Stinky success: Scientists identify the chemistry of B.O.

Sweat keeps us cool. But the stuff that seeps out from our armpits can create a powerful stink. Deodorants can help combat that odor. But to really tackle it, we first need to know what causes it. A new study finds that bacteria found only in our armpits make an enzyme that turns our sweat into smelly B.O.

Glands in our skin make two types of sweat. Eccrine (EK-kreen) glands located all over the body release watery sweat. It cools us as it evaporates. Apocrine (APP-oh-kreen) glands under our arms and in our private parts also release liquid. Although it might seem like garden variety sweat, it’s different from the watery eccrine type.

The apocrine liquid doesn’t actually stink. In fact, it doesn’t have any odor. But when nearby bacteria go to work, that sweat takes on a distinctive foul scent.

Michelle Rudden and Gavin Thomas wanted to know how apocrine sweat turns into that off-putting body odor. These biologists work at the University of York in England. Two years ago, their team identified the bacteria responsible for creating body odor. Many species of staphylococci (STAF-ih-luh-KOK-ee) live on our bodies. But this team turned up one that lives only where there are apocrine glands. Called Staphylococcus hominis (STAF-ih-luh-KOK-us HOM-in-iss), it’s even named for us. (That hominis refers to humans.)

Enzymes are molecules that drive reactions. Rudden and Thomas were looking for an enzyme that would break down apocrine sweat. This could release compounds that might move through the air — right to someone’s nose. To find it, they teamed up with York chemist Anthony Wilkinson. They wanted to find the enzyme that releases a thiol (THY-oll) called 3M3SH. This molecule creates the tangy, acrid smell that is the hallmark of B.O.

Exactly what makes thiols so stinky is a mystery, Rudden says. But it’s related to the sulfur atom in the molecule — the “S” in 3M3SH. “By itself, sulfur is odorless,” she says. But when bonded to another atom, it can give rise to an unpleasant scent. Our noses have what amounts to “a chemical sensor” to detect it, Rudden notes. Those receptors have played an important role in our history, she adds. They let our ancestors “sniff out rotten food or potential poisons,” she explains.

Breaking it down

The team started with the odorless molecule Cys-Gly-3M3SH. Cys stands for cysteine (SISS-teen) and Gly for glycine (GLY-seen). These two amino acids are basic building blocks of proteins. Apocrine glands release Cys-Gly-3M3SH. To become stinky 3M3SH, enzymes have to remove those two amino acids.

First, the researchers analyzed the genetic material, or genome (JEE-nohm), of each Staphylococcus species in the armpit. They found that all these bacteria had the gene for an enzyme that could remove glycine. Now they needed to find the one that removed cysteine.

The team incubated each staph species with Cys-Gly-3M3SH. The mixtures sat overnight at 37º Celsius (98.6º Fahrenheit) — human body temperature. By the next day, only two species had produced 3M3SH. One was the S. hominis known to cause B.O.

Looking again at the genomes of the armpit bacteria, the team spotted a possible cysteine-clipper. Called ShPatB, this enzyme occurred only in the two staph species that produced 3M3SH. To find out if this enzyme removed cysteine from 3M3SH, the team inserted genes for the enzyme into other bacteria — ones that don’t cause body odor. Then they incubated these modified bacteria overnight with Cys-Gly-3M3SH. By morning, they had made stinky thiol. It showed they had found the enzyme key to creating B.O.

The team then studied this enzyme’s structure. They found it has a pocket. Cys-3M3SH fit perfectly inside it. This made the enzyme very efficient at breaking cysteine’s bond to the thiol.

Evolutionary hangover

The team then did an “evolutionary” analysis of the enzyme. That means they looked at when the gene for this enzyme might have evolved in staph bacteria and in which species it ended up. Only a handful of closely related species now have it, they learned. It could be that a common ancestor picked up a similar gene (called PatB) from another bacterium, the team now suggests. In fact, bacteria often swap genes. Over time, the PatB enzyme likely evolved a pocket to fit the molecules from apocrine sweat glands. With that, the ShPatB gene was born.

How long ago did this happen? Possibly as far back as 60 million years, the team now reports. That’s when primates were beginning to separate from their nearest relatives.

B.O. may have been an important communication molecule among ancient primates, Rudden says. But it doesn’t seem to benefit modern humans, she adds. Our underarm stink may well be an “evolutionary hangover” from our ancient ancestors.

The team described its findings in Scientific Reports on July 27.

This study shows “there is always something new to learn,” says Sylvie Garneau-Tsodikova. She is a microbiologist and biochemist at the University of Kentucky in Lexington. She was not involved in this study. “It is exciting to get a better understanding and details at the molecular level,” she says. She also finds it interesting that this stinky body odor has been around longer than humans have existed as a species.

Now that the York team has identified the chemistry of human body odor, they can develop new types of deodorants, says Rudden. These will “stop B.O. production at the source without disrupting the armpit microbiome.”

And keep in mind, Rudden quips, when it comes to stinky underarms, “It’s really not you — it’s your bacteria!”

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