Kitchen Science Bacteria Challenge - Kitchen vs Toilet

For kitchen science we tried to see if there's any truth to the urban legend that there's more bacteria on a kitchen worksurface than on a toilet seat...
13 May 2007

Interview with 

Gillian Fraser, University of Cambridge


Bacterial culture as sampled from from a toilet seat


Ben Valsler met up with Gillian Fraser, from the department of Pathology at Cambridge University, to explore the bacteria in kitchens and in toilets.

Ben -   For Kitchen Science this week, we decided to do something a little different, so really it's not kitchen science as much as it is kitchen and toilet science.  I'm afraid this is one that you can't do at home, but what we've decided to do is see if there's any truth in the story that there are more bacteria on your kitchen work surface than there are on your toilet seat.  I met up with Cambridge University bacteriologist Gillian Fraser to see how we could test this out.

Gillian -   So when we met a few days ago in a rather nice toilet, what we planned to do was to take some samples from the toilet seat. We were then going to inoculate these samples onto some growth media that would allow us to grow the bacteria that were present on the toilet seat.

Ben -   So we took two swabs, which are rather like a long cotton wool bud, and we've rubbed on over a toilet seat and one over a kitchen work surface.  And then by inoculating you mean to rub the swab over some agar in a little dish and see what grows on it?

Gillian -   Yes, so some of the bacteria picked up on the swabs would definitely be able to grow on this media, as you can see if we look at the plates we've got lots of individual bacterial colonies growing.

Ben -   So what exactly are bacteria?

Gillian -   Well bacteria are single celled organisms and they're in our environment everywhere, they're pretty ubiquitous.  Many bacteria are found on plants, animals and of course, you and me.

Ben -   I've heard that actually there are more bacterial cells inside me than there are human cells; how does that happen, why am I not ill?

Gillian -   that's true, we actually carry many bacteria on our skin surfaces, also in our guts there are a huge number of bacteria.  These bacteria live together with us as commensals; under normal circumstances they don't cause us any damage.  In fact they're very good for us and can actually protect us from bacteria that cause disease.  So by colonising our surfaces, they can prevent the bad bacteria from being able to stick to those surfaces and grow.The bacteria that cause disease are actually quite similar in many respects to the good bacteria.  However these bad bacteria have acquired traits that enable them to damage the cells in our body, to for example release nutrients from our cells that can help the disease causing bacteria to grow.  So for example, they might produce a toxin that lyses (bursts) the cells in our bodies.

Ben -   So the good bacteria have evolved to get the nutrients they need through working with us, but the bad bacteria get the nutrients they need by taking advantage of our cells, breaking them up and taking what we have?

Gillian -   That's exactly right.  Another difference is that our body normally will not raise an immune response, will not try to kill off, the good bacteria normally associated with us.  But when a new bacterium, a pathogenic or disease causing bacterium comes along, the body will try and clear it by generating antibodies, and suchlike.  This immune response is also a way that damage is indirectly caused during a bacterial infection.

Ben -   So do antibiotics kill off all bacteria regardless of whether they're good or bad?

Gillian -   Well some antibiotics have quite a specific target range of bacteria that they will kill, but other antibiotics, broad spectrum antibiotics, can kill off a wide range of different bacteria.  These can include the good bacteria.  In the news recently you might have seen there's an increasing problem with an organism called Clostridium difficile also known as C. diff, which can gain a foothold in your gut after prolonged antibiotic treatment has killed off your normal gut bacteria.

Ben -   Which of course is very dangerous for people who are, for example, elderly, frail or suffering with another illness.

Gillian -   That's right, but that's not the only problem with prolonged and widespread antibiotic use.  We also see that pathogenic bacteria can become resistant to antibiotics, and this of course is a huge problem, everybody's heard about Methicillin Resistant Staphylococcus aureus.

Ben -   MRSA?

Gillian -   That's right.  A big problem now in hospitals.

Ben -   So the plates that we have here, one from our kitchen and one from our toilet seat.  I can see from here that one of them clearly has a lot more speckled patches on it than the other one, which looks fairly clean really.  Which one is which?

Gillian -   So the one with lots of bacterial colonies on it came from, not surprisingly, the toilet seat.  It looks like there's roughly about 5 times as many bacterial colonies have grown up on this plate compared to the plate where we inoculated the sample from the kitchen.Agar plate 1

Ben -   Can we look a bit further into this and see what sorts of bacteria we're getting?

Gillian - I was able to take some of the individual colonies from these plates and sub-culture them, which means to plate them out again onto different types of growth media which can select for the bacteria that would normally survive within your gastrointestinal tract, in your intestines.  Both of them have an important ingredient, bile salts, which would normally kill most bacteria except things like E. coli, which is normally found in your gut, and also pathogens like Salmonella and Shigella.

Ben -   The plate you've used for the original cultures is just a pale yellow colour, I think most people will have seen an agar plate that looks like that.  One is almost a tobacco stain dark orange, whereas the other one is quite a rich green.  Are these colours here to tell us anything or is that just the colour they are?

Gillian -   No, they actually tell us something.  The thing that gives the plates the different colours are pH indicators.  We put pH indicators in to tell whether the bacteria can use certain sugars that are in the plates as foods, because this can allow us to differentiate, to tell apart, different kinds of bacteria.  So E. coli would give you lovely dark red colonies, whereas salmonella would give you pale yellow to orange colonies.As you can see from our loo seat sample, colonies have grown on our selective agar, so we know that probably we've got some enteric bacteria in here (bacteria which can survive inside your intestines).  We also know by looking at what colour the pH indicators have gone (so what colour the colonies are) that these bacteria don't ferment lactose.  We know they're not E. coli, they're not enterococci or enterobacter, which are normally found in your gut.  They could be something like salmonella or shigella, which can both give you diarrhoea.

Agar plate 2

Ben -   So there you go, in our test we found many more bacteria on a toilet seat than we did on a kitchen work surface, and even some indication that the bacteria that were there could survive inside your intestines.

We asked members of our forum to vote on which plate they thought was from the kitchen.  You can see the results here. Kitchen science will be back to normal next week with an experiment you can try out at home.  Until then though, my thanks to Gillian Fraser and goodbye from me.


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