"Semi-synthetic" bacteria with an expanded six-letter genetic code have been successfully engineered by scientists.

These findings could lay the foundations for efforts "to impart life with new forms and functions", the researchers said in their report published in the PNAS journal.

The DNA code of all living organisms is made up of four base "letters" which pair up to form a double helix: G, T, C, A.

However the Escherichia coli bacteria created in the lab by the researchers contains a new synthetic pair made up of two bases called X and Y.

The semi-synthetic organism was able to keep the X and Y base pair after dividing 60 times, leading researchers to believe the bacteria could hold onto the synthetic pair indefinitely.

The team from the Scripps Research Institute in California said this was only the beginning for stable forms of semi-synthetic life.

"We can now get the light of life to stay on," senior author Professor Floyd Romesberg said.

"This suggests that all of life's processes can be subject to manipulation."

How did they do it?

The researchers first successfully created the synthetic X and Y base pair in 2014.

But while the synthetic base pair could be inserted into a living organism, the E. coli did not hold onto the pair for their entire lifespan and died soon after receiving it.

This is a standard DNA molecule with four base letters — the scientists have expanded it to included a synthetic base pair. ( Pixabay )

"If the semi-synthetic organism is going to really be an organism, it has to be able to stably maintain that information," Professor Romesberg said.

So, over the next few years, the team worked on modifying the E. coli to hold onto the synthetic pair indefinitely.

The scientists modified a nucleotide transporter, which transports pieces of the synthetic base pair into the bacteria's DNA and inserts it into the right place, to make it less toxic.

The team also changed their Y base so it would be better recognised by the enzymes that synthesise DNA molecules during replication.

They also used the CRISPR-Cas9 gene-editing tool to engineer the E. coli to mark any cells that dropped the X and Y as foreign invaders, effectively eliminating those cells.

Should we be concerned?

Sorry, this video has expired Molecular and synthetic biologist Nicola Patron explains CRISPR

First of all, there is no need to worry just yet — Professor Romesberg said these molecules have not been designed to work in complex organisms, such as humans.

He also said the actual applications for the synthetic base pair are zero right now — organisms can't currently read or process the base pair into something of value.

The base pair does however prove the concept of life forms being able to take on and keep "alien" bases.

The next step for the researchers will be to study how their six-letter genetic code can be transcribed into RNA, the molecule in cells needed to translate DNA into proteins.

The team also want to continue improving on the technique to hopefully engineer bacteria that can produce new kinds of proteins useful for medicines and materials of the future.