We use them to clean the dishes we eat off, but your kitchen sponge is a damp, crumb-filled environment that is perfect for bacteria to thrive in. Should you opt for a washing up brush instead?
Many species of bacteria know how to rough it out. Some thrive deep within the Earth's crust, or inside boiling hydrothermal vents, and others even call frozen tundra their home.
Ask most bacteria where they'd really like to live though, and a kitchen sponge would probably top the list.
Yes, it turns out that the very tool we use to clean our plates and glasses is packed full of microbial life. Sponges are bacterial heaven. They're warm, damp, and full of nutritious food crumbs for the microbes to feast on.
In 2017, Markus Egert, a microbiologist at Furtwangen University in Germany, published new data on the bacterial microbiome of used kitchen sponges. He discovered a whopping 362 species of microbes in those sponges. In some places, the density of bacteria reached up to 54 billion individuals per square centimetre.
"This is a very big amount, it's similar to the number of bacteria that you would find in a human stool sample," says Egert.
Sponges are full of holes and pockets – each providing a niche for a community of microbes to settle down in.
Lingchong You, a synthetic biologist at Duke University, and his team, used computers to model the complex environment of a sponge for a 2022 study. He found that sponges with pockets of varying sizes encouraged the most microbial growth. His team then replicated these results by growing different strains of E.coli in cellulose sponges.
"They found out that having a variety of different pore sizes in kitchen sponges is something that really matters [for encouraging bacterial growth]," says Egert. "This makes sense because, for microbes, you have individualists [such as] bacteria that like to grow on their own, and you have bacteria that need the company of others. Inside a sponge you have so many different structures or niches that everyone gets happy."
Sponges definitely make good homes for bacteria. However, it does not necessarily follow that these utensils are also a health risk to us. Bacteria exist everywhere – on our skin, in the soil, and in the air around us. Not all are harmful, in fact many perform vital jobs. The important question is, therefore, are the bacteria found in sponges even worth worrying about?
In Egert's 2017 study, he sequenced the DNA of the most common species. Although it was not possible to identify the exact species of each bacterium, five out of ten of the most prevalent species were closely related to bacteria known to cause infections in people with compromised immune systems. Special cleaning measures such as heating in a microwave or rinsing with hot, soapy water didn't really help either, as although it eliminated some bacteria, it allowed other, more resistant strains, to thrive.
"Our hypothesis is that cleaning measures might lead to a kind of selection process, where the few survivors can grow up to large numbers again," says Egert. "If you do this a couple of times, then this might lead to a selection of bacteria that are better adapted to cleaning."
It's important to note that none of the bacteria found by Egert are linked to food poisoning or severe sickness. In fact, 90% of hospitalisations from food-borne illnesses can be traced to just five pathogens, three of which are bacteria – Escherichia coli, Salmonella and Campylobacter. Thankfully, these are quite rare in sponges.
"We only found potentially pathogenic bacteria, so bacteria that can be dangerous to people with weak immune systems, or the elderly or children," says Egert. "Usually, for a healthy person, the bacteria inside the kitchen sponge are not harmful."
In 2017, Jennifer Quinlan, a professor of food safety at Prairie View A&M University in the US, and her colleagues, collected kitchen sponges from 100 homes in Philadelphia. She found that only 1-2% of those sponges contained bacteria linked to food poisoning in humans. The ones that did contained only small amounts of harmful bacteria.
This finding was backed up by a 2022 study in which Solveig Langsrud, a scientist at the Norwegian food research institute Nofima, compared the bacteria found in washing up sponges and brushes. She found a common set of harmless non-pathogenic bacteria in both kinds of utensil, including Acinetobacter, Chryseobacterium, Enhydrobacter, Enterobacteriaceae and Pseudomonas. However, brushes contained much less bacteria overall.
These potentially pathogenic bacteria can then also be transferred from your sponge to the plates and utensils or surfaces
"The vast majority of bacteria on sponges do not cause illness, they're just going to make it smell. They're going to make it unpleasant over time," says Quinlan. "Having said that, there is a possibility that if you use a sponge to wipe up raw meat or raw chicken juice that you could have some of those pathogenic bacteria on there, and studies have found that pathogens can be isolated from kitchen sponges."
So, while the bacteria growing in your sponge are not usually harmful, if dangerous bacteria such as Salmonella arrive on the scene, the sponge's structure makes it an ideal place for this pathogen to grow.
There's evidence this is the case. In Langsrud's study, when the researchers introduced Salmonella to kitchen sponges, they thrived, but when they added this bacteria to brushes, they died. That could be because brushes will often dry out more effectively between uses, killing the Salmonella bacteria, while sponges can still remain damp inside if they are used daily.
These potentially pathogenic bacteria can then also be transferred from your sponge to the plates and utensils or surfaces.
So, how often should we change our kitchen sponge? Quinlan argues that from a hygiene perspective, ideally you should replace it on a weekly basis, but there are things you can do to prolong its service.
"There's two easy ways of cleaning them. You can put them in the dishwasher at the end of the evening, or you can microwave them for a minute until you can see steam coming off. That will kill the majority of the pathogens."
Certainly, studies have shown that putting your sponge through the dishwasher or the microwave can reduce the bacterial load and can be more effective than soaking your sponge in bleach. But as Egbert's study demonstrated, this can then select for more resistant strains and so cleaning will become less effective over time.
Putting a sponge in boiling water and disinfectant will also kill most bacteria, although some may survive, especially those that form protective gooey biofilms. But this approach has been found to be effective at reducing potential pathogens such as Salmonella.
Other tips include not storing the sponge in the sink, so that it is allowed to dry in between uses, and really squeezing out the moisture and getting rid of food crumbs. However, some may consider using a different tool entirely in order to clean dishes.
"I wouldn't use kitchen sponges at all. It actually does not make sense to me to use such an item inside the kitchen environment," says Egert. "A brush is much better because it contains less bacteria and it dries out more easily. It's also easier to clean."
BBC