Repurposing bacterial secretion systems for the design of living therapeutics

The targeted delivery of therapeutics to sites of disease remains a major challenge. Almost all drugs, whether administered orally or intravenously, can cause systemic side effects. To circumvent these issues, synthetic biology-based approaches are being used to engineer bacteria to secrete therapeutic protein payloads directly at sites of disease. In the case of Gram-positive bacteria, the most straightforward approach has been to expand the repertoire of proteins recognized by their native protein secretion systems, which enable secretion across their cell membrane. However, for Gram-negative bacteria, these same secretion systems deposit proteins into the periplasm, the space between the two lipid bilayers of their complex outer envelope. Here, we review the ways that commensal and probiotic Escherichia coli are being engineered with additional secretion systems, including some originating from pathogens, outfitted to secrete desired therapeutic protein payloads. We discuss the potential advantages and limitations of each secretion system and potential areas for further investigation. Using examples of variants developed for the treatment of inflammatory bowel diseases, we provide a case study focused on the secretion systems, payloads, and transcriptional regulatory pathways that have been introduced. These studies demonstrate how advances in understanding bacterial secretion systems and synthetic biology are addressing therapeutic payload delivery challenges.