Explainer: The bacteria behind your B.O. | Science News for Students

Explainer: The bacteria behind your B.O.

Our armpit sweat isn’t stinky until certain bacteria come to dine
Aug 22, 2019 — 6:40 am EST
a photo of a woman with sweat showing through her shirt, her arms are raised above her head

Relax. Everybody sweats. But what causes that distinctive sweaty smell? Blame your bacteria.

Dandamanwasch/iStock/Getty Images Plus; adapted by L. Steenblik Hwang

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.

a very sweaty lady
A good workout in the gym can leave you wet, but it’s not stinky. Body odor develops only when certain underarm secretions become altered by bacteria living on the skin.
PeopleImages/E+/Getty Images

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.”

Power Words

(more about Power Words)

apocrine gland   A sweat gland in mammalian skin that secrete oil compounds in people. Humans have apocrine glands in their armpits and private areas. Other animals, such as horses, have apocrine glands all over the body and use their secretions to keep themselves cool.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

B.O.     Short for body odor. It’s the pungent scent that usually is associated with sweaty underarms.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

component     Something that is part of something else (such as pieces that go on an electronic circuit board or ingredients that go into a cookie recipe).

diet     The foods and liquids ingested by an animal to provide the nutrition it needs to grow and maintain health. (verb) To adopt a specific food-intake plan for the purpose of controlling body weight.

eccrine glands   These are sweat glands in mammalian skin that mainly secrete salty water. In mammals such as mice and cats, eccrine glands are on the pads of the feet. Their sweat helps these animals keep their grip on slippery surfaces. In humans, eccrine glands occur all over the body, and pump out sweat to help us cool our bodies.

gland     A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

lipid     A type of fat.

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

microbiology   The study of microorganisms, principally bacteria, fungi and viruses. Scientists who study microbes and the infections they can cause or ways that they can interact with their environment are known as microbiologists.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

pheromone     A molecule or specific mix of molecules that makes other members of the same species change their behavior or development. Pheromones drift through the air and send messages to other animals, saying such things as “danger” or “I'm looking for a mate.”

precursor     A substance from which some later thing is made. It may be a compound that will change into something else as a result of some chemical or biological reaction.

puberty     A developmental period in humans and other primates when the body undergoes hormonal changes that will result in the maturation of reproductive organs.

secrete     (noun: secretion) The natural release of some liquid substance — such as hormones, an oil or saliva — often by an organ of the body.

secretion     A liquid substance — perhaps a hormone, an oil or saliva — released by the body, often by an organ, such as the eye, pancreas or skin.

side effects     Unintended problems or harm caused by a procedure or treatment.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

staph     Short for Staphylococcus. It's a family of bacteria that is responsible for a number of serious human infections. One member of the family also contributes to the distinctly pungent smell of human sweat.

transporters      (in cellular biology) The structures on biological membranes through which ions and small molecules can enter cells.


Journal:​ ​​G.S. Minhas​ ​et​ ​al.​ ​ Structural basis of malodour precursor transport in the human axilla.​ ​​eLife.​ ​​Vol.​ ​7,​ ​July 3, 2018. doi:​ ​10.7554/eLife.34995.