A pile of simple polyurethane sponges, commonly used in the kitchen for cleaning dirty dishes. They ...

Did you ever to stop to think about how much bacteria is in your kitchen sponge?

While kitchen sponges are soaked in water and soap every day, they are not as clean as one may think. Turns out, they’re the biggest breeding grounds for active bacteria than anywhere else in your house–surpassing even your toilet–according to a study published late last month in Scientific Reports.

“Kitchen sponges are likely to collect, incubate and spread bacteria from and back onto kitchen surfaces, from where they might eventually find their way into the human body, e.g. via the human hands or contaminated food,” the researchers said.  “In addition, direct contact of a sponge with food and/or the human hands might transfer bacteria in and onto the human body, where they might cause infections, depending on their pathogenic potential and the environmental conditions.”

Using 14 used kitchen sponges that were collected from private households in Germany, researchers extracted DNA samples from the sponges after separating their bottom and top parts with sterile instruments.

By looking at the DNA as well as RNA in samples, the team identified 362 different species of bacteria. Of those bacteria, the scientist found that five represented “pathogenic potential” and accounted for 41.9 percent of the dataset. Some of the bacteria categorized as a risk group included Acinetobacter johnsonii, Acinetobacter pittii, and Acinetobacter ursingii. Five other non-pathogenic species founded included species such as Brevundimonas diminuta, Chryseobacterium haifense, and Pseudomonas cremoricolorata.

When asked what methods were used to clean their sponges, the sponge users told the researchers that they either heated the sponge in a microwave or rinsed it with a hot, soapy water. But along with microwave treatment, it has been shown that cleaning a sponge by boiling it is a “reasonable hygiene measure” because it has been “shown to significantly reduce the bacterial load of kitchen sponges,” according to the study.

“However, our data showed that regularly sanitized sponges (as indicated by their users) did not contain less bacteria than uncleaned ones. Moreover, “special cleaning” even increased the relative abundance of both the Moraxella– and Chryseobacterium…”

Chryseobacterium hominis and Moraxella osloensis, are two of the ten dominant bacteria that the researchers described as being closely related to a risk group. Besides being classified as such, Moraxella is the bacteria that is known to generate malodor in laundry and may be the reason why kitchen sponges sometimes emit a smelly scent as well.

While boiling your sponge or microwaving it may seem like a good way to sanitize your kitchen sponge, researchers found contradictory information. They found that those sponges that were sanitized regularly did not contain fewer bacteria than those that were not. In fact, those sponges that were sanitized regularly showed significantly greater proportions of Chryseobacterium hominis, and Moraxella osloensis.

“When people at home try to clean their sponges, they make it worse,” Markus Egert, a microbiologist at the University of Furtwangen in Germany, and one of the authors of the study told the New York Times.

No matter what cleaning method was tried, none achieved a general bacterial reduction of more than about 60 percent, the researchers said.

This recent study revealed to the research team that kitchen sponges harbor more bacteria than priorly believed. They also concluded that current sponge sanitation methods may not be adequate in reducing the bacterial load in sponges but might only increase some species of bacteria. While there are no studies out currently that provide more in depth details on the effectiveness of different sponge cleaning methods, the researchers recommend that kitchen sponges be changed regularly, such as on a weekly basis.

Feature Image via Flickr/Horia Varlan