Sneaky Peek

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.

An example of a bacterial biofilm

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.

Wondergut Wisdom

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

Principal Reference

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

This Post Has 6 Comments
  1. Dear Joanna,
    I am the first author of the article “Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species”. Thank you very much for sharing our work. Your report is the most accurate I have read so far among the hundreds that have appeared on the web. I am very happy that our discoveries helped you in explaining one aspect of your daily life.
    Best wishes,
    Massimiliano Cardinale

  2. Whilst admiring the microbial diversity of your kitchen sponge, we must not be fooled into forgetting that, along with your hands, cleaning cloths and sponges are the most risky surfaces in your home for spreading infection. Studies of the preparation of chickens contaminated with Campylobacter or Salmonella showed that these pathogens are widely spread, sometimes in large numbers, to hands, food contact hand contact surfaces and cloths. The study also showed that cleaning the food prep surfaces with a clean cloth only served to transfer the germs to the cloth and spread them to other surfaces which were wiped as part of the cleaning up process. In the UK around 60% of chickens we buy are contaminated with Campylobacter or salmonella – and up to 1 in 20 UK homes prepare a meal with a contaminated chicken every day. The estimated numbers of cases of campy infection per yr is 280,000. Around 30% of all food poising cases occur in the home
    This microbiome study of 15 sponges taken at random from the home is interesting – but underestimates the infection risks associated with using cloths and sponges. Good hygiene means hygienic cleaning of surfaces in the places and times that matter. The places that “matter” most are hands, hand contact and food contact surfaces – and cleaning cloths/sponges. The time that matter are during food handling, toilet visits, taking care of pets, home healthcare. In assessing our newly discovered understanding of different microbiomes and the possible beneficial implications for out health, we must always do risk:benefit assessments of the possible consequences of deliberately exposing ourselves to “dirt”. For sponges the risks far outweigh the benefits – Margot is far from being a friend in your home
    1. Developing hygiene practice for the home – the IFH risk-based approach to home hygiene (targeted hygiene).
    2. Cogan, T.A, Bloomfield, S.F. and Humphrey, T.J. (1999) The effectiveness of hygiene procedures for the prevention of cross contamination from chicken carcasses in the domestic kitchen . Letter in Applied Microbiology, 29, 354-358

    1. Thank you so much for commenting Sally. I agree with you – this is a tiny study and we know that our hands and kitchen sponges are the major source of bacteria in the kitchen. A sensible level of hygiene is important and I have added in that when we have chicken (which is rarely) I actually dispose of my sponge afterwards (mainly out of concern for my older relatives). However, given the number of people who prepare chicken and eat it daily, the level of food poisoning from Campylobacter/Salmonella is a very small proportion of those people and, of course, Campylobacter/Salmonella aside, nearly all of us are wiping down our surfaces with kitchen cloths as this is basically better than not wiping them down at all and the majority of us do not get ill as a result. These interactions are inordinately complex and don’t just depend on which pathogens are around but also the state of our health/gut microbiome too. I guess the point I am trying to make in this article is that, whilst good hygiene around sponges is essential, they will still contain billions of bacteria (indeed washing them regularly doesn’t seem to really affect this) and the vast majority of sponge-users who are being sensible will never become ill as a result of their kitchen sponge. We need to guard against the pathogens within reason but for me, the risk doesn’t warrant the environmental cost of using disposable cloths. And I think there is a risk with cleaning too much with bleaches and anti bacterial gels that we wipe out commensals too which, just by being there, help to prevent the pathogens dominating. I am in no way suggesting a gung ho approach. My son (7) named the cloth Margot as a result of realising there was an entire ecosystem existing within it – just as our pets carry both commensals and pathogens so do our kitchen cloths. The picture, when it comes to bacteria, is not black and white. Best wishes Jo

