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Kitchen sponges are breeding grounds for bacteria. And, no, microwaving dirty sponges doesn't really help.

Kitchen sponges
Sponges have been identified as the key reason that kitchens tend to host more microbes than bathrooms.

It’s long been known that kitchen sponges are reservoirs of bacteria. They are warm, wet and absorbent — an excellent breeding ground for microorganisms.

In fact, sponges have been identified as the key reason that kitchens tend to host more microbes than bathrooms.

A recent study from Germany, however, reports that kitchen sponges may harbor even more bacteria than we thought — a lot more. Researchers genetically analyzed 14 used sponges from different households and found they contained an astounding 362 different types of bacteria.

Fortunately, most of the bacteria are not harmful to humans. But some are.

The study’s authors also discovered that practices commonly suggested for keeping sponges clean, such as microwaving or boiling them, are not all that helpful.

They recommended instead that we replace our kitchen sponges with a new one every week.

Advanced technologies

In the study, the German researchers did not identify the bacteria on the 14 sponges by culturing them in a lab, as had been done in previous research on this topic. Instead, they used a genetic sequencing technique that is able to identify many more bacteria, including some that can’t be grown on culture plates.

The researchers also used a sophisticated type of imaging — known as a confocal laser scanning microscopy — to get a detailed look at the number and density of the bacteria.

These techniques revealed that the used kitchen sponges contained billions of bacteria on their surfaces and within their interior cavities — specifically, 54 billion bacterial cells per cubic centimeter.

The testing also identified 362 different “operational taxonomic units” (basically species) of bacteria.

“What surprised us was that five of the 10 which we most commonly found belong to the so-called risk group 2, which means they are potential pathogens,” said Markus Egert, a molecular biologist at Furtwangen University, in a released statement. (A pathogen is a microorganism that can cause human disease.)

The most concerning of these were Acinetobacter johnsonii, Moraxella osoloensis and Chryseobacterium hominis. All three can lead to human infections, although primarily in people with weakened immune systems.

More than a third of the bacteria identified in the samples were, like Moraxella osoloensis, from the family Moraxellaceae. These microbes are typically found on human skin, so they most likely got on the sponges from the hands of the people using them in the kitchens. (By the way, this particular family of bacteria are why laundered clothes and sheets sometimes have an unpleasant, “stinky” odor, especially if the items have not been dried quickly.)

The good news, however, is that bacteria that cause food poisoning — specifically, salmonella, proteus and campylobacter — were not found on the samples. 

Limitations and implications

The study’s biggest limitation is, of course, that it analyzed only 14 sponges collected in one region of Germany. A larger sampling from other areas of the world might have much different results.

Still, the study’s findings offer a good reminder to practice careful hygiene when cleaning kitchen surfaces, particularly since the study also found that attempts to clean sponges by microwaving or washing them led to only a short-term decrease in the number of germs.

In fact, the study’s authors say such attempts at sanitizing sponges might cause potentially harmful bacteria to grow back in even greater numbers.

Instead of attempting to clean kitchen sponges, they suggest replacing the items weekly.  Health officials also recommend using paper towels to clean up kitchen surfaces — or cloth towels that are then laundered on the hot cycle of a washing machine. 

FMI: The study was published online in the journal Nature Scientific Reports, where it can be read in full

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Comments (7)

Never a fan

I've never been a fan of using sponges in the kitchen, especially in the roles of countertop cleaning, dishwashing, etc. This study confirms my admitted bias against their use.

I much prefer the combination of a bristle brush and a washcloth, both of which can actually be sanitized to a significant degree. Dishwasher detergent commonly contains an antiseptic in the form of a hefty percentage of bleach, and I run my kitchen bristle brush through the dishwasher on a regular basis. The wash cloth can be tossed in the washing machine, using hot water, and then thoroughly dried.

Allow them to dry?

Of course, I don't know the effect on bacteria, but I find that squeezing excess water out of the sponge when I'm finished does a remarkable job of helping the sponge dry overnight. Unsqueezed sponges tend to stay wet & begin to stink fairly quickly.

Dry those sponges

I totally agree with Brian S. That's exactly what I do. After I use a sponge for dishes or wiping countertops (I use a different color sponge for those two jobs to keep them apart) I rinse and wring them out a few times and let them air dry. They will eventually wear out mechanically from use, but that happens BEFORE I detect any noticeable oder.

Drying

This is my thought as well... if the sponge is completely dried between uses, does that cut down the bacteria levels? The idea of tossing sponges each week is untenable from a resource-use perspective.

Drying won't kill many types of bacteria

The Escherichia coli Genetic Stock Center stores many of their strains on slips of dried paper. They can be rehydrated and grown, little the worse for wear. Other species have a similar resistance to drying.

Many types of bacteria (although not E. coli) form highly resistant spores under harsh conditions. They revert back to growing bacteria when conditions improve.

Finally, just because the sponge seems dry to you, doesn't mean that it doesn't have tiny microdrops of water that are large enough for microscopic bacteria to live in.

Bottom line: Drying will be effective for some species of bacteria, but not for others. It's still safest to replace them on a regular basis, much as that seems wasteful. It's better than risking your family's health.

Please don't fear monger with stories like these

I have been rinsing, squeezing out and leaving sponges to dry for over 30 years. My mother has been doing the same for 60. In addition to my personal, anecdotal evidence, I have never heard of one illness or death that has been tied specifically to "sponges not being replaced regularly." Where are the statistics to indicate this is a legitimate threat and not simply fear mongering? The writer chooses to cite only one study which she then admits could be totally meaningless.

Oops. My bad. This story is in the "opinion" section and we live in the age of "alternative facts" anyway, so I guess it's alright to publish whatever unsubstantiated drivel that comes along.

Microwaves do help

180f for 180 seconds kills everything. Throw it in a cup with water, 5 minutes on high. Sanitized.