The idea is that the wild-type Streptococcus that already lives in your mouth produces lactic acid, which is bad for your teeth. The modified bacteria produces

The idea is that the wild-type Streptococcus that already lives in your mouth produces lactic acid, which is bad for your teeth. The modified bacteria produces ethanol instead, which is neutral for your teeth. I don't believe there's any assertion that the ethanol is helpful to your teeth, just that you're going to have some bacterial colony in your mouth, so they are trying to change the byproducts of that colony to something less bad.