene products that increase fitness in the gut and gut-like environments. The methods will be tested in in vitro culture, small intestinal organoid co-culture, and in vivo in the mouse gut to determine enrichment of the biotherapeutic population, enhanced production of the therapeutic small molecule, peptide, or protein of interest, and phenotypic outcome in systems of increasing verisimilitude to the human gut.