      1. Dear Jo – I need to make a few more points
        1. My concern is not about sponges as a SOURCE of germs, but as a VECTOR of pathogens. You are talking about the resident microrbiome of sponges – I am talking about their function, which means that there is significant risk of them picking up pathogens by very nature of their function in our lives. Hygiene is about preventing the spread of these pathogens from a “infected” source (people. food pets etc) via a vector to a susceptible person. This means breaking the chain chain of infection by hygienic cleaning of key vector surfaces in the places and times that matter). We call this targeted hygiene. The 2 most important vectors of infection in the home are the hands and cleaning cloths/sponges. I chose Campylobacter as an example but I could equally have included norovirus, cold virus, rotavirus, Staph aureus etc etc. Targeted hygiene to break the chain of infection is described in our simple e learning resource at Please do have a look
        2. I cannot accept the following statement because it’s anecdotal i.e. based on your knowledge of, maybe 2000 at the most?, sponge users – you say “ nearly all of us are wiping down our surfaces with kitchen cloths as this is basically better than not wiping them down at all and the majority of us do not get ill as a result” . In the UK there are 70 million people of which a quarter million get campy and 3 million get norovirus infections every year – so you cannot claim that the majority of us do not get ill. It’s like saying my friends and family have never had a car accident in the last 10 years – so I have decided to stop wearing a seat belt. In public health terms both hygiene-related disease and car accidents are significant population health risks, which require interventions to reduce risks. Hygiene is everyone’s responsibility. Unless we all practice good (targeted) hygiene, based on scientific evidence rather than our anecdotal knowledge we will never reduce the need for antibiotic prescribing – which is now a health priority. Same goes for cutting down car accidents – its everyones responsibility. The evidence suggests that the benefits of routine hygiene practices which ensuring that we do not spread germs via cloths outweigh the risks.
        3. Targeted hygiene means applying a “procedure” to risk surfaces at critical times, which is enough to reduce contamination (if it happens to be present) to a safe level. Whilst I agree, for the reasons you say, that indiscriminate daily cleaning with bleaches and anti bacterial must be discouraged, evidence shows that in some situations “wiping with detergent” is not enough to reduce pathogens to a safe level (and like the car seat belt you never know when it is needed so it has to be a habit) – and in these situations cleaning an disinfection is needed to break the chain of infection. We also address this in our learning resource.

        1. This is great Sally – thank you for taking the time over this. The resource is interesting – thank you for sending it, although it doesn’t specifically deal with the kitchen sponge issue.
          1. To be honest, I think we are both arguing from the same side. I understand that the issue of the sponge, due to its role in the kitchen, is as a spreader of microbes – both pathogenic and commensal as opposed to simply being a “sink” of microbes so to speak. That is why I am interested in it because our perceived ideas around the role it plays may actually be opposite to the role it really plays – ie it spreads microbes, including pathogens sometimes, rather than ridding our kitchen of them. I agree that intelligent, targeted hygiene is important – both in hand washing, surface cleaning and sponge cleaning/replacing. I think my article makes this clear. But it remains the case that because of its design and the role it plays in our kitchen, the actual function of a kitchen sponge may differ from our perception of its function and I am interested in people realising this which, on the one hand involves knowing that sensible hygiene measures are important but on the other hand also involves accepting that microbes are everywhere. And this means it is important to also focus on looking after our internal health and the health of our gut microbiome to give us the best chance against the inevitable exposure to pathogens. Also, this is an area of science in which there is such a huge amount to learn and which is developing so quickly. Scientific knowledge is the best we have but it is imperfect – antibiotics is a case in point. We use kitchen sponges and standard hygiene processes because it is the best we can do for now. But they are imperfect. Hopefully, at some point in the future, we will have sponges or sprays that are inhospitable to or target pathogens but support commensals.

          The useful things that reading your work has clarified is that after cleaning surfaces and implements with hot water and our usual detergent, rinsing them afterwards is important and easy to do. Drying is also important – I am glad about this as it gives me another reason to remind my husband to dry the surfaces after he has wiped them! In terms of our kitchen sponge, this means rinsing it out after use and leaving it somewhere to dry, rather than leaving it in a pool of water in the sink. And this is something that we do.

