You may have covered this in biology class, but here is a refresher: What's the ideal condition for growth of bacteria and mold? A dark, perpetually damp and relatively undisturbed environment. And here's the bad news. That's exactly what your dorm's bath mat provides.
Fight the grunge by tossing those bath mats in the washing machine once a week. It doesn't matter if they are made entirely of rubber or vinyl-backed carpet fibers. Peel them off the floor and wash them. Just don't do it in a massive combo-load that includes your sheets and skivvies because you'll risk cross-contamination. Instead, use a liberal amount of detergent and as high a water temperature as the material will allow. Then thoroughly air-dry the mats before returning them to the bathroom floor. For extra insurance, scrub that filthy floor first [source: Butler].