Scientists have long cast shadows over the humble kitchen sponge. The household’s staple food sneaks into sinks among dirty dishes and soggy leftovers, sucking up and amplifying microbial forces capable of invading clean food spaces. The smart kitchen goer is allowed think they’ve locked this situation in – a simple toss through a sanitizing dishwasher cycle or a sizzling swirl in the microwave…and done. Sudsy germ explosion averted.
Nice try, says science.
In a comprehensive study of 14 household sponges and their microbial inhabitants published in Scientific Reports, researchers confirmed that kitchen sponges are indeed household abominations. In addition, sterilization attempts seem to only temporarily free up sponge space for potential pathogens, which rapidly recolonize the purulent scrubber.
According to the authors, led by Markus Egert of the University of Furtwangen in Germany:
From a long-term perspective, sponge cleaning methods do not appear to be sufficient to effectively reduce the bacterial load in kitchen sponges and may even increase the proportion of [disease-linked] bacteria… We therefore recommend that you regularly (and easily affordable) replace kitchen sponges, for example, on a weekly basis.
Now, the researchers haven’t soaked up every possible sponge science data point. But their study offers the most comprehensive look yet at the pesky sink dwellers.
By taking top and bottom samples from 14 household sponges used in Germany, the researchers extracted genetic material to sequence and identify the microbial inhabitants. They also used a method of tagging active, breeding microbes with fluorescent markers and visualizing them in the sponge material using 3D confocal laser scanning microscopy.
Squeeze data
The sequencing collected more than 220,000 raw DNA sequences, representing 9 phyla, 17 classes, 35 orders, 73 families and 118 genera of microbes. As other, smaller studies reported, the researchers found that bacteria runs in the family Moraxellaceae dominated sponge space, accounting for about 36 percent of microbes in samples. These germs are typical of human skin and have been found everywhere in kitchens that are often cleaned with sponges: counters, refrigerator shelves, faucets, and stoves. They can also be found on dirty laundry and are known to give clothes a bad smell.
The researchers found otherwise Proteobacteria, Bacteroidetesand Actinobacteria were primary phyla. Five of the 10 most common operational taxonomic units – sort of like species – were closely related to bacteria associated with moderate disease.
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(A) Kitchen sponges, due to their porous nature (visible under binoculars (B) and water soaking capacity), are ideal incubators for microorganisms. Scale bar (B): 1 mm.
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Sponge samples.
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Pie charts showing the taxonomic composition of the bacterial kitchen sponge microbiome as provided by pyrosequencing of 16S rRNA gene libraries from 28 sponge samples (top and bottom samples from 14 sponges, respectively). For better readability, only the 20 most common orders and families are shown.
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3D model of the bacteria in the sponge sample 6b. Volume renderings of a confocal stack with sponge autofluorescence (cyan); Gam42a-stained bacteria (blue) and EUB338MIX-stained bacteria (red); where Gammaproteobacteria appear purple due to the overlapping of red and blue, while other bacteria remain red only.
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Analysis of bacteria in sponge sample 9b. Maximal projections of confocal stacks, with EUB338MIX-stained bacteria in red (A) and sponge autofluorescence in cyan (B); (C) is the overlap; and (D) is the 3D model.
In terms of concentration, previous studies had a sponge germ density around 107–109 bacterial colony forming units per sponge. Assuming that only about one to three percent of bacteria can be grown into colonies in labs, those estimates agree with the new data. Egert and his colleagues found densities as high as 2.5 x 1010 and 5.4×1010— that’s 25 to 54 billion — bacterial cells per cubic centimeter of sponge. The 3D visualizations showed a ubiquitous distribution of microbes throughout the sponge, with dense biofilm-like structures on the sponge surface.
The researchers noted that some sponge owners said they cleaned them regularly, either by microwaving them or by washing them with hot soap. Those sponges didn’t have fewer microbes than the others, but they did tend to have more bacteria associated with those associated with disease.
“Presumably, resistant bacteria survive the remediation process and rapidly recolonize the vacated niches until they reach similar abundance as before treatment,” the authors concluded. “Whether this has consequences in terms of clinical relevance remains to be demonstrated,” they add. But the work highlights “an astonishing bacterial colonization of kitchen sponges” that they hope will “create even more awareness for kitchen sponges as hygienically relevant microbial incubators.”
Scientific Reports2017. DOI: 10.1038/s41598-017-06055-9 (About DOIs).