Armpit microbiomes: body odour and bacteria

How are smelly molecules created on human skin, and should we really be washing it all off?
20 July 2021

Interview with 

Gavin Thomas, University of York




With the summer heat we’re experiencing here in the UK, some may suspect they’ve been a bit smellier than usual? Well, that’s down to the bacteria that live in our armpits. We’ll hear more in a moment how understanding them better could lead to superior deodorants. First though, let’s hear from the Royal Society’s Keith Moore about how microscopes - and the formerly invisible world of microbes they enabled visitors to see - have always been a huge hit over the years...

Keith - Microscopes were a really important part of how science was demonstrated in the 19th century. And one of the great pioneers, Robert Koch the germ theory pioneer, sent specimens of tuberculosis, leprosy, and other bacteria for exhibition by William Watson Cheyne in 1882. And of course, later Koch would go on to win a Nobel Prize for his tuberculosis research. Visitors to the summer science exhibition would be able to look down microscopes and use them. They would see interesting things down there, perhaps minerals or small creatures, but scientists attending might be interested in the instrument itself. So instrument makers would demonstrate their wares at the Royal Society in the hopes that scientists might buy one of those instruments.

Of course a lab wouldn’t be a lab without a microscope these days, and some scientists use theirs to stare at what’s living in the average armpit. The University of York’s Gavin Thomas does just that, as he told Eva Higginbotham...

Gavin - It's the microbes that make us smell. And to understand that we need to think a little bit about what sweat is and the different types of sweat that there are. So if you look at our armpit, it's a very rich environment with different types of sweat glands, but the type of sweat glands that we normally see on our open skin, which are called eccrine sweat glands that make the kind of salty water we know that we use for thermoregulation, they are found in the underarm, but also another important type of glands called the apocrine glands. And these apocrine glands make different types of sweat, which contains a range of different chemicals, but also the odourless precursors that are acted on by bacteria to form the molecules that we know as body odour.

Eva - And what kind of bacteria are they that live in the armpit?

Gavin - We've discovered some particular bacteria called Staphylococci that were important in this process. There's lots of different types of bacteria that live in the underarm. We like to think of it as a bit of an oasis on the skin. So the skin has different parts, the open skin, scalp, for example, your feet and your underarms, they're all different environments. And they all have their own populations of microbes. So the underarm has a particularly rich set of microbes, very diverse set of microbes, and only some of those are actually able to make body odour. And we kind of pinpointed the ones that are able to do that.

Eva - And do those bacteria like to be in the armpits because they like to eat the molecules that are in that sweat you were talking about.

Gavin - Yeah, we think so. I mean, we haven't formally proven this yet, but we know that these odour precursors, when the bacteria take them up, they take them up first into their cells and then they actually partly break them down, and they eat the things that they're releasing. So they release nutrients from those molecules and then the final molecule that they can no longer use, they then spit it out. And that molecule is the volatile molecule that forms one of the most pungent components of body odour, which is called a thioalcohol. So we've been working on discovering how these particular bacteria recognise this odourless precursor, take it up, break it down, producing what we recognize as BO.

Eva - And does everyone make the same molecules and so smell the same? Because some people just are smellier than others.

Gavin - Yeah. That's a great question. There is a genetic basis behind this in terms of the amount of those odorless precursors I just mentioned, and how much you've actually made. So we know that there are some populations of people in the far east to have a genetic mutation in the enzyme that actually secretes the precursors into the underarm. So these people don't make, or don't secrete, as much of the precursors. And therefore aren't able to make so much thioalcohol. Now they do still smell, but they smell differently to a Caucasian person because they're missing that key component of the body odour. Body odour is made up of a range of different molecules. The thioalcohols that we study are the most pungent component. So without them, they actually smell slightly different to what a Caucasian person does. And also the actual population of microbes in the underarm clearly has an effect because we know that some people have produced higher or lower levels of actual body odour. So at least two things influence that.

Eva - And can we try and use this understanding of this breakdown of these molecules in these specific bacteria to try to make better deodorants and so people can smell less?

Gavin - Yeah. So that's what we're trying to do. And that's what our colleagues in Oxford are looking at. So because we've discovered how the bacteria take up those precursors and how they break them down, we can try and design targeted drugs that will inhibit that particular process. So that, we think, is better. If you look at current deodorants and antiperspirants which either kind of mask body odour or prevent the precursors being secreted, many of them contain a kind of general anti-microbial things like triclosan. These will kind of effect all the microbes in the underarm. And what we really want is to design something that will just specifically hit the microbe which we've discovered, which is called Staphylococcus hominis that makes the odour, so this information should be able to help us make a much smarter deodorant.

Eva - So we'd like some kind of very specific armpit antibiotic that would just knock out the specific, bad-smelly ones.

Gavin - Well, not even an antibiotic in that sense, we don't even need to kill the bacteria. We just need to stop that particular process. So the idea is that we really go in and stop the transporter from taking up the precursor. We're not killing the bacteria and we shouldn't be changing the microbiome too much. So it's a really, really kind of guided weapon to inhibit the particular process that leads to odour production.

Eva - So there you go. We often think about trying to keep our gut microbiome happy, but perhaps we should be thinking more kindly of our underarm microbiome. One of my questions is what is the point of all of this? Is there any reason that the bacteria should be in our armpits breaking down these molecules in this very specific way? How could this have evolved?

Gavin - Yeah, that's a great question. I mean, that's what we're really trying to understand now because we understand which bacteria do it, how they do it, but not why they do it. So our research has suggested that the production of this thioalcohol molecule is, is a pretty ancient process. So it certainly predates our split from other higher apes. So it's been among us all the way through human evolution, and we know that other mammals use microbially-derived volatile chemicals to communicate with each other.

Eva - Is that what a pheromone is?

Gavin - Exactly. That is the definition of a pheromone. It's a volatile chemical that's used to communicate between animals of the same or potentially different species. So these might be human pheromones in our evolutionary past. They might've been important to humans in terms of communicating.

Eva - And do you think they could be important to humans in communicating now? Is there anything we could be doing by being quite clean these days and washing our armpits and applying deodorant? Could there be some benefit that we're missing out on?

Gavin - That's a difficult question to answer. I mean, we know that if you go back a couple of hundred years, we all had a strong odour that would have been part of our personality, our character. And then the Victorians come along. They associate odour with illness and therefore as part of cleaning up the cities and the environment they get rid of smells. Smells are linked to disease. And that, on the whole, is a very sensible thing to have done. So we have an environment now where we have a lot fewer smells in our environment. And again, in the early 20th century, a lot of soap companies really rammed home the idea that BO was bad to sell their product. So it's difficult. Is this conditioned in us is, is BO really actually bad? And again, I always like to make this nice quote that Napoleon famously apparently wrote to Josephine when he was returning from the battlefield where he said, "I'm coming home, don't wash." So clearly for Napoleon, the unwashed smell of Josephine had a positive association for body odour. Whether there's any direct benefit now, it's difficult to say

Eva - So more research to be done then. So Gavin, what can people see if they check out your stall online?

Gavin - So we've got a great arcade game called backman. You can play a Staph. hominis, it's the bacterium I've been talking about. You can go around and try to defeat the other skin microbiome and eat odour precursors. And on the website, you can read much more about the science behind what I've just been talking about. And also there's a quiz there too. And also some videos from the Oxford team saying about how they're developing these new drugs to try and reduce body odour.


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