Did you know that the number of bacteria inside 2 cm3 of kitchen sponge tissue is greater than the number of human beings that have ever lived on our planet?
The common-place kitchen sponge captured global headlines when a study , authored by myself and colleagues from Gießen and Munich, uncovered just how much bacteria the trusty sponge actually hosts.
The study was the first molecular or cultivation-independent study on the microbiome of used kitchen sponges. In other words, we took the sponges straight from the kitchen and were not reliant on growing the bacteria in a laboratory.
Admittedly, with just 14 sponges under investigation, our study was very small and the results and generated hypothesis need to be confirmed. Nevertheless, our data supports the long-held belief that kitchen sponges are hugely contaminated and are a hygiene-risk in the kitchen.
But, if this is well-known, why did the research capture the minds of the global media? I have put forward three key reasons below.
1. The sponge hosts a population beyond our wildest imagination
The study determined local cell densities of up to 54 billion per cm3 – this is a similar density to human stool samples.
Further cultivation-based studies by our group proved that cell densities in used kitchen sponges can reach up to 1012 CFU per gram. Weighing approximately 10 g, the total number of microorganisms inside a sponge might reach 1013, which is the estimated number of microorganisms of a human being. From a microbiome point of view, the kitchen sponge appears to be a fully-fledged member of the household!
For humans, seeing is believing, and microbial contaminations are easily overseen or neglected, because bacteria and other microorganisms are invisible. The study enabled us to visualize the active bacteria like never before using Fluorescence-in-situ-Hybridization (FISH) of 16S rRNA and Confocal Laser Scanning Microscopy (CLSM). For the first time, people could directly see the bacteria sitting inside a kitchen sponge, forming biofilm-like structures.
2. Quantifying the pathogenic bacterial community
In the 14 investigated sponges we found 362 different species of bacteria. This level of diversity is similar to the bacterial diversity in many niches of the human body, such as the intestinal tract. In addition, 5 out of the 10 relatively most abundant species were potentially pathogenic and thus represent a potential threat for YOPIs – young, old, pregnant or immunocompromised persons.
Many species we detected were known from other domestic habitats, in particular from washing machines. For instance, we identified a high presence of Moraxella osloensis, a bacterium very well known to cause bad odors . So, if your sponge at home starts stinking, this bacterium might be the cause.
3. Unwelcome bacteria actually thrive on regularly cleaned sponges
The most shocking finding was that higher shares of malodor-causing and potentially pathogenic bacteria were found in sponges that were regularly cleaned, either by microwaving or by washing them out with hot, soapy water. In other words, the more we clean, the more unwelcome bacteria flourish!
We interpreted this finding as a selection phenomenon. The billions of microorganisms form hard-to-crack biofilm-like structures inside a kitchen sponge and probably all domestic sanitization methods eventually fail in sterilizing a sponge.
Although millions of microorganisms get killed, a few will survive and flourish again – probably because they are better adapted to resist the applied cleaning or sanitization method. In the long term, this selection might lead to a kitchen sponge microbiome with adverse features for human health and well-being, such as higher shares of malodor-causing bacteria.
All of the findings supported the preconception that the typical antimicrobial cleaning procedures lead to the survival of extremophilic microorganisms – organisms that survive in extreme environments – that negatively affect human health .
Personally, I believe that probiotic cleaning and hygiene strategies, i.e. strategies that involve living, beneficial microorganisms, could be of help here – but this might be a topic for another day.
So, what should you do with your dirty kitchen sponge at home? In an ideal world, sponges would not be used in kitchen environments anymore, because they provide ideal living conditions for microorganisms: constant wetness, many nutrients and a huge and well-protected surface to proliferate on.
If you do want to keep using them, you can try to reduce the microbial load. In my view, the best way is cleaning in the washing machine at 60°C using a heavy-duty detergent containing bleach.
We found that while newly bought sponges were virtually free of microorganisms, within a few days they are massively colonized. Therefore, long-term use and cleaning of a sponge should be avoided and after a few weeks of use you should remove it from the kitchen. This does not mean that you have to throw it away though. To help reduce waste, the sponge can be used for duties without the domestic hygiene needs: cleaning your toilet, garden pots, litter box, car tires…until it dies naturally.
You think a sponge cannot die? Obviously inspired by our research and the overwhelming, microbial diversity of kitchen sponges (which might also be seen as a reservoir for domestic immune-stimulation), a blog-writer named Joanna recently reported that her son raised their sponge to the rank of a full family member. It even got its own name: Margot .
And for those concerned about bacteria in the rest of their home, I recently published a popular, easy-to-read science book on the microbial world of humans and their homes .
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 Cardinale M. et al. (2017). Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species. Sci Rep. 5791. doi: 10.1038/s41598-017-06055-9.
 Kubota et al. (2012). Moraxella species are primarily responsible for generating malodor in laundry. Appl Environ Microbiol. 78(9):3317-24. doi: 10.1128/AEM.07816-11
 Egert M. (2018). Honor Thy Lodgers? – Structure and Function of the Human Built Environment Microbiome. Tenside Surfactants Deterg. 55 (5): 364-368. doi: 10.3139/113.110577