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  • Alex Cloherty

Mellow Yellow

As the world seems to be falling apart before our eyes, I decided to go for a lighter topic this week on Microbial Mondays. I hope it brings a smile to your face.


The topic of this week's post was inspired by that task which is, to me, the very epitome of the ordinary and mundane: laundry. I was washing a load of white clothing, and noticed that on some older items, the armpits were looking a little dingy. During the current pandemic, I've spent a lot of my down-time reading about the environmental impact and ethics of the fashion industry, and teaching myself how to better care for my clothing. So, I viewed these yellow stains as a challenge. My plan of action to beat the stains was as follows: 1) figure out what causes the stains, 2) use my knowledge of the cause to get rid of the stains, and 3) use my knowledge of the cause to prevent further stains.


If you've continued reading this far, you might be thinking, well step 1 is easy, it's bacteria. That's what I'd always heard before, too, but being a biologist, I wanted to know which bacteria, and how is it that the yellow stain isn't washed away at high temperatures, as it should be if only bacteria are responsible for the yellowing phenomenon. I did some quick searches for scientific papers, but (perhaps unsurprisingly, it's probably not a topic that would receive much research funding) there aren't too many people out there studying armpit bacteria. It took me about an hour to find this one paper. Evidently I should have been using the more tasteful keyword "underarm", rather than "armpit".


After reading a few papers, I found something suprising. It turns out, bacteria are probably not the culprits in the yellow-stain situation. But before I give the culprit away, what's the situation in the armpit anyways?


Armpits are host to lovely, multi-species communities of bacteria. According to this paper, the main players in the armpit game are Staphylococci, Corynebacterium, Clostridia, Alicyclobacilli. These bacteria love to grow in 'pits because they are offer a cornucopia of bacteria-food, in the form of various human secretions. In your armpits, there is not only sweat, but also sebum. Both of these secretions are, in the absence of bacteria, more or less transparent and odorless. These secretions also have functions - sweat is important for temperature regulation, and sebum helps keep your skin and hair from drying out.


I have a hypothesis about why we think of sweat and sebum as being yucky in this day and age. I have no data at all to back this up, but I think it's because we spend too much time inside. When I go on camping trips, I definitely sweat, and probably sweat more than I do on a typical workday in the lab. But, I'm less bothered by it. I think this is because when I'm on a camping trip, the outdoor elements seem to magically blow away some sweat and sebum. Sun evaporates it and rain washes it away, before bacteria can multiply in it. But anyways, back to armpits.


As I mentioned above, sweat and sebum aren't inherently smelly. It's when bacteria come into the picture that things get smelly. Here, sebum is described as being made up of "triglycerides, free fatty acids, wax esters, squalene, cholesterol esters, and cholesterol", which is basically Christmas dinner for bacteria. And where there is lots of food for bacteria, there will be lots of bacterial babies on the way. But those bacteria babies won't make yellow pitstains on their own. This is where humans come in.


For hundreds of years, humans have been coming up with ways to make ourselves smell better. There are endless scented products available now that we spray on or rub into our skin, two of which are pretty specific to the pits: deodorants and antiperspirants. Both of these products are intended to counteract the smell of bacteria growing in armpits, but there is one key difference. While deodorants simply aim to cover up the smell, antiperspirants aim to do as their name implies and actually prevent the sweating and smelliness.


One of the many chemicals used in antiperspirants is aluminum salts, which are a double-hitter: they both control sweat and can kill bacteria. But, they have a side effect. It is the mixing of this aluminum with sweat that leaves behind yellow stains on clothing. Those stains aren't due merely to bacterial growth, but rather to the way we've tried to get rid of the microbes.


So, the best way to get rid of these stains isn't to scrub off bacteria, it's to use other (acidic) chemicals like vinegar that are able to more or less dissolve the stuff stuck in the fabric. And the best way to prevent those stains? Being a little smellier! (Just kidding, aluminum-free deodorant does exist)


Until next week,

~ Alex


A note about the sources...

As I mentioned, there aren't very many scientific papers published on the topic of armpit bacteria, and even fewer about pit stains. These papers, and the references cited within them, were my main sources for this blog post. If you have more sources or more up-to-date information, feel free to reach out and let me know!


  1. Urban, J., Fergus, D. J., Savage, A. M., Ehlers, M., Menninger, H. L., Dunn, R. R., & Horvath, J. E. (2016). The effect of habitual and experimental antiperspirant and deodorant product use on the armpit microbiome.PeerJ,4, e1605. https://doi.org/10.7717/peerj.1605

  2. Acikgoz Tufan, H., Gocek, I., Sahin, U. K., & Erdem, I. (2017). A novel washing algorithm for underarm stain removal. IOP Conference Series: Materials Science and Engineering, 254(8).


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