Your Sponge Is Foul

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I like doing dishes but hate the vile blobs used to do them. Sponges are deeply disgusting, and though you may try to convince me otherwise I have always felt this way, based on little more than my senses (i.e. after a few days of use, they tend to smell, look, and in all likelihood, taste bad).

This is not terribly surprising. We use highly absorbent blobs to scrub off food waste, and then cast them aside—maybe soapy and wrung-out but basically still soaked through with food waste—so they can sit around damp at room temperature and harbor all kinds of life. When confronted with some especially bad ones, I’ve sometimes felt that inflicting the sponge upon the dirty dishes might actually make them “dirtier,” in some relevant sense. In desperation, once their grossness gets too palpable to ignore, we might run them through the dishwasher or throw them in the microwave, hoping this will mitigate the obvious grossness, and start the process over again.

I felt validated in my sponge-disgust while reading a recent study published in the journal Scientific Reports, with the eye-catching title of “Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species.


As the authors note, there was already a rich literature around Life On Sponges before they began their study:

Despite common misconception, it was demonstrated that kitchen environments host more microbes than toilets12,13,14. This was mainly due to the contribution of kitchen sponges (Fig. 1A, B), which were proven to represent the biggest reservoirs of active bacteria in the whole house ...


Markus Egert, the University of Furtwangen microbiologist who led this research, made that same point a little more colorfully. Reflecting on the fact that 82 billion bacteria can occupy a single cubic inch of used sponge, he told the New York Times: “That’s the same density of bacteria you can find in human stool samples ... There are probably no other places on earth with such high bacterial densities.”

The article is fascinating and worth reading in full, especially if you like looking at neat three-dimensional graphics that visualize the bacteria teeming on the surface of a sponge’s tissue. By analyzing the genetic material present in used kitchen sponges, they exposed, among other interesting things,

  • the sheer diversity and quantity of bacteria that exist within the kitchen sponge,
  • the prevalence of potentially pathogenic bacteria within that bustling hive of life,
  • the inefficacy of usual methods of household sponge sanitation—though “cleaning” killed off some forms of bacteria, it promoted the growth of potentially pathogenic bacteria.

To be clear: this study is pretty modest in its claims and does not make any attempt to link presence of these potential pathogens to any particular human illnesses, and suggests that future studies focus on actual pathogenicity of kitchen sponges. It does, however, suggest that hospitals, nurseries, schools heed the existing evidence and dispose of sponges regularly. It recommends that a kitchen sponge user swap out weekly.

If, for reasons of thrift or waste reduction, you’d rather not burn through a new sponge every few days, there may be better alternatives. I have always preferred brushes with nylon bristles because they seem to dry better and trap less crap. I asked Egert about his personal preferences. While his study only examined synthetic sponges, he cautioned that sponges made out of biodegradable materials like cellulose might be even more prone to microbial contamination because they can be “directly eaten” by bacteria. “In contrast, tools that soak less water, dry faster, have a smaller (inner) surface, such as your brush, might indeed be better for regular cleaning,” he said. “We also use brushes at home and regularly clean them in the dish washer...more research is definitely needed here!”


As Egert admits, there’s much more to learn here, but his work confirms what your nose and eyes can tell you: Your sponge is gross, you should definitely reconsider the way you “clean” it, and are probably due to throw it right now.