The surprising structural reason your kitchen sponge is so disgusting

It turns out that the common kitchen sponge is a better incubator for bacterial communities than a laboratory petri dish, because the structure of the environment in which they grow affects interactions between microbial species.

According to a new study, it’s not just the trapped leftover food that makes the microbes thrive inside of it, but the structure of the sponge itself. Some bacteria prefer to live in a diverse community, while others prefer to exist only with bacteria like themselves, so an environment that allows both kinds to live their best lives leads to the strongest biodiversity.

Soil provides this sort of optimal mixed-housing environment, and so does your kitchen sponge. Published in Nature Chemical Biology, these results have implications not just for kitchens, but also for industries that make products using bacteria.

In nature, bacterial communities mix to varying degrees and soil provides all the nooks and crannies needed for different populations to grow without much interaction from their neighbours. But when humans grow bacterial species to manufacture products – like alcohol, biofuel and medications – we just throw them together into a structureless goop on a plate or inside a big vat.

“Bacteria are just like people living through the pandemic – some find it difficult being isolated while others thrive,” says Dr Lingchong You, professor of biomedical engineering at Duke University in the US. “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 maximise its overall coexistence.”

In a series of experiments, the scientists showed that various microbial species can affect each other’s populations depending on factors of their structural environment like complexity and size.

They barcoded the genomes of about 80 different strains of E. coli so that they could track their population growth, and then mixed the bacteria in various combination on laboratory growth plates. These plates had a variety of potential living spaces for the bacteria – ranging from six large wells to 1,536 tiny wells.

The design of these wells could mimic the different environments that bacteria might prefer to grow in; the large wells approximated environments in which many microbial species can mix freely, whereas the small ones were similar to spaces where they could keep themselves isolated.

Interestingly, regardless of the habitat size, the results were the same: the bacteria evolved into a community of only one or two surviving strains. But it was the intermediate-sized wells that resulted in the greatest diversity of those that survived.

“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),” explains You. “But the intermediate portioning allowed a maximum diversity of survivors in the microbial community.”

This indicates why a kitchen sponge is such an optimal habitat for microbes: it mimics the degrees of separation found in healthy soil by providing different layers of separation, in combination with different sizes of communal spaces.

To prove this the researchers also ran the experiment with a strip of regular household sponge and found that it was an even better incubator of microbial diversity than any of the previous laboratory equipment used.

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

These results create a framework for scientists working with diverse bacterial communities to test which structural environments might work best for their research. It also has implications in industry, as structural environments must be considered by those that use bacteria in their processes.