This post is part of a series. For Part 1, Danger Lurks in Your Kitchen – The 5 Nastiest Places, click here.

By: Heather Williams

When you think about the dirtiest room in your house, the mind generally wanders to the bathroom.  While that is certainly an area where bacteria are known to mingle, it isn’t the room with the most bacteria.  The answer will probably surprise you.  The Kitchen hosts the largest microbiome in your entire home.  Kitchen environments are host to more microbiomes than your bathroom toilet.  This is largely due to the presence of the innocent looking household sponge.  In fact, sponges are only second in highest bacterial load in your whole house only to drain traps.  If that disturbs you, you are not alone.  German researchers delved into exactly what is going on in your kitchen sponge.

Microbiome, and Why It Matters to You

Researchers analyzed the microbiome found in used kitchen sponges to identify the relative amount and types of bacteria found in this household item.  A microbiome is not necessarily a bad thing, but a general term describing a community of microorganisms that inhabit the environment compose of bacteria, fungi, and viruses.  Some microbiomes are healthy, keeping pathogens in check, like the microbiome found on your skin.  But there are also bad microbiomes that contain an unproportionate number of harmful bacteria and viruses.  They analyzed newly bought unused sponges alongside 28 used sponges from private households.  Households provided a brief history of the sponge use and age as well as any cleaning activity performed on the sponge.

What’s on That Sponge?!

The kitchen sponge is an excellent breeding ground for bacteria.  Both harmless and harmful bacteria enjoy this atmosphere.  The warm, moist environment with small crevices throughout the body allows many places for bacteria to colonize and grow.  This all happens at the microscopic level, too small for our eyes to see.

 Bacteria present in the microbiome of a kitchen sponge could change based on what they are used to clean, the microbiome on the skin of the people using the sponge, and a variety of other factors.  Common bacteria found on your kitchen sponge includes Campylobacter ssp., Enterobacter cloacae, Escherichia coli, Klebsiella ssp., Poteus ssp., Salmonella ssp., and Staphylococcus ssp..  Image analysis demonstrated bacterial densities of up to 5.4 X 1010 cells per cm2 on the sponge.  If that sounds like a big number, consider that it is 7 times the number of people inhabiting the planet Earth!  That’s a lot of bacteria!

You might believe that adding soap to the sponge must cancel out anything that might be lurking within.  But consider that your kitchen sponge not only acts a reservoir, but also a disseminator of harmful microorganisms, which can lead to cross-contamination of food, hands, and surfaces.  This means that when you wipe down the counter or the sink with an older sponge acting like a bacterial incubator, you are spreading the bacteria around, transferring it to all of those surfaces.

A Sponge Life

Think about the journey your sponge goes through.  It sits idle, slowly drying out from the last use.  Right there on your sink counter.  It takes quite a while for a sponge to fully dry up.  The nature of the sponge is to hold water.  It’s a sponge.  That’s what sponges do.  Meanwhile, bacteria are happily reproducing.  We can’t see it, but it’s happening.  In fact, you may even notice a change in smell of the sponge after a few days of use.  It is probably not the water that you are smelling.  It’s a whole colony and microbiome growing in its new home.

The family meal is now complete.  It’s time to do the most dreaded kitchen chore. The dishes.  You run water in the sink, wet the sponge, and lather up with some dish soap.  The clean up begins.  Plates, spoons, forks, glassware, cutting boards.  Gasp!  Was there raw chicken on that cutting board?  It’s all getting clean.  That is what soap is for, right?  The dishes have been washed, the sponge is wrung out and placed in its home on the sink counter.  It stands ready until it is dutifully needed to be used again.

But wait?  Did you wash your hands after washing the dishes?  Was this before or after your dried and put the dishes away?  With that bacteria in the sponge smearing all over the place, some of it is left behind on surfaces waiting for its next host.  Maybe it’s a towel.  Maybe it’s a person.

You look at the sponge and think, “We’ve had this sponge for a while.  It’s about time to clean it.”  Maybe you boil water and throw the sponge in to sterilize it.  Maybe you toss it in the dishwasher.  Maybe you even decide to microwave it.   Whatever method you choose, unfortunately will not likely help.

De-bugging the Sponge

Researchers found that contrary to what you expect, regular cleaning of kitchen sponges does more harm than good.  This activity seems to affect the microbiome structure in a negative way.  In fact, a greater proportion of Chryseobacterium haminis and Moraxella osloensis were found in sponges that were regularly sanitized.

Methods such as boiling and microwaving sponges showed a reduction in bacterial load by 60% in laboratory testing initially, but this same statistic was not achieved in used sponges.  It seemed that while bacteria were initially killed, those that survived replicated and new bacteria recolonized at a fast rate, repopulating the microbiome of the sponge.

“Presumably, resistant bacteria survive the sanitation process and rapidly re-colonize the released niches until reaching similar abundance as before the treatment.”  Researchers explain this effect is similar to what happens in the gut after antibiotic therapy.  The rapidly recolonized and many times harmful bacteria recolonize faster than the non-pathogenic bacteria.

To Sponge or Not to Sponge

At this point you might be reconsidering the ongoing use of a kitchen sponge in your home.  While some may opt for a “one time use and launder dish cloth” or some other disposable method, if you like your sponge keep your sponge.  Simply toss it out fairly regularly and skip cleaning it.