Believe it or not, the filthiest, most germ-infected surface on earth isn’t a pig sty, restaurant dumpster or public gas-station toilet seat. It’s right inside your house, and everything you think you know about cleaning it is wrong.
A Scientific Reports study published in July examined 14 used kitchen sponges from private homes and was able to identify 82 billion bacteria — comprising 362 different species — living in just one cubic inch of space. That’s seven or eight times more bacteria than the number of people living on earth.
“[Used kitchen sponges contain] the same density of bacteria you can find in human stool samples,” said Dr. Markus Egert, a microbiologist at Germany’s Hochschule Furtwangen University and co-author of the study. “There are probably no other places on earth with such high bacterial densities.”
However, some feel this isn’t necessarily something to get worked up about. One article claims the sample size is too small to reach a broader conclusion, and that shoelaces and cell phones are just as bacteria-laden. Another claims that even environments considered sterile can make people sick. That said, it’s hard to refute the idea that used sponges provide a perfect ecosystem for breeding bacteria: They’re warm, they’re wet, and they provide a hearty variety of food sources from stuck-on leftovers.
Many believe boiling a kitchen sponge or putting it in the microwave or dishwasher for a couple of minutes results in sterilization, but according to the study, this is the worst possible method. (“When people at home try to clean their sponges, they make it worse,” said Dr. Egert. ) While the weaker bacteria is killed by the microwave’s heat, the stinkiest and most pathogenic bacteria survive and, with reduced competition from less potent bacteria, multiply with abandon.
“The remaining species, which are, for unknown reasons, more resistant to the cleaning methods than the ones that get killed, proliferate again and grow up to higher shares than before,” said Dr. Egert. “It might be similar to the use of antibiotics, where some bacteria can survive due to resistance against the drug.”
Further, according to the study, up to 10 bacterial species found in the sponges — including E. coli, Klebsiella, Staphylococcus and Salmonella — can cause disease and infections in those with weaker immune systems, including children and the elderly.
One of the most prolific microbes discovered was Moraxella osloensis, a bacterium found on human skin that’s recognized in the odor of dirty laundry, especially when left to hang indoors. When sponges are boiled or microwaved, Moraxella osloensis survives and replicates, making the implements even stinkier than they were before.
For those concerned about pathogenic microbes on eating surfaces, Dr. Egert recommends throwing kitchen sponges away after about a week. (“Don’t be afraid of your sponge, but be aware that it contains billions of potentially pathogenic germs,” Egert says.) For the especially frugal, he says running it through a washing machine in hot water with bleach and detergent may render it safe to use somewhere less “hygiene-sensitive.” Like your toilet.