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August 15, 2017

What can destroy the bacteria in your kitchen sponge? Pretty much nothing, study suggests

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    A new study urges people to replace their sponges regularly, cautioning that common sponge-cleaning strategies, such as running it through the microwave or dishwasher, are useless against certain germs—but others are pushing back, saying the research is nothing more than fearmongering.

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    Study: Throw out that sponge

    For the study, published in Scientific Reports, researchers assessed 14 used sponges and identified 362 different species of bacteria living in them. According to the researchers, the microbes were densely packed together: Each cubic inch of sponge-space hosted about 82 billion bacteria.

    Markus Egert, a microbiologist at the University of Furtwangen and lead researcher for the study, said, "That's the same density of bacteria you can find in human stool samples." He added, "There are probably no other places on earth with such high bacterial densities."

    To their surprise, the researchers found that "regularly sanitized sponges"—those that were washed in soapy water, run through the dishwasher, or zapped in the microwave—"did not contain less bacteria than uncleaned ones." In fact, they found the sanitized sponges tended to harbor more of a particular bacterium called Moraxella osleonsis osloensis. While the bacterium is largely harmless, it can pose a danger to people with compromised immune systems, the New York Times reports.

    According the Times, that microbe is also linked to the unpleasant smell found in dirty laundry—and possibly the funky smell a sponge emits after long use. As a result, the researchers advised people to consider throwing out their sponges every week or so, when they begin to smell. "When people at home try to clean their sponges, they make it worse," Egert explained.


    But not everyone agrees with the advice—or the study. Writing in Forbes, Judy Stone, an infectious disease specialist, said the study is "scare mongering," noting that "unless you are seriously immunocompromised," you don't need to worry about making sure your sponge is "disinfected" so long as it is sanitized.

    Stone cited the small sample size of sponges—a limitation acknowledged by the study authors—and pointed out that Moraxella is "normal skin flora." Further, she pointed out that "we do not live in an otherwise sterile environment. We, and all our surroundings, are covered in bacteria."

    For instance, she pointed out that according to Charles Gerba, a microbiologist from the University of Arizona, cellphones have 10 times more bacteria than toilet seats. Furthermore, a study of the cellphones of 200 health care workers found that 95 percent of their phones carried hospital-acquired bacteria.

    Stone argued that most people don't need to keep throwing away old sponges because most people don't need to have sterilized sponges—they should be safe with merely sanitized sponges. To do so, she cited two methods that USDA says will kill 99.9 percent of bacteria: microwave the sponge for two minutes (making sure it is damp and has no metal on it) or running it through the dishwasher with hot water and a dry cycle. 

    Study aimed 'to create awareness ... not fear,' researcher says

    For their part, the researchers stressed that the study was designed to improve how the bacterial populations of sponges are measured—not to assess disinfection measures. Egert added that he hopes to compare various methods of disinfection in a follow-up study.

    Ultimately, Egert said the study was "mainly thought to create awareness, ... not fear." For those who remained concerned about dirty sponges, however, he recommended that for regular cleaning, "tools that soak less water, dry faster, [and] have smaller inner surfaces might indeed be better."

    Egert added, "You should not become hysteric and afraid of your kitchen sponge now. ... But if you're already ill or have ill people at home, you should be more careful" (Klein, New York Times, 8/4; Stone, Forbes, 8/6; Klein, New York Times, 8/11).

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