Who do I call?

Apparently the people who installed our bathroom either didn't use waterproof grouting, or didn't seal it. So we have some sort of mold/mildew in the grouting. We've tried various cleansers, and it's hard to remove and keeps coming back. Neither one of us has seen this before, it's not like we don't clean it, we do, but the problem keeps returning. We've tried vinegar, bleach, lysol tub and shower cleaner, tile-x, no-name brand cleansers, even just detergent and water. The vinegar worked ok, the lysol best of all, but even that hasn't kept it totally at bay. At this point given our lack of time and expertise, I'm ready to pay someone to take care of the problem, which probably means re-grouting the bathroom. (Actually, both bathrooms, even thought the front shower hasn't had this problem due to infrequent use.)

So who do I call to have something like that done? A plumber? A contractor? Someone else?

Thanks,
-Clueless first-time home owner.