A replicating ‘alien’ life form has been created by scientists who introduced DNA molecules not found in nature to a common bacterium.

The Escherichia coli (E. coli) bugs are able to grow and reproduce as normal despite containing two extra letters in their genetic code.

In future, the research could lead to creation of microbes capable of manufacturing entirely new proteins with potential applications in medicine and nanotechnology.

But the rapid advance of ‘synthetic biology’ also raises the worrying prospect of new life forms escaping from the laboratory with unpredictable consequences.

Nature’s genetic code consists of a DNA alphabet of just four ‘letters’ – adenine (A), cytosine (C), guanine (G) and thymine (T).

An additional letter, uracil (U), is found in DNA’s close cousin molecule, RNA.

The A, C, G and T building blocks form ‘base pair’ partnerships whose sequences write the text of the ‘book of life’.

Lead scientist Floyd Romesberg, from the Scripps Research Institute in La Jolla, California, US, said: “Life on earth in all its diversity is encoded by only two pairs of DNA bases, A-T and C-G, and what we’ve made is an organism that stably contains those two plus a third, unnatural pair of bases. This shows that other solutions to storing information are possible and, of course, takes us closer to an expanded-DNA biology that will have many exciting applications – from new medicines to new kinds of nanotechnology.”

The work, reported in the journal Nature, involved overcoming a billion years of evolution to get the expanded genetic alphabet into living bacteria.

It started in 2008 when Romesberg’s team of researchers succeeded in replicating unnatural base pairs in a test tube. They also managed to transcribe the semi-synthetic DNA into RNA, a first step towards translating a new genetic code into a protein.

But performing the same trick in the complex environment of a living cell presented much greater challenges.

To solve the problem the scientists first added the artificial base pair molecules – d5SICS and dNaM – to a fluid solution outside the cell.

Then they used a special transporter molecule, made by a species of micro-algae, to import them into the bacteria.

“That was a big breakthrough for us – an enabling breakthrough,” said co-author Denis Malyshev, also from the Scripps Research Institute.