Genetic Entanglement Enables Ultrastable Biocontainment in the Mammalian Gut

Imbalances in the mammalian gut are associated with acute and chronic conditions, and using engineered probiotic strains to deliver synthetic constructs to treat them is a promising strategy. However, high rates of mutational escape and genetic instability in vivo limit the effectiveness of biocontainment circuits needed for safe and effective use. Here, we describe STALEMATE (Sequence enTAngLEd Multi lAyered geneTic buffEring), a dual-layered failsafe biocontainment strategy that entangles genetic sequences to create pseudoessentiality and buffer against mutations. We entangled the colicin E9 immunity protein (Im9) with a thermoregulated meganuclease (TSM) by overlapping the reading frames. Mutations that disrupted this entanglement simultaneously inactivated both biocontainment layers, leading to cell death by the ColE9 nuclease and the elimination of escape mutants. By lengthening the entangled region, refining ColE9 expression, and optimizing the TSM sequence against IS911 insertion, we achieved escape rates below 10^-10 as compared to rates of 10^-5 with the nonentangled TSM. The STALEMATE system contained plasmids in E. coli Nissle 1917 for over a week in the mouse gastrointestinal tract with nearly undetectable escape rates upon excretion. STALEMATE offers a modular and simple biocontainment approach to buffer against mutational inactivation in the mammalian gut without a requirement for engineered bacteria or exogenous signaling ligands.