Researchers at the University of York traced the source of underarm odour to a particular enzyme in a certain microbe that lives in the human armpit.
To prove the enzyme was the chemical culprit, the scientists transferred it to an innocent member of the underarm microbe community and noted – to their delight – that it too began to emanate bad smells.
The work paves the way for more effective deodorants and antiperspirants, the scientists believe, and suggests that humans may have inherited the mephitic microbes from our ancient primate ancestors.
“We’ve discovered how the odour is produced,” said Prof Gavin Thomas, a senior microbiologist on the team. “What we really want to understand now is why.”
Humans do not produce the most pungent constituents of BO directly. The offending odours, known as thioalcohols, are released as a byproduct when microbes feast on other compounds they encounter on the skin.
The York team previously discovered that most microbes on the skin cannot make thioalcohols. But further tests revealed that one armpit-dwelling species, Staphylococcus hominis, was a major contributor. The bacteria produce the fetid fumes when they consume an odourless compound called Cys-Gly-3M3SH, which is released by sweat glands in the armpit.
Humans come with two types of sweat glands. Eccrine glands cover the body and open directly onto the skin. They are an essential component of the body’s cooling system. Apocrine glands, on the other hand, open into hair follicles, and are crammed into particular places: the armpits, nipples and genitals. Their role is not so clear.
Writing in the journal Scientific Reports, the York scientists describe how they delved inside Staphylococcus hominis to learn how it made thioalcohols. They discovered an enzyme that converts Cys-Gly-3M3SH released by apocrine glands into the pungent thioalcohol, 3M3SH.
Thomas said: “The bacteria take up the molecule and eat some of it, but the rest they spit out, and that is one of the key molecules we recognise as body odour.”
Having discovered the “BO enzyme”, the researchers confirmed its role by transferring it into Staphylococcus aureus, a common relative that normally has no role in body odour. “Just by moving the gene in, we got Staphylococcus aureus that made body odour,” Thomas said.
“Our noses are extremely good at detecting these thioalcohols at extremely low thresholds, which is why they are really important for body odour. They have a very characteristic cheesy, oniony smell that you would recognise. They are incredibly pungent.”
The research, a collaboration with Unilever, raises new possibilities for deodorants that target only the most active BO-producing microbes while leaving the rest of the underarm microbial community untouched. “If you can have a more targeted approach that selectively knocks down Staphylococcus hominis, it could be longer lasting,” said Thomas.
Michelle Rudden and others on the study next looked at the genetic relationships between dozens of Staphylococcus species. The analysis suggests, tentatively, that only a handful inherited the BO enzyme from an ancient microbial ancestor about 60m years ago.
Since apocrine glands only secrete BO-making compounds from puberty onwards, the odours may have played a role in shaping humanity. “All we can say is this is not a new process. BO was definitely around while humans were evolving,” Thomas said. “It’s not impossible to imagine these were important in the evolution of humans. Before we started using deodorants and antiperspirants, in the last 50 to 100 years, everyone definitely smelled.”
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