‘Zombie’ cells created by transplanting genomes into dead bacteria

A living synthetic cell was created by transplanting a complete genome into a dead bacteria, bringing it back to life. This breakthrough could help synthetic biology fulfill its enormous, but still distant, promise of engineering organisms to create sustainable fuels, pharmaceuticals and new materials.

Synthetic biology involves tweaking biological systems or creating new ones to introduce new functions, such as rewriting yeast DNA so that organisms produce desirable chemicals. In an effort to create more versatile engineered microbes, researchers in 2010 synthesized a bacterial genome and then transplanted it into a living cell, creating what they called the first synthetic cell.

But there was a problem. It was very unclear whether the cell was actually governed by the synthetic genome rather than its original genome, because bacteria frequently take up genetic material from the environment and add it to their own genome in a process called horizontal gene transfer.

To get around this problem, John Glass of the J. Craig Venter Institute (JCVI) in La Jolla, California, and his colleagues decided to first kill the host cell — or at least its genome.

The researchers turned to a chemical called mitomycin C, used as a chemotherapy drug to kill cancer cells by damaging their DNA, and tested it on cells of a simple bacteria. Mycoplasma capricolum.

“The cell is still healthy, but since it can no longer reproduce and the genome is no longer functional, it is destined to die or it is already dead,” explains Zumra Seidel, also a member of the JCVI team.

Then they added a synthetic version of the genome of another bacteria, Mycoplasma mycoidto dead cells using a technique they call whole genome transplantation.

Some bacteria began to grow and divide normally and genetic testing showed that they carried the synthetic genome. This makes them the first living synthetic bacterial cells constructed from non-living parts, say the researchers, who call them “zombie cells” because they have been reanimated after death.

“We take a cell without a genome and it’s functionally dead. But by adding a new genome, that cell is resurrected,” says Glass.

Kate Adamala of the University of Minnesota calls the work a technical breakthrough. “They place a genome payload into a non-living recipient, so they don’t get any help from the host’s own repair mechanisms. They’ve essentially reset that cell,” she says. “It’s incredible work.”

It also blurs the line between life and non-life, Adamala explains. “The business model of any living cell is to metabolize and replicate. These features have become the hallmark of life. [cell’s genome] this makes for very little residual metabolism and certainly does not replicate. What then is the true mark of life?

Team member Elizabeth Strychalski of the National Institute of Standards and Technology in Gaithersburg, Maryland, suggests that biology can systematically operate across a porous boundary between life and death. “I hope it gets people thinking about how life is a series of processes and if we apply an engineering mindset to it, we can look at our living system and ask ourselves what processes we actually need to achieve the end goal we’re trying to achieve.”

So far, the technique has only been tried on Mycoplasmabut the team sees it as proof of principle that could enable the more rapid creation of synthetic organisms that function as mini chemical factories, making therapeutic drugs or carrying out environmental remediation work.

“For a long time, we’ve had the ability to assemble very large pieces of synthetic DNA, but we haven’t been able to deliver them to a place where they can do useful things,” says Strychalski. “It’s like having a script for a Shakespeare play, but not being able to perform it.”

Akos Nyerges of Harvard Medical School says this work addresses a major challenge in synthetic biology. “This technology makes genome transfer a more predictable and reliable strategy, potentially paving the way for many subsequent applications in other species,” he says.

Moving towards more complex organisms such as yeast or E.coli can be difficult, because these organisms have a cell wall that Mycoplasma and larger genomes are missing, but Glass is optimistic the technique will achieve that as well.

“If it works for one type of organism, it will probably work for another,” he says, and his lab is studying ways to remove and replace cell walls. “Under good growing conditions, E.coli will create a new cell wall,” he says.

There is always the possibility of biosafety issues with synthetic biology, Nyerges says. THE Mycoplasma The species used in the study are pathogens in goats and cattle, but he says none of the modifications are expected to increase virulence.

Strychalski says existing best laboratory practices ensure the risk of pathogen escape is minimal.