‘Deadman’ and ‘Passcode’ kill switches keep synthetic bacteria in check

Two new containment strategies to keep genetically modified bacteria from escaping into the environment are reported in a study published online this week in Nature Chemical Biology. The synthetic gene circuits-called ‘Deadman’ and ‘Passcode’-are customizable and may have a variety of industrial and environmental applications.

Genetically engineered bacteria can potentially be used to accomplish a range of important tasks, such as monitoring toxins in rivers and improving crop fertilization. However, before modified cells are used in these ‘real world’ scenarios, scientists also want to make sure that the cells cannot grow on their own, which could disrupt native environments. This kind of control mechanism is typically called a ‘kill switch’ as the bacteria are meant to die without a necessary input provided by the scientists.

In this study, James Collins and colleagues report two new kill switches that can efficiently kill Escherichia coli. The ‘Deadman’ switch builds on past work creating a bacterial strain that needs an external chemical to prevent a continuously expressed toxin from killing the cell, with the alterations reported here making the system faster and more reliable. The ‘Passcode’ switch uses hybrid transcription factors, in which one section of the transcription factor protein responds to a chemical signal and another controls a specific segment of DNA, to enable researchers to mix and match the chemical signals used to control the cell. The authors note that this customization should allow researchers to tailor bacteria according to the specific application desired.

Finally, they suggest that the Passcode switch may also be particularly useful as a tool for protecting intellectual property, since unauthorized growth of bacterial strains without the appropriate passcode molecules would induce cell death.