Kill switches for engineered microbes gone rogue

Stable autonomous kill switches ensure biocontainment of living microbes designed as devices for medicine or the environment. New research outlines two new types of kill switches that address these challenges. The new kill switches are self-sufficient and highly stable in bacterial populations that evolve, and they last over many generations.

They can ensure that only bacteria with intact synthetic gene circuits survive, or confine bacteria to a target environment at 37°C (body temperature) while inducing them to die at lower temperatures, as demonstrated during bacterial exit from a mouse intestinal tract.

For the kill switch, the "Essentializer," researcher’s leveraged previously engineered "memory element" that allows E. coli bacteria to remember an encounter with a specific stimulus in their environment.

The memory element, derived from a bacteria-infecting virus called bacteriophage lambda, either remains silent or reports the occurrence of a signal by permanently turning on a visible reporter transgene that the scientists can trace. The signal can be any molecule, for example, an inflammatory cytokine in the gut or a toxin in the environment.

In their recent study, the team devised a way that ensures the memory element is not lost from the genome during the evolution of the bacterial population over more than a hundred generations. During that time, the genomes of individual bacteria acquire random mutations, which also could potentially occur in the memory element, destroying it in their wake.

The researchers introduced the Essentializer as a separate element at another location in the bacterium's genome. As long as the memory element remains intact, either of the two bacteriophage factors that control its function also inhibits the expression of a toxin gene encoded by the Essentializer.

However, the toxin gene remains somewhat "leaky," still producing residual amounts of toxin that can kill the cell. To keep those residual toxin levels at bay, the researchers included a second gene in their kill switch, which produces low levels of an anti-toxin that can neutralize small amounts of the toxin.

See:

Pamela A. Silver et al. Rational Design of Evolutionarily Stable Microbial Kill Switches. Molecular Cell, November 2017 DOI: 10.1016/j.molcel.2017.10.033

Posted by Dr. Tim Sandle