“Zombie” cells created by transplanting genomes into dead bacteria

A living, synthetic cell was created by transplanting a complete genome into a dead bacterium, bringing it back to life. The breakthrough could help synthetic biology fulfill its huge but still distant promise of engineering organisms to create sustainable fuels, medicines 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 make desired chemicals. In an effort to create more versatile engineered microbes, in 2010 researchers 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 has been very difficult to be sure whether a cell is actually controlled by a synthetic genome rather than its original genome, because bacteria often absorb genetic material from the environment and add it to their own genomes in a process called horizontal gene transfer.

To work around this problem, John Glass at 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, which is used as a chemotherapy drug to kill cancer cells by damaging their DNA, and tested it on the cells of a simple bacterium. Mycoplasma capricolum.

“The cell is still healthy, but because it can no longer reproduce and the genome is no longer functional, it is destined to die or is already dead,” says a member of the team Zumra Seidelalso at JCVI.

They also added a synthetic version of the genome of another bacterium, Mycoplasma mycoidesto dead cells using a technique they call whole genome transplantation.

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

“We take a cell with no genome, and it’s functionally dead. But by adding a new genome, that cell is resurrected,” says Glass.

Kate Adamala at the University of Minnesota calls the work a technical breakthrough. “They put the genomic payload into a non-living recipient, so they don’t get any help from the host’s own repair mechanisms. They’ve basically boot-booted the cell,” he says. “It’s an amazing job.

It also blurs the line between life and non-life, says Adamala. “The business model of any proper living cell is to metabolize and replicate. These functions have become the hallmarks of life. [cell’s genome] in this it does very little residual metabolization and most certainly does not replicate. What, then, is the true sign of life?”

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

The technique has only been tested so far Mycoplasmabut the team sees it as a proof of principle that could allow synthetic organisms to be created more quickly to act as mini chemical factories, producing therapeutic drugs or carrying out environmental remediation.

“For a long time, we’ve had the ability to assemble very large pieces of synthetic DNA, but we haven’t been able to get 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 actually perform it.”

Like Nyerges at Harvard Medical School, says the work addresses a major challenge in synthetic biology. “This technology makes genome transfer a more predictable and reliable strategy, potentially opening up many downstream applications in other species,” he says.

Towards more complex organisms such as yeast or E. coli can be challenging because these organisms have a cell wall that Mycoplasma missing and larger genomes, but Glass is optimistic that the technique will succeed in those as well.

“If it works for one type of organism, it will probably work for another,” he says, and his lab is investigating ways to remove and replace cell walls. “In the right growing conditions, E. coli it creates a new cell wall,” he says.

With synthetic biology, there’s always the potential for biosafety issues, Nyerges says. The Mycoplasma the species used in the study are pathogenic in goats and cattle, but he says none of the modifications are expected to increase virulence.

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

topics:

• biotechnology /
• microbiology