Mildew and mold result from too much moisture on a surface.
Tiles dry quickly, so they aren’t prone to the problem.
Grout won’t grow mold or mildew, either, unless it has broken down enough to become damaged or porous.
Mildew in grout is a good indicator that it’s time to re-grout; the old stuff just can’t keep moisture out anymore.
Here’s the irony: Products with bleach are the number one cause of grout breakdown.
For mildew and mold problems, replace old grout before moisture seeps so far into the wall or flooring that it causes serious damage.
To keep mildew and mold from coming back, make sure the tile and grout are wiped after each shower or bath and stop using products containing bleach and other harsh chemicals.