WASHINGTON — Scientists are expanding the genetic code of life, using man-made DNA to create a semi-synthetic strain of bacteria — and new research shows those altered microbes actually worked to produce proteins unlike those found in nature.

It’s a step toward designer drug development.

One of the first lessons in high school biology: All life is made up of four DNA building blocks known by the letters A, T, C and G. Paired together, they form DNA’s ladder-like rungs. Now there’s a new rung on that ladder.

A team at The Scripps Research Institute in La Jolla, California, expanded the genetic alphabet, creating two artificial DNA “letters” called X and Y. A few years ago, the researchers brewed up a type of E. coli bacteria commonly used for lab research that contained both natural DNA and this new artificial base pair — storing extra genetic information inside cells.

The next challenge: Normal DNA contains the coding for cells to form proteins that do the work of life. Could cells carrying this weird genomic hybrid work the same way?