In effect, the bacteria have a genetic code of six letters rather than four, perhaps allowing them to make novel proteins that could function in a completely different way from those created naturally.

“If you have a language that has a certain number of letters, you want to add letters so you can write more words and tell more stories,” said Floyd E. Romesberg, a chemist at Scripps who led the work.

Image Floyd Romesberg, a chemist, led the work at the Scripps Research Institute in California. Credit... Scripps Research Institute

Dr. Romesberg dismissed concern that novel organisms would run amok and cause harm, saying the technique was safe because the synthetic nucleotides were fed to the bacteria. Should the bacteria escape into the environment or enter someone’s body, they would not be able to obtain the needed synthetic material and would either die or revert to using only natural DNA.

“This could never infect something,” he said. That is one reason the company he co-founded, Synthorx, is looking at using the technique to grow viruses or bacteria to be used as live vaccines. Once in the bloodstream, they would conceivably induce an immune response but not be able to reproduce.

One possible use of an expanded genetic alphabet is to allow cells to make new types of proteins.

Combinations of three nucleotides in DNA specify particular amino acids, which are strung together to make proteins. The cell, following those instructions, strings amino acids together to form proteins. With rare exceptions, living things use only 20 amino acids.

But there are many amino acids that could possibly be used in proteins, potentially adding new functions. Ambrx, a San Diego company that has filed to go public, is incorporating novel amino acids into certain proteins that are used as drugs in an effort to make the drugs more potent in killing tumors or make treatments last longer in the bloodstream.