Genetic engineers have designed strains of E. coli that can survive only in the presence of a compound that doesn’t exist in nature—meaning the bacteria’s odds of escaping the lab alive are limited. Dan Mandell, a research fellow in genetics at Harvard Medical School who co-authored a paper about the findings in Nature this week, explains. David Guston, co-director of the Consortium for Science, Policy and Outcomes at Arizona State University, discusses the ethical and safety implications of the work.