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:
Posted by Dr. Tim Sandle