          2. My response on this was not anecdotal but based on the figures you gave me. There are 70 million people approx. in the UK. And you told me that approx. 1 in 20 households are eating contaminated chicken daily. I guessed that from 70 million people, there are maybe 20 million households – that is 1 million households daily eating contaminated chicken and 250,000 infections a year. So that is 365 million households eating contaminated chicken every year and 250,000 people getting infected. That, by any standards is a relatively small rate of infection in relation to exposure. It is still a significant issue, I see that, especially in terms of trying to minimise antibiotic consumption, but in terms of exposure to infection rate ratio – it is interesting – along with the fact that you state that only 30% of these infections actually occur in the home. So 365 million households, 250,000 infections but only 75,000 originating from homes. I see that norovirus infection is higher. But even so, what I see is that many, many more people are exposed to pathogens than are infected by them. Which means many of us manage to inhibit an infection when others cannot. In no way am I saying that this means it is ok not to practice good hygiene – I am not saying that at all. I know that good hygiene practices are part of our protection. I think my article makes that clear but also that the things we are doing that we think are hygienic may not be – wiping surfaces in the first place (because our sponges contain so many bacteria), washing cloths (which appears not to get rid of microbes) and indeed, using disinfectants etc which rid us of the protection of the commensals. So that leaves us accepting that we live in a microbial world, practising good hygiene but at the same time realising that, with the best will in the world, we cannot get rid of them all, nor would it be healthy to do so. Of equal importance is looking after our health in terms of the state of our gut microbiome, diet, sleep, stress etc as this also helps in our resilience against pathogens.
          3. It is so tricky with the disinfectant isn’t it? I would be concerned about using this daily, even on high risk surfaces, just so that I am in a good habit, mainly because I am concerned that by decimating commensals too, they leave a void that makes it too easy for some remaining opportunistic pathogens to take a hold they would otherwise be unable to achieve. And then I would be in a worse situation than if I had just used my usual detergent, hot water and a clean cloth (remember my article says I wash and dry these and tea towels 2 or 3 times a week).

          I guess what I find fascinating about it all is that yes, we have best practice in terms of cleaning and breaking the chain of spreading pathogens but microscopic life is EVERYWHERE – in us and on us and around us. We harbour what we perceive to be “pathogens” in our nasal cavities, our throats, our guts and in some of us they never get to a level to do us any harm. In others, something triggers them proliferating. Similarly, some of us don’t get food poisoning when exposed to food pathogens when others do. None of this obviates the need for good cleanliness and not just around the kitchen, hand washing is key too (but again, anti bac gel etc rids us of the commensals on our hands and I am concerned this is problematic – so we use hot water and soap). The point of my article was to make us all think a bit more about this whole issue and I think between us we have really done that! And to make us realise that our behaviours around this are too simplistic for our growing understanding of microbes. Microbes were around before we were and will probably be around after we are gone and I think it is good for us humans to remember that however we try, I am not sure we will ever really have the upper hand.
          I really appreciate you taking the time to share your knowledge on this topic with us Sally. It can only ever be a good thing for us all to be more aware around these issues. If you would prefer to contact me directly my email is Best wishes Jo

          1. Of course we agree that the key question is “how do we protect ourselves against infections, whilst at the same time re establishing our exposure to vital microbes in our environment. The evidence currently suggests, for the latter, the need is for a change in our lifestyles – natural childbirth, breast feeding, outdoor activity etc – to build a healthy microbiome together with less antibiotic nd better diet to sustain a healthy microbiome. There is no evidence that home cleanliness is a factor, but that does not mean it is not. So the way to tackle maximising prevention of infection is through targeted hygiene – in the place and times that matter – with effective hygiene interventions. In blogs and in the print media etc, I have recently seen enthusiastic proponents of increasing good bacteria exposure in ways that also increase risk of exposure to pathogens – the messages were “stop washing your hands”, “go back to hand dishwashing instead of using a dishwasher”, and “clean surfaces with a bacteria laden kitchen sponge”. People just do not seem to want to understand targeted hygiene because they think that “germs” come from things that look and smell “dirty” – not from food, people and pets – and that hands, food contact surfaces and cloths/sponges are the way they are spread
            We must also get over our obsession against disinfectants. Handwashing or using alcohol handrubs is intended to get rid of the transient pathogens that we pickup, it reduces, but far from decimates the skin microbiome – it leaves behind millions of good microbes which soon re-establish a full population – and we are only treating the critical point i.e. our hands – not our whole bodies. We tend to tar all microbiocides with the same brush – but there is a growing view that we should, where they are needed, use microbiocides like bleach and other oxidising agents and alcohol which disappear quickly and leave no active residue.

            Whilst I am absolutely against indiscriminate use of microbiocides, I could argue that the evidence suggests that not using a disinfectant where it is needed increases the risk of infection – and the need for antibiotic prescribing – so prudent use of disinfectants could help to reduce the problem of antibiotic resistance.
            As you say – and I said in a recent article – in future we are going to have to view our microbial world very differently –

            At the moment, it is very trendy for microbiomists, allergists and environmentalists/greens to write blogs and media articles which extol the need to become part of our global microbiome and dismiss hygiene as unimportant. Someone has to start championing hygiene – antibiotics won’t be there to protect us in the future if we carry on in this way

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