The genetically modified microbe has been programmed to swim down fine cracks in concrete and once at the bottom it produces a mixture of calcium carbonate and a bacterial glue. This glue combines with the filamentous bacterial cells, ultimately hardening to the same strength as the surrounding concrete and essentially “knitting” the building back together.
I see a Stephen King novel here. The microbes come under control of a remote, malicious entity and begin to destroy concrete rather than repairing it.
Or worse, they start to knit together windows and doors so that they don’t open, seal vents in buildings, close drains and our civilisation sinks into oblivion.