You need to bring it up with them. make it so its a weekly rotation for cleaning. If they don't follow through, perhaps offer to change chores with them if they can't handle toilets/tubs. To be passive aggressive, dump their items (clothing and towels) wet, on their bed or put all their products/items into a bin back in their room if they don't clean that week; "Well, if you're not able to clean up after yourself like I/we do, then you're not able to use it." There's also other ways, like perhaps spray liquid ass before their shower on the weekend's it's their turn if they don't clean it at all! Obviously these examples become more and more petty until you get actual, responsible adults to be roommates with.

Without a villian their can be no hero.