What's the worst that could happen?

Roads that actively try to kill you.

Oh, I wasn't even thinking in that direction. I just meant bacteria that produce concrete as a byproduct of their normal life processes, getting loose into the environment and creating concrete everywhere. That other thing would be fun too, though.

In field tests, freeze-dried bacteria were applied to the surface of 3-foot by 3-foot (about 1-meter by 1-meter) plots, and urea and calcium chloride were sprayed on top.

So a construction worker with kidney stones peed on it.
Sounds about right.

Its just small humans then.

These bacteria are soil bacteria, so they're already in the environment. It seems like they produce biocement in small quantities and under specific conditions, so the real innovation here seems to be packaging them in a "just add water"-style container