The human armpit has a lot to offer bacteria. It's moist, it's warm, and it's usually dark.
But when the bacteria show up, they can make a stink. That's because when some kinds of bacteria encounter sweat they produce smelly compounds, transforming the armpit from a neutral oasis to the mothership of body odor. And one group of bacteria is to blame for the stink, researchers say.
The researchers took bacteria commonly found in the armpit and added an odorless molecule found in human sweat. "These odorless molecules come out from the underarm, they interact with the active microbiota, [and] they're broken down inside the bacteria," explains Dan Bawdon, a postdoctoral researcher at the University of York in England, who led the study.
When the bacteria break down the sweat they form products called thioalcohols, which have scents comparable to sulfur, onions or meat. "They're very very pungent," says Bawdon. "We work with them at relatively low concentrations so they don't escape into the whole of the lab but ... yes, they do smell. So we're not that popular."
The thioalcohol molecules evaporate from the underarm, which is what makes the armpit smelly. So Bawdon and his advisor Gavin Thomas, a senior lecturer in microbiology at the University of York, measured how much thioalcohol each bacteria species produced. In the end, they could point to Staphylococcus hominis as one of the worst offenders. They announced their findings Monday at the Society for General Microbiology's annual conference in Birmingham, England.
The two researchers are hoping that their findings will change the way that we engineer deodorants fight body odor. Most deodorants block sweat glands or kill off underarm bacteria. Blocking the sweat glands sometimes leads to irritated or swollen skin. And given all the new research into the complexity of the human microbiome, the researchers are a little anxious that deodorants may kill good bacteria, too.
It's hard to say whether the bacteria in the armpit are helping the human body the way that gut bacteria or skin bacteria do. "But it kind of makes sense to not kill everything," says Thomas. "As we know from antibiotics, if we can design something specific that's probably going to be a more sensible approach."
He and Bawdon envision a deodorant that would keep armpit bacteria from producing thioalcohols. They borrowed their bacteria from Unilever, a company in the Netherlands and the United Kingdom that produces personal care products. The company provided a small amount of funding so that it can use the research results to make next-generation deodorants.
But before such a deodorant shows up on the shelf, the researchers need to make sure that there aren't other smelly processes taking place in the armpit. There may be other molecules that make the armpit smelly, and the researchers haven't yet finished their quest to describe them all.
"It's an extremely exciting time to be a microbiologist," Thomas says. Of those many denizens of the armpit, he says, "We haven't yet really figured out why they're there and exactly what they're all doing."