Kitchen Sponges—Hot Real Estate for Germs


A new study looks at the microbial burden of sponges and dispels our beliefs on cleaning them.

There is no nice way of saying it; kitchen sponges are pretty gross. They perform a dirty job, and so it’s to be expected that these kitchen tools take the brunt in terms of microbial load. Although we’ve always considered the sink and counter-top to be a source for germ colonization, more attention is being put on the role of sponges in spreading bacteria.

The availability and use of sponges may be a more modern invention; however, just like the toilet, they have become a hub for germs. Not only are sponges used for cleaning and therefore, frequently moist, they are also considered “microbial incubators” as they are constantly inoculated with new microbes through their use or through their holding place within the sink or counter.

For a recent study, scientists investigated the microbial load on such sponges, hoping to analyze the microbiome (catalog or community of microbes in our bodies). They tested 28 sponge samples from 14 used kitchen sponges by utilizing a 454-pyrosequencing of high-throughput 16s rRNA gene sequencing, a technique that would allow them to truly see the diversity of microbes within the sponges.

What did they find?

Imaging analysis showed that there were tiny cities of bacteria (aka local densities) of up to 5.4*1010 cells per cm3, which is a lot. “Such bacterial densities,” the authors state, “are found only in feces.”

Bacterial concentration appeared to be in the internal cavities and on the surfaces. How many organisms were found? The researchers found 9 phyla, 17 classes, 35 orders, 73 families, and 118 genera (try saying that 5 times fast!). The most dominant phylum were Proteobacteria, Bacteroidetes, and Actinobacteria. The analysis found that there was impressive colonization by Acinetobacter, Moraxella, and Chryseobacterium species, and of those 3, Chryseobacterium hominis and Moraxella osloensis revealed larger proportions in sponges that were regularly sanitized. That’s a pretty scary notion—those sponges that were cleaned frequently had higher bacteria related to pathogens!

The researchers also found that attempts to sanitize or clean the sponges by boiling or microwaving were not as effective as previously thought. “Regularly sanitized sponges (as indicated by their users) did not contain less bacteria than uncleaned ones. Moreover, ‘special cleaning’ even increased the relative abundance of both the Moraxella— and Chryseobacterium—affiliated OTUs” (operational taxonomic units, aka groups of closely related organisms). The researchers also highlighted that this revelation means that resistant bacteria surviving the sanitation process will rapidly re-colonize.

What should we do if sponges are, in many ways, worse off when we clean them regularly?

Throw them away and plan on using a new sponge frequently (ie, weekly). Kitchens are prime residential property for those microbes looking to colonize and studies have shown that some of the most diverse colonizing bacterial communities enjoy infrequently-cleaned surfaces, such as refrigerator doors, fans above stoves, etc. Although we’re always considering the role of microbial foot-traffic in healthcare, it’s easy to forget the role that our homes play. Infection control is vital in healthcare, but we often ignore the importance of following practices like environmental cleaning, hand hygiene, and in this case, tossing our sponges more frequently, in our own abodes.

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