It's the Pits

Your armpits carry millions of tiny microbial passengers. Watch scientists study the ecosystem of the armpit and learn how deodorants can affect not only your social life, but also your microbial life.

RALEIGH — This is a story about pits. 

No, not the pits needed to make famous North Carolina barbecue. We're not talking about construction pits, or fruit pits either. 

We're talking about another kind of pit that everyone has. 

You guessed it. Armpits. 

When you think of armpits, a common thought is that they are kind of gross, or kind of smelly. But the thing is, on a cold day or a hot day, the armpit is a warm, damp place. And if you’re a bacteria, that’s a happy place; a place to thrive. 

So when you're talking about armpits, it’s not exactly the person that has the odor; it’s the bacteria living on the person that produces the odor. The bacteria metabolize the sweat, and give off a smell as this occurs. 

Of course, to stop that smell, people wear deodorants and antiperspirants. But those underarm hygiene products don’t just affect your social life. 

“Some of our research dealt with finding out how the use of deodorants, antiperspirants, or even no product influences what microbes live on your skin,” explains Dr. Julie Horvath, Associate Research Professor at North Carolina Central University. The University has been in collaboration with the North Carolina Museum of Natural Sciences on this research. “We’ve found that the products also significantly change the type and quantity of microbial life that lives on you.” 

More specifically, the chemicals alter the microbiome of the human armpit. So before learning which microorganisms can live in your armpit, it’s important to first understand the effects of underarm hygiene products. 

Deodorants contain things that kill off microorganisms, such as ethanol or other antimicrobial agents. They kill the microbes that create the odor. The products might also contain a fragrance to mask any excess odor.  

Antiperspirants contain aluminum based salts that get into sweat glands, and block the glands so that sweat is not produced. If there's no sweat, there’s no food for the microbes producing the odors. 

In conducting their study, researchers with NCCU and the NC Museum of Natural Sciences took daily armpit swabs from 17 men and women over the course of eight days. Some participants used deodorants, some used antiperspirants and some used no product at all. The study subjects followed their normal hygiene routine on day one, quit using all products on days two through six, and then only used antiperspirant on days seven and eight. 

The researchers then cultured all of the samples and discovered that, while the numbers of microbes varied widely in the first days of the study, by day six, the amount of bacteria for all participants was about the same. 

“We take the swabs with the bacteria and place some of the bacteria in these trays, which contain an artificial food source, and then let them grow,” says Dr. Julie Urban, at the North Carolina Museum of Science. We're at the museum, and she pulls the tiny round trays out of an incubator. The trays all contain a gel, which has hundreds of circles growing on it. The circles are large and small, and a variety of colors. 

“Those are colonies of bacteria that have grown and multiplied,” explains Dr. Urban. “Now we can not only look at them, but we can do DNA sequencing to identify the specific bacteria and better understand how various products affect microbial diversity over time.” 

The first finding that surprised researchers was how the armpit microbiome changed on days seven and eight of the study, when participants began using antiperspirants. Researchers found very few microbes in any sample, verifying how drastically the products reduced microbial growth. 

“You could almost think of this like a field and a fire went through and it wiped out all life,” says Dr. Horvath. “When you wear product you are wiping out most of the life on your skin, so just like the weeds are the first to grow back after a fire, it’s the staph bacteria on humans.” 

Staphlococcaceae bacteria, or staph, is a very common and very fast growing bacteria. 

The samples also turned up a greater diversity of microbes in the armpits of people who wore antiperspirants. At first, researchers say they were surprised by the findings, because those products are designed to block sweat glands. 

“But it makes sense once you think about it because by using those types of products, you are introducing, or selecting for a very different environment for bacteria to grow,” says Dr. Urban. 

Researchers plan to do more studies with more people, but initial finds confirm that using antiperspirant and deodorants can completely rearrange the microbial ecosystem on your skin, including what’s living on us and it's level of accumulation. 

It’s not clear whether those changes are positive or negative, because until now, the microbes that are there and the effects of daily habits on those microbes were somewhat of a mystery.

Researchers recognize that their work so far at least has people thinking about the effects of products on their skin and the microbes that live there, in addition to the effects on what is a main concern for many: whether or not their armpits smell.

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