What gets dirtier the more you clean it? A sponge

Those using hot water or the microwave to sanitise sponges may be surprised to hear this could be worsening the situation. This was revealed in a recent study published in Scientific Reports and conducted by Furtwangen University, Justus Liebig University and the Helmholtz Centre.

Professor Markus Egert of Furtwangen University, who headed up the study, looked at 14 used sponges from the city of Villingen-Schenningen under the microscope — and 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,” said Professor Egert.

The researchers found bacteria such as Acinetobacter johnsonii, Moraxella osoloensis and Chryseobacterium hominis. They are typical on human skin and can cause infections, which makes them particularly dangerous for the elderly and those with weak immune systems. Bacteria in the family Moraxellaceae made up about 36% of microbes in samples. This is not only responsible for the smell of dirty laundry, but by using sponges riddled with bacteria, it can also be found on many surfaces around the kitchen.

Germany has 40 million private households, so assuming there is at least one sponge per household, there could be 40–80 million mini bacteria-breeders there alone.

However, while many would argue this is all the more reason to sanitise them, sponges that were regularly washed showed significantly higher levels of bacteria.

While cleaning sponges may produce a short-term solution that temporarily minimises the number of germs, the numbers that grow back are even higher than before.

“Presumably, resistant bacteria survive the sanitation process and rapidly recolonise the released niches until reaching a similar abundance as before the treatment,” the authors concluded.

Sponge material has multiple pores that harbour these germs and provide ample space for them to repopulate. Professor Egert suggested the concentration sometimes reached more than 5 x 10¹⁰ cells per cubic centimetre. Although these levels are only usually present in faecal samples, Professor Egert stated that faecal bacteria was scarcely detected; however, they remain a cause for concern in the 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,” he said.

Hygiene instincts may encourage people to clean their sponges regularly, but Professor Egert concluded that sponges are “microbial incubators in the household” and that it is important to replace them on a weekly basis.