Concrete is the most popular building material on the planet, probably because it's very strong, easy to work with and cheap, being made mostly of rocks. There are some durability issues with concrete, though, and to make it stronger, Dutch scientists have added self-healing capsules to help buildings heal cracks on their own.
Bacteria can fix cracks in concrete the same way that humans can, by filling them in with more concrete materials, or in this case, limestone. To get the bacteria into the concrete in the first place, you just mix in some nutrients and lots of little ceramic spheres full of inactive bacterial spores, which remain inactive until they get wet. When a crack in the concrete opens up and the spores get a drink of water, they get to work turning the calcium in the nutrients into limestone, filling in cracks.
These bacteria, being bacteria, aren't big or powerful enough to fix large or wide cracks: the maximum crack size that they can fill is a width is about 0.5mm. That's not really a problem, though, because outside of a major earthquake, all big cracks start as small cracks, and most of them develop into big ones as water gets into them. So, if the bacteria can seal up small cracks as they form, you'll be preventing bigger cracks from happening later on.
Adding bacteria and nutrients and stuff to concrete mixes isn't free, and it could add as much as 50% to the materials cost. This sounds like a lot, but the concrete itself usually only accounts for 1-2% of the total cost of building something. Maintaining that building, on the other hand, is far more expensive, so spending a little extra on some helpful bacteria up front could save lots and lots of money in the long term.
Bacteriacrete is currently undergoing optimization and outdoor durability testing, but it could be commercialized in as little as two years.