Bacteria that secrete minerals are a well-known tool for "healing" cracked limestone statues, and the process got a Dutch scientist thinking. He theorized that concrete seeded with bacteria and a substance they transform into calcium carbonate would create a material that's able to seal cracks as they form. It turns out he's mostly right, the Economist explains.
The bacteria employed have to be able to withstand highly alkaline environments, and it took some testing to show that they remain alive and able to produce minerals even after the cement has hardened. The little cells do seal up small cracks, though they only survive for a few weeks. But researchers are working on another theory: that encasing the bacteria in clay extends their lifespans.