It has often been said that your kitchen sponge hosts more germs than your toilet.
Turns out it's true. German researchers presenting their studies in the latest edition of Scientific Reports have shown a piece of sponge the size of a sugar cube can host as many as 54 billion bacteria, a small number of which may be pathogenic, illness-causing bacteria.
The researchers, from the Faculty of Medical and Life Sciences, Institute of Precision Medicine (IPM), Microbiology and Hygiene Group, Furtwangen University, say their work revealed "an amazing bacterial colonisation of kitchen sponges, and visualised its extent for this common microbial hot spot for the first time".
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"Within a domestic environment, kitchens and bathrooms have a high potential to function as 'microbial incubators', due to the continuous inoculation of new microbial cells, by food handling and direct body contact to the domestic surfaces."
"Despite common misconception, it was demonstrated that kitchen environments host more microbes than toilets. 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."
An earlier study, by Ojima and coworkers (2002), showed that kitchen sponges had the second highest load of coliforms of the whole house, after the drain traps. "Further works showed the presence of specific pathogenic bacteria in kitchen sponges, including Campylobacter, Enterobacter cloacae, Escherichia coli, Klebsiella spp, Proteus spp, Salmonella, and Staphylococcus."
But this most recent study showed that kitchen sponges "harbour a higher bacterial diversity than previously thought" but "obligate human pathogens" are in the minority.
The researchers say it is not that easy to get rid of the germs, no matter whether you use commercial cleaning products, microwave the sponge or pop it into boiling water. While all these things do significantly reduce the bacterial load, they don't kill all the germs.
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"The results were contradictory, for example showing effectiveness in the laboratory, but not in 'used' kitchen sponges, and no method alone seemed to be able to achieve a general bacterial reduction of more than about 60 per cent."
The researchers also noted that kitchen sponges not only act as reservoir of micro organisms, including "bacteria with a probable pathogenic potential", but can also spread these on your kitchen surfaces, such as the benchtop and taps. This can lead to cross–contamination of hands and food, which is considered a main cause of food–borne disease outbreaks.
They also noted one type of bacterium, called Moraxella osloensis, causes the sponge to smell, rather like laundry that has been left wet too long. But, paradoxically, when the sponge is cleaned, these bacteria increase in abundance, so the cleaned sponges will smell more often. Regular cleaning also increased the abundance of another type of bacteria.
One good thing to come from the survey was the fact that no bacteria could be detected in a collection of newly bought, unused kitchen sponges.
In light of the results, and the fact that sponge sanitation methods appear not sufficient to effectively reduce the bacterial load, and might even increase certain bacteria, the team suggests "a regular (and easily affordable) replacement of kitchen sponges, for example, on a weekly basis".