Hi Poohbear,
I would try a very small amount of bleach and a mild dish washing soap I use Dawn that works best for me just make sure it is a very small amount of bleach no more than a teaspoon followed by the dish soap rinse then hang it to dry over night Please make sure you sterlize the mouth piece as well Hope this works for you. I had a camelbak and thats how I cleand mine. All the best.
John B.