A few filthy pans, and your kitchen sponge can turn into a gunky, foamy mess pretty quickly – but how long should you continue using said gunky, foamy mess? A recent study has placed these kitchen essentials under the microscope and found them to be awash with bacteria, suggesting that a regular refresh mightn't be a bad move for one's health.

The study was carried out by researchers from Germany's Furtwangen University, Justus Liebig University and the Helmholtz Centre, and is claimed by the scientists to be the first comprehensive study of kitchen sponge contamination. It involved placing 14 used sponges from households in Germany's Villingen-Schenningen area under the microscope, where the team found 362 different types 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," explained Egert.

Among these were Acinetobacter johnsonii, Chryseobacterium hominis and Moraxella osoloensis, all of which can cause infections, particularly for people with a weakened immune system. The latter can also cause sponges to smell. On the upside, fecal bacteria and others that cause food poisoning and dysentery were hardly detected at all.

But what's really interesting about the study was the finding that washing out kitchen sponges might do more harm than good. Those sponges that users claimed to clean regularly by placing them in the microwave or rinsing them out contained much higher levels of potentially pathogenic bacteria. The scientists say that this is likely a consequence of the sponge's porous structure, which creates an ideal environment for communities of microorganisms to grow.

"Sometimes the bacteria achieved a concentration of more than 5 times 1010 cells per cubic centimeter," explained Egert. "Those are concentrations which one would normally only find in fecal samples. And levels which should never be reached in a kitchen. These high concentrations can be explained by the optimal conditions the bacteria find in the sponge: besides the large surface area for growth, there are high levels of moisture and nutrients from food residue and dirt."

The researchers say that the environments most susceptible to the wrath of soiled sponges are where people with weakened immune systems are present, like hospitals and retirement homes. But as a general rule, they suggest throwing away your sponge once a week rather than washing it out mightn't be a bad idea.

The research was published in the journal Scientific Reports.