A kitchen sponge sitting on dirty dishes in the sink
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Your kitchen sponge harbors zillions of microbes. Cleaning it could make things worse

That sponge in your kitchen sink harbors zillions of microbes, including close relatives of the bacteria that cause pneumonia and meningitis, according to a new study. One of the microbes, Moraxella osloensis, can cause infections in people with a weak immune system and is also known for making laundry stink, possibly explaining your sponge’s funky odor. Researchers made the discovery by sequencing the microbial DNA of 14 used kitchen sponges, they report this month in Scientific Reports. Surprisingly, boiling or microwaving the sponges didn’t kill off these microbes. Indeed, sponges that had been regularly sanitized teemed with a higher percentage of bacteria related to pathogens than sponges that had never been cleaned. This could be because pathogen-related bacteria are more resistant to cleaning and rapidly recolonize the areas abandoned by their susceptible brethren—similar to what happens to our gut after an antibiotic treatment, the scientists say. When the researchers put the sponges under the microscope, they discovered that a single cubic centimeter could be packed with more than 5 x 1010 bacteria, which corresponds to about seven times the number of people inhabiting Earth. Such bacterial densities, the scientists say, are found only in feces. But don’t worry—the solution to a clean sponge is simple: Just replace it every week.