Explainer: The bacteria behind your B.O.

There are some aspects of being human that just aren’t very glamorous. One of them, without question, is our body odor. Most people sweat when it gets hot outside or we exercise. But that reek emanating from our armpits and private parts? That’s not from a hearty workout. In fact, it’s not from us at all. Our distinct funk comes thanks to bacteria living on our skin.

Bacteria take innocent, non-smelly chemicals and turn them into our human stank, a recent study shows. The results suggest that while our body odor might be unappreciated now, in the past it may have been part of an individual’s allure.

Our armpits sport glands — groups of cells that produce secretions — called apocrine (APP-oh-kreen) glands. These are found only in our armpits, between our legs and inside our ears. They secrete a substance that might be mistaken for sweat. But it’s not that salty water that seeps out, all over our bodies, from other eccrine [EK-kreen] glands. The thick secretion released by apocrine glands is instead full of fatty chemicals called lipids.

If you take a whiff of your underarm, you might think this secretion stinks. Scientists have been trying to figure out the source of our signature scent. They have put forward many different molecules as the source of body odor, notes Gavin Thomas. He’s a microbiologist — a biologist who specializes in one-celled life — at the University of York in England.

Scientists used to think that hormones might cause our sweaty smell. But “it doesn’t look as if we make those in the underarm,” Thomas says. Then scientists thought our sweaty smell might come from pheromones (FAIR-oh-moans), chemicals that affect the behavior of other animals. But those didn’t seem to matter much either.

In fact, the thick secretions from our apocrine glands don’t smell very much on their own. This is where the bacteria come in, says Thomas. “Body odor is the consequence of bacteria in our underarms.”

Bacteria are real stinkers

Bacteria coat our skin. Few have stinky side effects. Staphylocci (STAF-ee-loh-KOCK-ee), or staph for short, are a group of bacteria that live all over the body. “But we found [this] particular species,” Thomas reports, “which only appears to grow in the underarm and other places where you have these apocrine glands.” It’s Staphylococcus hominis (STAF-ee-loh-KOK-us HOM-in-iss).

Thomas looked at the diet of S. hominis while he was working with other scientists at the University of York and at the company Unilever (which produces body products such as deodorant). This germ takes up residence in your pits because it loves to dine on a chemical from the apocrine glands. Its favorite dish is called S-Cys-Gly-3M3SH. S. hominis pulls it in through molecules — called transporters — in its outer membrane.

The molecule has no smell on its own. But by the time S. hominis is done with it, the chemical has been transformed into something called 3M3SH. This is a type of sulfurous molecule called a thioalcohol (Thy-oh-AL-koh-hol). The alcohol part ensures that the chemical escapes easily into the air. And if it’s got sulfur in its name, that hints it’s likely to stink.

What does 3M3SH smell like? Thomas gave a group of non-scientists in a local pub a whiff. Then he and asked them what they had smelled. “When people smell thioalcohol they said ‘sweat,’” he says. “Which is really good!” It means that the chemical is definitely a component of the body odor we know and loathe.

Thomas and his colleagues published their findings in 2018 in the journal eLife.

Other staph bacteria also have transporters that can suck up the odorless precursor from our skin. But only S. hominis can make the stink. That means that these microbes probably have an extra molecule — one other staph bacteria don’t make — to chop up the precursor inside S. hominis. Thomas and his group are now working to figure out exactly what that molecule is and how it works.

And there’s still more to the story

3M3SH is definitely a part of our distinctive sweaty scent. But it’s not working alone. “I’ve never smelled someone and thought ‘Oh, that’s the molecule,’” says Thomas. “It’s always going to be a complex of smells. If you smell somebody’s underarm it’s going to be a cocktail [of scents].” The other ingredients in that cocktail, though, vary from person to person. And some of them still await discovery.

B.O., it seems, is a partnership between our apocrine glands and our bacteria. We produce 3M3SH, which has no smell. It serves no purpose, except to act as a delicious snack for the bacteria that turn it into the stink in our sweat.

That means that our bodies may have evolved to produce chemical precursors, just so the bacteria could gobble them up and make us stink. If true, why would our bodies aid bacteria to make these smells. After all, we now spend so much time trying to make those smells disappear.

In fact, Thomas says, those odors may have mattered far more in the past. People are very sensitive to the stink of sweat. Our noses can sense 3M3SH at only two or three parts per billion. That’s two molecules of the chemical per billion of molecules of air, or the equivalent of two drops of ink in a 4.6-meter (15-foot) diameter backyard swimming pool.

What’s more, our apocrine glands don’t become active until we hit puberty. In other species, smells like these are involved in findings mates and communicating with other members of a group.

“So it doesn’t take a huge leap of imagination to think 10,000 years ago maybe smell had a much more important function,” Thomas says. Until a century ago, he says, “We all smelled. We had a distinct smell. Then we decided to shower all the time and use a lot of deodorant.”

His research has made Thomas a little more appreciative of our natural fragrance. “It makes you think it’s not such a bad thing. It’s probably quite an ancient process.”