I've been using the foam stoppers on my 2 liter starter flasks for at least a couple of years with zero problems. The open cell foam is essentially a maze of very tiny tunnels. Bacteria have no means of self propulsion. ie, they can't crawl, so any that enter the labyrinth soon get trapped and none make it through to the wort. Gases, however, pass freely in both directions. ie CO2 out and O2 in. The stoppers are washable and can be reused. I give them a soak in hot Oxyclean followed by a cold water rinse then use Starsan as the final sanitizing step. The stoppers offer some added protection over the foil method, but I've never had a problem doing it that way either. I have not found it necessary to autoclave or boil the stoppers.