According to the New York Times, mold needs two things to grow, water and food, meaning that anything with organic particles can feed the mold. This can include dirt, skin cells, cotton, cardboard, dust, hair, paper, and wood.
Toilets naturally include many conditions that mold requires to thrive. It is important to keep areas where mold might grow clean and well ventilated. Since a toilet has water in it, the focus will have to be on removing any food sources for mold. A mix of one part bleach to ten parts water is effective at cleaning mold, and should be mixed just before cleaning and used immediately.