Why Your Kitchen Sponge Is the Ideal Breeding Ground for Bacteria

Researchers have discovered why the common kitchen sponge is such a good incubator for bacteria.

A new study has found that it's not just leftovers that make dish sponges breeding grounds for bacterial cultures—the structure of the sponge itself plays a role.

In a series of experiments, researchers from Duke University in North Carolina examined how the complexity and size of the structural environment around colonies of microbes affect their population dynamics.

They found that for bacteria that thrive in diverse communities and for bacteria that prosper when alone, the humble kitchen sponge was an optimal environment—better than any equipment their lab could offer.

Lingchong You, professor of biomedical engineering at Duke, said in a statement: "Bacteria are just like people living through the pandemic—some find it difficult being isolated while others thrive.

"We've demonstrated that in a complex community that has both positive and negative interactions between species, there is an intermediate amount of integration that will maximize its overall coexistence."

The research could help industries that use bacteria for tasks such as cleaning up pollution to work out which structural environments they should use.

In nature, the way communities of microbes mix varies according to medium. Soil, for example, provides nooks and crannies for different populations to grow with only limited interaction with neighboring cultures. The surface of a leaf has a similar structure.

However, when we use bacteria in the production of commodities such as alcohol or medication, we tend to throw different species together in structureless goop on a plate, in a petri dish or even in a giant vat.

The experiments conducted by You and his team, detailed in a paper published in the journal Nature Chemical Biology, demonstrate that these industries could prosper by taking a more structural approach to storing bacteria.

To track the growth of a population of bacteria, the Duke researchers genetically engineered 80 strains of E. coli to glow different colors. Some of the populations were integrated in living spaces that had six large wells, allowing them to mix freely. Others were placed in more than 1,500 tiny wells, mimicking conditions where communities are kept separate.

You said: "The small portioning really hurt the species that depend on interactions with other species to survive, while the large portioning eliminated the members that suffer from these interactions—the loners. But the intermediate portioning allowed a maximum diversity of survivors in the microbial community."

The team found that regardless of the size of the wells, the ultimate outcome was the same: in each type of well, only two species survived.

This is because in the smaller wells a handful of strains evolved into a single community in which only two prospered, whereas in the larger wells a broad range of species was whittled down to just two by the experiment's conclusion.

Not only do the results point the way in the development of structures to encourage bacterial growth, they indicate why the kitchen sponge is such a good environment for bacteria—it mimics the different degrees of separation found in healthy soil.

To confirm this, You and his team reran the experiment with a kitchen sponge and found it was a better incubator than any of the other environments they had tested.

You said: "As it turns out, a sponge is a very simple way to implement multilevel portioning to enhance the overall microbial community. Maybe that's why it's a really dirty thing—the structure of a sponge just makes a perfect home for microbes."

Bacteria Sponge
Left: Species of bacteria engineered to glow so they can be identified. (Right): stock image of a kitchen sponge. Researchers have discovered that the structure of a dish sponge helps bacteria grow. Andrea Weiss, Zach Holmes and Yuanchi Ha, Duke University/GETTY