In addition to the naturally occurring nucleotides adenine, thymine, guanine and cytosine, which form the rungs of DNA's double-helix structure, the bacterium carried two more base-pair partners.

For more than a decade, scientists have been experimenting with so-called unnatural base pairs, or UBPs, saying they may hold the key to new antibiotics, future cancer drugs, improved vaccines, nanomaterials and other innovations. Until now, however, those experiments have all been conducted in test tubes.

"These unnatural base pairs have worked beautifully in vitro, but the big challenge has been to get them working in the much more complex environment of a living cell," lead study author Denis Malyshev, a molecular and chemical biologist at Scripps, said.

The new genetic material did not appear to be toxic to the bacteria, and it only remains in the organism's genome under specific lab conditions. In a natural environment, the molecules - nucleoside triphosphates - degrade and disappear in a day or two. Once they disappear, the bacterium reverts back to its natural base pair arrangement.

Still, experts said insertion of the synthetic materials into genome was a milestone.

"This definitely is a significant achievement," said Ross Thyer, a synthetic biologist at the University of Texas at Austin, who was not involved in the research. "What I'm most excited about is how this will help us answer some bigger evolutionary questions: Why has life settled on a specific set of bases?"

Malyshev and colleagues went about creating the semi-synthetic bacterium by genetically engineering a stretch of ring-like DNA known as a plasmid. The engineered plasmid contained usual complement of A,T, G and C nucleotides, as well as two man-made molecules, which join to form a new rung on the DNA ladder.

Getting the bacteria to maintain those molecules in their DNA was far harder. Like all genetic material, the new molecules degrade over time, and the have no way of producing the foreign synthetic materials.

If this man-made genetic material was to survive within the bacteria and be passed on during reproduction, the study's authors reasoned that they would have to surround the cells with a solution containing the new material. They would also have to create a doorway through which the synthetic molecules could enter the cell.

"The resulting bacterium is the first organism to propagate stably an expanded genetic alphabet," the authors wrote.

The next step will be to give experimental cells a reason to want to keep the synthetic genetic coding.