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........

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