The toxic stuff comes from them dying when exposed to oxygen creating an ammonia spike.
They convert nitrates to nitrogen gas and complete the nitrogen cycle.
The best way to culture them is in a deep sand bed in a remote sump that can be disconnected from your system should a problem arise. By deep, I mean 8+ inches. It is typically not a good idea to have a DSB in your display. I recommend that you research deep sand beds before you incorporate one in your system. These bacteria can also live deep in the pores of rocks.