Controlling genetically engineered bacteria by simple ‘on’ and ‘off’ switches

To prevent genetically modified bacteria from escaping into the wider environment, researchers have developed safeguards in the form of two so-called “kill switches,” which they call “Deadman” and “Passcode.” These kill switches can cause synthetic bacteria to die without the presence of certain chemicals. Image: MIT News

To prevent genetically modified bacteria from escaping into the wider environment, researchers have developed safeguards in the form of two so-called “kill switches,” which they call “Deadman” and “Passcode.” These kill switches can cause synthetic bacteria to die without the presence of certain chemicals.
Image: MIT News

One of the milestones of the bioengineering era is the development of genetically modified bacteria, which are ‘engineered’ to carry specific genes of interest that would help carry out certain tasks. Researchers can incorporate into the bacteria genes with required properties like immunity or recognition of toxins in river waters or something which can improve crop fertilisation.

But before these bacteria can be safely let loose, scientists need to exercise caution and find a way to prevent them from escaping into the wider environment, where they might grow and cause harm.

Researchers at the Broad Institute of MIT and the Wyss Institute of Harvard, recently discovered a way to efficiently program the bacteria to switch between their active and their inactive state whenever needed. They developed safeguards in the form of two so-called “kill switches,” which can cause the synthetic bacteria to die without the presence of certain chemicals.

James Collins, Professor of the Medical Engineering and Science in MIT’s Department of Biological Engineering and Institute for Medical Engineering and Science (IMES), who led the research, revealed that there have been many research efforts during the past year to control bacteria proliferation by reprogramming their whole organism to die in the absence of specific nutrients (i.e. amino-acids).

He explained that this approach was very expensive and laborious, with relatively high failure potential. “In our case, we are introducing standalone circuits that can be popped in to any number of different organisms, without needing to rewire or change much of the genome in order for it to accommodate the switch,” he says.

The team has developed two types of such ‘switches’: the ‘Deadman’, and the ‘Passcode’. They both lead to programmed bacteria cell death but in a different way. Their advantage is that they are stand-alone, so there is no need for constant supervision by their creator scientists.

The Deadman switch

This switch was motivated by the so-called deadman brakes on old trains, which required a conductor to be in constant contact with the handle or pedal in order for the vehicle to move forwards, Collins says.

This switch is part of a bacterial strain that, in the presence of an external chemical, inhibits a constantly expressed toxin from killing the cell. This part is essentially a ‘switch’, which can be active (‘on’-state) or inactive (‘off’-state) based on the expression of two transcription factor genes; the switch can flip between the two states when either one of the transcription factors is expressed. Collins and his team managed to control the transcription factor expression indirectly, through the control of another small molecule. This molecule keeps the switch ‘off’ when it is present, but when it is removed the switch turns ‘on’.

“If the system does get flipped, by removing the small molecule, it would express toxins at a very high level that could then quite rapidly and readily kill off the bug,” he says.

The Passcode

The Passcode is similar to a logic AND gate; it receives as input a set of signals and when these signals form a specific, pre-defined combination, the gate opens and leads to cell proliferation and survival.  If the transcription factors detect that the right combination of small molecules are present in the environment, then the bacteria will survive.“If any of the required inputs are not correct, then the bug will die,” Clement Chan, a postdoc in Collin’s laboratory, says.

The passcode combination needed for the cell to survive scales up to the control of expression of specific transcription factors, again using small molecules, and can be changed by the researchers. This is like having unique access to a device that only you know how to use!

The switches could also be used to protect a company’s intellectual property, Chan says.

According to Collins, the two kill switches have been successfully tested in the E.coli bacterium and researchers now aim to incorporate them into therapeutic and diagnostic tools, in order to target a number of bacterial infections.

The original paper was recently published in the journal Nature Chemical Biology, and can be found here. To read more, click here.

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