Mol. Cell 68, 686–697.e3 (2017) https://doi.org/10.1016/j.molcel.2017.10.033

Credit: ELSEVIER

Bacterial kill switches involve induction of lethal genes unless certain environmental conditions are met, and can be used to contain genetically engineered microorganisms and prevent the spread of their engineered genes. Stirling et al. have now developed two new kill switches for use in Escherichia coli, termed 'essentializer' and 'cryodeath', that utilize a toxin–antitoxin system in which toxin CcdB is expressed if certain environmental conditions are not met, while antitoxin CcdA is constitutively expressed to counteract any leaky CcdB expression when those conditions are met. The essentializer system maintains the presence of a previously developed 'memory element' that records and maintains a signal output through the use of two transcription factors. If the memory element is lost and neither transcription factor is present, the essentializer system induces production of CcdB and the cell dies. Meanwhile, the cryodeath system represses CcdB expression at 37 °C, but induces it at lower temperatures. Application of cryodeath in the mammalian gut ensures that the engineered bacteria are no longer viable once excreted into the environment. By extensively screening libraries of genetic elements that make up these kill switches, the authors have developed systems that are particularly stable to evolutionary pressures and that can be maintained for many generations without continuous input of exogenous survival factors.