When you use reusable bags you have to be aware of contamination. This mostly happens with raw meats, but all bags should be cleaned from time to time. If I buy meats I always throw the bag in the wash even, if I don't see anything. I know this can be a problem for canvas bags, but it is important for your future purchase safety.
A study at the University of Arizona, in Tucson by Charles P. Gerba PhD, said that "some bags had more E coli that underwear"! Gross right? This happen from meats mostly. He went on to say that the bacteria levels increased when bags sat in hot cars, the heat increases the growth of the bacteria. That does not mean we revert back to plastic bags, but start washing our bags. I have washed ConservingNows bags over and over and they still look brand new. I love that no matter where I go I have my own bags ready.