For tougher stains on really grimy white grout, use oxygen bleach as a grout cleaner. You’ll find this cleanser is most often sold in powdered form; popular brands include OxiClean and Charlie ...

Mold or mildew are organic stains that thrive in the damp corners ... Then scrub with the brush, making sure to get the bristles into the grout itself. Rinse thoroughly and let the area dry.