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Sponges are essential for thorough cleaning. We use them to scrub dishes, our kitchen counters, and basically any grimy surfaces that need a scour. But sponges can also harbor as many as 45 billion microbes per square centimeter, and if you’ve been using your scrubby for more than a week, some scientists say it may be time to get a new sponge. We spoke to a microbiologist to find out why sponges are such a hotbed for bacteria—and the best way to keep your dishwashing gear clean.
How much bacteria lives in your kitchen sponge?
Trina McMahon, PhD, a microbiologist and professor at the University of Wisconsin–Madison, tells me that sponges are the ideal environment for bacterial growth. They get wet frequently, often don’t dry out properly, and as a result, layers of bacteria can form within little crevices. According to a study published in 2017, there could be as many as 362 kinds of bacteria living in your kitchen sponge.
“You’re giving them food by getting food on the sponge,” McMahon says. “And they’re warm, because you’re using hot water, so it really is all the perfect ingredients for bacteria to grow.”
How harmful is the bacteria?
A paper published in the Journal of Environmental Health in 2012 found that kitchens and sponges housed even more bacteria than bathrooms did. And unless you’re using antibacterial dish soap, hand-washing won’t sanitize your dishes completely—however, scrubbing with soap, writes journalist Ashley Laderer for Insider, will “lift [bacteria] off surfaces so that they can be washed away by water.”
McMahon tells me not to be alarmed. For the most part, she says, “The bacteria that are on your sponge are not pathogens. They’re not going to make you sick.” Old, dirty sponges may smell bad—but they’re unlikely to have harmful bacteria.
Still, it’s wise to practice good kitchen hygiene. A study by Judy Ikawa and Jonathan Rossen found that sponges and dishcloths can serve as a vehicle for spreading bacteria over kitchen surfaces, which can increase the risk of cross-contamination and foodborne illness when food is mishandled. Salmonella via sponge? No thank you.
What’s the best way to sanitize your sponge?
To stem the growth of bacteria, McMahon recommends sanitizing sponges frequently by microwaving them, boiling them, or soaking them in a solution of diluted bleach. These methods won’t kill off all the microbes on your sponge, but will still result “in a more than 99.9% reduction in the number of bacteria,” write Ikawa and Rossen.
Does putting sponges in the dishwasher achieve the same effect? “The microwave creates so much heat. It’s like boiling it,” McMahon says. “I don’t think that the dishwasher gets quite hot enough or really kills a lot of stuff.” Microwaving a wet sponge, though, generates steam within its pockets, creating enough heat to kill off bacteria. Dishwashers are designed for cleaning hard surfaces like dishes, but may not be as effective for soft, porous sponges.
To sanitize your sponge, run it under water for a few seconds to avoid a fire hazard, then microwave it for a minute on high. You can also place it in a pot of water and boil it for five minutes, or soak it in a solution of bleach (Ikawa and Rossen tested with a ratio of ¾ cup per gallon), also for five minutes. If your sponge contains metal, avoid the microwave at all costs and stick to the last two methods.
Okay, but how often should I replace my sponge?
“I would recommend every week to every two weeks. I tend to replace mine every two weeks,” McMahon tells me. “I think the best indicator is just whether the sponge smells bad or is slimy.” She throws her sponges away when they look gross, and says that most microbiologists she knows would probably say the same thing.
Ultimately, practicing good hygiene and making the effort to sanitize your sponge often is the best way to reduce the microbial activity not just on your scrubby, but all around your kitchen. And when in doubt? Throw it out.
