By this point, it shouldn't surprise you to know that the world is full of bacteria. But the numbers can still be baffling. Take your body: It probably has 37 trillion cells or so, and maybe the same number of bacterial cells.
Now think about your kitchen sponge.
I don't care if you want to think about it or not. You're thinking now. And based on a new study from a team of German researchers, what you're hopefully picturing is something horrible. By horrible, I mean their estimates show that a sugar-cube sized piece of the most bacteria-dense part of the sponge would have 54 billion bacterial cells.
"Despite common misconception, [past research has] demonstrated that kitchen environments host more microbes than toilets," the authors write in the paper, published recently in the journal Scientific Advances. "This was mainly due to the contribution of kitchen sponges, which were proven to represent the biggest reservoirs of active bacteria in the whole house."
(pause for screams)
These researchers study the microbiome of the built environment, meaning the types of bacteria that exist in places humans interact with. They're especially interested in bathrooms and kitchens, places where we spend a lot of our time and dump a lot of bacteria through things like cooking and pooping. Given that we touch sponges, let them sit out, and then literally just wipe them on things, the researchers were interested in just what was going on.
It was a bacteria party, of course.
The bacteria is red, the sponge is blue (Cardinale et al)
The team DNA-sequenced 28 samples from 14 sponges, finding 118 genera (that's the plural of genus, the classification above species) of different kinds of bacteria in total. They also looked at a sponge up close with a method called fluorescence in situ hybridisation coupled with confocal laser scanning microscopy, or FISH-CLSM.
Even after sanitising the sponges, new bacterial colonies seemed to pop up. "No [sanitisation] method alone seemed to be able to achieve a general bacterial reduction of more than about 60%," the authors wrote. "Our data showed that regularly sanitised sponges (as indicated by their users) did not contain less bacteria than uncleaned ones."
Pathogenic bacteria, those that make you sick, made up a small minority of the bacteria in the sponge, according to the paper. But sanitising methods could increase the potential for risk group two bacteria, those that can cause treatable or preventable disease.
There's more work to be done and limitations to the study. Its authors call out that further study is needed to determine just how pathogenic the bacteria in the sponge are, and there ought to be a controlled study on how sanitation changes the sponge's microbiome. Additionally they calculated local bacteria abundances, but didn't average the numbers out over a whole sponge. They also note that this field doesn't have a whole lot of other research in it.
We're not trying to scare you — bacteria are awesome, and most of the ones in this study are fairly harmless.
But the authors do suggest replacing your sponge once a week.