Kitchen sponges contain great numbers of bacteria, some of which may be related to species that can cause infections in humans. Some of our sponges may even contain small numbers of bacteria that are known to cause harm in humans. Boiling them or microwaving them is likely to reduce the bacterial count to some extent but washing them in a regular wash probably won’t and it may make them smell more (thanks to Moraxella osloensis). Read on for more information.
Knowledge about the microbial world is developing at a rapid pace and it appears to confirm that a diverse collection of microbes in our gut (mainly in our colon) is a really good thing for our health. To help ensure that we develop this, we need to eat a wide range of whole foods, expose ourselves to the natural world (basically get our hands dirty) have pets, exercise and ideally minimise our stress levels. But where does the need for bacterial diversity leave us in terms of the conundrum of the kitchen sponge?
Our kitchens harbour a remarkably diverse pool of microbes – we spend a lot of time there, handling food and touching surfaces and utensils. And of course, it tends to be a warm place with moist regions, creating a great environment for bacterial activity.
There is some evidence to suggest that our kitchen harbours more microbes than our toilet does. But where bacteria are concerned, sheer numbers need not be a reason for concern – volume is only part of the picture. It is the preponderance of pathogens (infection or disease causing microbes) or lack of them that is the real issue. The vast majority of bacteria in the kitchen will be commensal (harmless) to humans.
One of the main sources of bacteria in the kitchen is the kitchen sponge. Kitchen sponges are porous, moist and contain food particles and nutrients for bacteria to utilise. Not surprisingly then, bacteria are distributed throughout our kitchen sponges – concentrated on the sponge surface and the cavities inside the sponge. On the surface, bacteria create their complex biofilms – I imagine this to be like a soft bacterial scab – the bacteria adhere to each other and to the sponge surface. A small German study published this month (although it appears to have been carried out in 2012) and which I will refer to as the Sponge Study, showed that these bacteria are alive and well and going about their bacterial business – on and within our kitchen sponges.
Kitchen sponges, by virtue of their role in our lives, act as very effective distributors of the microbes existing within them. The Sponge Study tested just 14 sponges, that were split in half horizontally (to give 28 samples). In some parts of the sponge, they found as many as 10 billion bacteria per cm3 of sponge. In total, they found 362 distinguishable groups of bacteria.
The class Gammaproteobacteria dominated the sponge community – representing 54% of the microbes present. This class contains a high number of important pathogens but also plenty of commensal (harmless) bacteria. Of the 10 most abundant groups found in the kitchen sponges, 5 of them were closely related to bacteria categorised as a risk to humans. Let’s be clear about this though – the ones actually identified, (e.g Acinetobacter johnsonii, Moraxella osloensis) were not pathogens themselves. Being related to a criminal does not make you a criminal. The researchers, themselves say “Relatedness based on partial 16S rRNA gene sequences is only a weak indicator for the pathogenic potential of the identified bacteria, and we are not aware of any case in which an infection with these bacteria was explicitly reported from a domestic environment.”
Past research has found kitchen sponges to contain Campylobacter (a common cause of food poisoning), Enterobacter Cloacae (can be a part of our normal gut microbiome but is also linked to respiratory tract and urinary tract infections), E. Coli (can cause severe diarrhoea and vomiting) Salmonella spp (can cause food poisoning) and Staphylococcus spp (can infect cuts, for example).
The Sponge Study found no Campylobacter, very low levels of Enterobacteriaceae (and not pathogenic ones), no E. Coli, no Salmonella and only a tiny amount of Staphylococcus and Streptococcus.
And even if they had found some of these, would it matter? It is quite possible that many of us carry low numbers of certain strains of these bacteria within us and suffer no ill effects. Likewise, our kitchen sponges may also harbour low numbers of more harmful strains of these bacteria, with no ill effects to us.
The thing about microbiome research at the moment is that nobody yet knows what the hell is really going on. We are only just at the stage of simply identifying just how many pieces of the jigsaw there really are and which pieces might be edge pieces. We don’t yet know how many pieces the jigsaw has or which parts fit with which and a great many of the pieces are, metaphorically speaking, very similar colours, just to make it all the more complex.
Having attended a talk about the kitchen microbiome earlier this year (which also concluded that our kitchen sponges contain a great number of microbes, some of which may be harmful to us), I concluded that the pragmatic thing to do would be to make sure I wring out my kitchen sponge and leave it somewhere to dry out after use. I also started to wash my kitchen sponges and tea towels regularly (which for us means at least 3 times a week). However, the result has been that my tea towels and kitchen sponges smell worse than they did before I adopted this habit. Baffled by this, I can be seen regularly sniffing these items and looking rather perplexed.
The Sponge Study has given me some sort of explanation for this smelly phenomenon. Moraxellaceae (a family within the class Gammaproteobacteria (the majority of Moraxellaceae are harmless to humans)) seems to be a typical inhabitant of kitchen sponges. It has also been detected consistently on sink surfaces, taps, fridges and ovens – any surface that might be cleaned regularly with kitchen sponges. Moraxellaceae is also a normal part of our skin microbiome and, of course, we touch all of these surfaces, as well as the kitchen sponge, many times a day with our bare hands.
In the Sponge Study, those kitchen sponges that were regularly cleaned did not show significantly reduced bacterial numbers compared to those that were not. And, what is more, that special damp kitchen sponge/tea towel smell is apparently caused by the activities of Moraxella osloensis and its numbers may actually become enriched during a domestic wash. Which explains why the more we have washed our kitchen sponges and tea towels Chez Wondergut, the smellier they have become.
So where does this leave us, other than with smelly kitchen sponges?
Kitchen sponges contain great numbers of bacteria, some of which may be related to species that can cause infections in humans. Some of our sponges may even contain small numbers of bacteria that are known to cause harm in humans. Boiling them or microwaving them is likely to reduce the bacterial count to some extent but washing them in a regular wash probably won’t and it may make them smell more (thanks to Moraxella osloensis).
It makes sense to wring out our kitchen sponge when we have finished with it and leave it somewhere dry so that it is not constantly damp. I replace my sponge reasonably regularly and I always throw it away after we have washed up after preparing a chicken due to the risk of spreading Campylobacter from chicken to sponge to surfaces.
After this thoroughly absorbing romp, I am not sure I will be changing much on the kitchen sponge front, mostly because I suspect that, whilst a sensible level of hygiene in the kitchen is essential, a healthy gut microbiome will cope admirably with the occasional rear-guard action from the kitchen sponge. A more conscientious approach to kitchen sponge hygiene may be required in hospitals, nurseries, homes for the elderly and in any other place where those living there have vulnerable immune systems (which we know to be closely related to vulnerable gut microbiomes). In these situations, the safest thing to do would be to regularly replace the kitchen sponge with a new one (which, whilst not sterile, has a very low bacterial count).
Otherwise, admire your kitchen sponge for the complex ecosystem that it is. It is probably contributing to the bacterial diversity inside you and, assuming sensible levels of hygiene, probably not in a bad way. For those of us who do not have pets, perhaps we can view our kitchen sponge as performing a similar role for us as a very small dog or a cat in terms of carrying both commensal and pathogenic bacteria. My youngest son has even gone so far as naming ours Margot.
Text © 2017 by Joanna Webster
Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species: Massimiliano Cardinale, Dominik Kaiser, Tillmann Lueders, Sylvia Schnell, and Markus Egert Scientific Reports July 19 2017:7:5791