Synthetic biologists are fitting the genomes of microorganisms with synthetic gene circuits to break down polluting plastics, non-invasively diagnose and treat infections in the human gut, and generate chemicals and nutrition on long haul space flights. Although showing great promise in the laboratory, these technologies require control and safety measures that make sure the engineered microorganisms keep their functional gene circuits intact over many cell divisions, and that they are contained to the specific environments they are designed for.