Washing a sponge in the dishwasher also isn't enough to kill germs unless you have an extremely high-heat germicide cycle. You can soak them between cleanings in a solution of no more than 1 cup of bleach to 1 gallon of water, or vinegar and water. Soak them in the bleach solution for about 5 minutes or in the vinegar solution for 20 to 30 minutes. Rinse thoroughly and let air dry.