All of us have this predetermined perception about sponges that they are perfectly fine during all times of their life, to clean our dishes with. These sponges require a lot of precision and considering that they are in direct contact with your dishes, it is necessary that you take the required precaution.
Since sponges are required for cleaning away the mess from dishes, they themselves can serve to be breeding grounds for a lot of mold and bacteria. This is not something you would like on your sponge, which is why cleaning them up is the best way forward. Just pop the sponge into a microwave for half an hour and you will see all the bacteria go away. Just pull it out after a couple of minutes to ensure that you don't catch it when it is hot.