Microwaving Your Kitchen Sponge Won’t Save You from Microbes

A weeks-old kitchen sponge probably won’t kill you, but it could make you sick.

The nooks, crannies, and general dampness of a frequently used sponge is an ideal breeding ground for a host of different microbes. To keep the infestation—and smell—at bay, many people will pop a wet sponge in the microwave for a minute. But according to a new study, even that isn’t enough to kill them all. In fact, it could make things worse.

For best results, discard after one week.

Microbiologists in Germany put 14 different types of kitchen sponges to the test, collecting them from households in and around the city of Villingen-Schwenningen. (People voluntarily donated their sponges for science, apparently.) The researchers then cut the sponges up and sampled the top and bottom of each. They used DNA sequencing to determine which types of microbes were living on the sponge and a special type of laser-powered microscopy to see where they were breeding.

Here’s Beth Mole, writing for Ars Technica:

The sequencing harvested more than 220,000 raw DNA sequences, which represented 9 phyla, 17 classes, 35 orders, 73 families, and 118 genera of microbes. As other, smaller studies reported, the researchers found that bacteria in the family Moraxellaceae dominated sponge space, accounting for about 36 percent of microbes in samples. These germs are typical on human skin and have been found all over kitchen surfaces that tend to be cleaned with sponges—counters, fridge shelves, faucets, and stoves. They’re also found on dirty laundry and are known to give clothes a stinky smell.

Otherwise, the researchers found Proteobacteria, Bacteroidetes, and Actinobacteria were primary phyla. Five of the 10 most common operational taxonomic units—basically like species—were closely related to bacteria associated with moderate diseases.

Most of the donors admitted that they didn’t try to sterilize their sponges. But of those who did, their techniques could only eliminate 60% of the microbes at most. What’s more, attempting to clean the sponge only seemed to make more room for disease-causing microbes. “Presumably, resistant bacteria survive the sanitation process and rapidly re–colonize the released niches until reaching a similar abundance as before the treatment,” the researchers wrote.

The best solution, they said, is to get a new sponge, preferably every week.

What do we really mean when we say that we’re clean?