The researchers have tweaked it's genetic properties such that it only begins to germinate when it comes in contact with the highly-specific pH of concrete. Once the cells germinate, they are programmed to crawl as deep as they can into cracks in the concrete, where quorum sensing lets them know when enough bacteria have accumulated.Engineered Bacteria Can Fill Cracks In Aging Concrete (via JWZ)
That accumulation lets the bacteria know they've reached the deepest part of the crack, at which point the cells begin to develop into bacterial filaments, cells that produce calcium carbonate, and cells that secrete a kind of bacterial glue that binds everything together. Once hardened, the bacteria is essentially as strong as the concrete itself, restoring structural strength and adding life to the surrounding concrete.
The bacteria also contains a self-destruct gene that keeps it from wildly proliferating away from its concrete target, because a runaway patch of bacterial concrete that continued to grow despite all efforts to stop it would be somewhat annoying