They may be good for the environment, but reusable grocery bags are also a breeding ground for bacteria. Many responsible shoppers choose their groceries and put them into the same bags during each trip to the store. Bits of food and debris drop to the bottom and form a perfect haven for bacteria. New food becomes contaminated and provides new droppings for the germs already in the bag.
If you plan to use reusable bags, the best idea is to find some that are washable and wash them in hot water to kill those little buggers living in the bottom.