Yep, housekeeper recommends Tilex Mold and Mildew

OP here - thx much everyone!

For really bad grout, I mix dilute bleach with water and scrub with a toothbrush.

Tilex mold and mildew (the orange one) worked for us. It takes a few applications to get it really clean, and it smells vile, so I only use it when absolutely necessary, but it does the job.

Bleach + grout brush (they're a little hard to find, no idea why).

Once it's immaculate, use a grout sealer! Once your grout is sealed it won't mold or mildew as much. I assume you're having these issues because the builder/ tile guy used cheaper grout. There is stain resistant grout out there, but it's pricier.

Oxyclean solution - spray on, after 20 minutes scrub.... We have a rotating brush that attaches to a drill, it is amazing. Our grout has never looked better.

im interested in this too! i read in an online forum that resolve carpet cleaner actually works very well. you are supposed to apply it with a firm toothbrush and then rinse. i havent tried it yet but im hopeful! ive tried many of the products specifically sdvertised for grout but havent been happy with any of them.

My shower grout is getting gross. I use my bathroom fan and I wipe down the tile after the shower after I'm done to get rid of the extra moisture, but my grout is still mildewing. Anyone have experience with this? What's the best way to clean mildew out of grout lines? The grout is star Quartz if that makes any difference. TIA!