Xylitol is really cool that way. The bacteria waste energy trying to convert it to a form they can use, but they just end up with at a dead end with xylitol-5-phosphate. Once they realize that they need to get rid of it, they convert it back to xylitol (so it can cross the membrane easier and probably to keep the phosphate). Of course, that means the xylitol is back to fool some other nearby bacteria into wasting its energy.

They end up wasting their energy on work that won't help them reproduce and needing to waste more energy getting rid of it.