I heard the microwave thing too. I keep a spray bottle of vinegar/water/TTO and spray my sponges with that. I also run them thru the dishwasher with a full load. That gets them pretty clean!
I have noticed since I switched to vinegar cleaning, my sponges dont get stinky anymore......not like before.
The guy that was on Oprah a week or so ago, talking about germs, said you really shouldnt keep a sponge longer than a month. He said to soak sponges in a bleach solution, in fact, that was his answer to everything......sheets, toothbrushes.........yuck!