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Dr. Phillip Tierno is the author of The Secret Life of Germs and the director of New York University's microbiology department. He says that sponges and dishrags can hold literally billions of bacteria, so they should be kept no longer than one month.
"You tend to build up debris, food particles and fibers that you cannot eliminate," he explains. "If you de-germ by using a good household disinfectant after each use, you probably could use the sponge longer than a month."