• Sponges are the most bacteria-covered objects in most homes (more so than toilets), according to microbiologists.
• One recent study found even more microbial diversity on sponges than researchers expected.
• Sponge-cleaning methods like boiling and microwaving are less effective than most people think.

If the dishes are stacking up in the sink, you’re probably reaching for that sponge and soap.

But when was the last time you changed that sponge out? Remember, kitchen sponges are dirtier than toilets.

In fact, it’s common knowledge among microbiologists that the things you use to clean your dishes are the dirtiest objects in your home.

A study published in August in the journal Scientific Reports suggests those spongy bacterial colonies may be even more of a health hazard than we thought. Because of that, the researchers behind the report recommend replacing your sponge every week.

The researchers conducted a genetic analysis of bacteria on 28 samples from 14 used sponges. They wrote that this was the most comprehensive analysis yet of the microbiome — the community of bacteria — living on kitchen sponges.

Kitchens are where new bacteria are regularly introduced, both because of human traffic and food preparation. Sponges, which are often warm, wet, and contain traces of old food, are ideal breeding grounds for those bacteria.

The goal of the new analysis wasn’t to find pathogens, which make people sick, but just to see what was living on the sponges. The answer? Lots of things.

“Our work demonstrated that kitchen sponges harbor a higher bacterial diversity than previously thought,” the authors wrote.

They found that five of the 10 most common bacterial groups had pathogenic potential, including Acinetobacter johnsonii, Chryseobacterium hominis, and Moraxella osloensis. They also found pathogenic groups that could lead to a staph or strep infection, though those weren’t as abundant.

They compared their tests to newly purchased, unused sponges and found those to be basically bacteria free.

The other surprising result of the study was that cleaning sponges may be less effective than previously thought. Microwaving and boiling sponges can initially reduce about 60% of the bacteria on them, according to the study, but won’t sterilize them.

And even sponges the scientists tested that had been regularly cleaned in that way didn’t have fewer bacteria than the uncleaned sponges. The researchers think that resistant bacteria most likely survive the sanitation process then quickly repopulate the sponge, making it harder to remove them over time.

Philip Tierno, a microbiologist, previously told Business Insider that the best way to clean a sponge was to “put it in a little bleach solution.” However, the new study didn’t evaluate the effects of that method.

Although it’s a good idea to clean a sponge after each use, the researchers “suggest a regular (and easily affordable) replacement of kitchen sponges, for example, on a weekly basis,” according to the study.