The researchers bought Knox brand gelatin at a local supermarket and dissolved it in the solution with the bacteria. When they poured the mixture into molds and cooled it in a refrigerator, the gelatin formed its bonds — “just like when you make Jell-O,” Dr. Srubar said. The gelatin provided more structure, and worked with the bacteria to help the living concrete grow stronger and faster.

After about a day, the mixture formed concrete blocks in the shape of whatever molds the group used, including two-inch cubes, shoe box-size blocks and truss pieces with struts and cutouts. Individual two-inch cubes were strong enough for a person to stand on, although the material is weak compared to most conventional concretes. Blocks about the size of a shoe box showed potential for doing real construction.

“The first time we made a big structure using this system, we didn’t know if it was going to work, scaling up from this little-bitty thing to this big brick,” said Chelsea Heveran, a former postdoc with the group — now an engineer at Montana State University — and the lead author of the study. “We took it out of the mold and held it — it was a beautiful, bright green and said ‘Darpa’ on the side.” (The mold featured the name of the project’s funder.) “It was the first time we had the scale we were envisioning, and that was really exciting.”

When the group brought small samples to a regular review meeting with officials from Darpa, they were impressed, Dr. Srubar said: “Everyone wanted one on their desk.”

Stored in relatively dry air at room temperature, the blocks reach their maximum strength over the course of days, and the bacteria gradually begin to die out. But even after a few weeks, the blocks are still alive; when again exposed to high temperature and humidity, many of the bacterial cells perk back up.

The group can take one block, cut it with a diamond-tipped saw, place half back in a warm beaker with more raw materials, pour it in a mold, and begin concrete formation anew. Each block could thus spawn three new generations, yielding eight descendant blocks.

The Department of Defense is interested in using the reproductive ability of these “L.B.M.s” — living building materials — to aid construction in remote or austere environments. “Out in the desert, you don’t want to have to truck in lots of materials,” Dr. Srubar said.