Kitchen sponges are dirtier than toilets.
In fact, it’s common knowledge among microbiologists that the things you use to clean your dishes are the dirtiest objects in your home.
A recent study published in the Nature journal Scientific Reports suggests those spongy bacterial colonies may be even more of a health hazard than we thought. Because of that, the researchers behind the report recommend replacing your sponge every week.
In the new study, researchers conducted a genetic analysis of bacteria on 28 samples from 14 used sponges. They wrote that this was the most comprehensive analysis of the microbiome (the community of bacteria) living on kitchen sponges yet.
Kitchens are generally areas in which new bacteria are regularly introduced, both because of human traffic and food prep. Sponges, which are often warm, wet, and contain traces of old food, are ideal breeding grounds for those bacteria.
The goal of the new sponge analysis wasn’t specifically to find pathogens, which make people sick, but rather just to see what was living on the sponges. The answer? Lots of things.
“Our work demonstrated that kitchen sponges harbour a higher bacterial diversity than previously thought,” the authors wrote. They found that five of the 10 most common bacterial groups had pathogenic potential, including Acinetobacter johnsonii, Chryseobacterium hominis and Moraxella osloensis. They also found pathogenic groups that could lead to infection with E. coli, Staph, or Strep, though those were low abundance.
(They did compare their tests to newly purchased, unused sponges and found those to be basically bacteria free.)
The other surprising result of the study was that cleaning sponges may be less effective than previously thought. Microwaving and boiling sponges can initially reduce about 60% of the bacteria on them, according to the study, but won’t sterilize sponges completely.
And even sponges the scientists tested that had been regularly cleaned in that way didn’t have less bacteria than the uncleaned sponges. The researchers think that resistant bacteria likely survive the sanitation process, then quickly repopulate the sponge, making it harder and harder to remove those bacteria over time.
Microbiologist Philip Tierno previously told Business Insider that “the best way [to clean a sponge] is to put it in a little bleach solution.” However, the new study didn’t evaluate the effects of this method.
Although it’s a good idea to clean a sponge after each use, the researchers “suggest a regular (and easily affordable) replacement of kitchen sponges, for example, on a weekly basis.”
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