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.
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.