Dr Chris Callewaert, post-doctoral researcher at Ghent University, Belgium, and founder of the body odour research communication firm Dr. Armpit, described his research on the underarm microbiome during a presentation given at Probiota in Milan early last month.
He explained that the main causes of body odour are due to the production of sweat, lipids and hormones secreted from different types of sweat glands. But he said the resulting odour depends on the underarm microbiome composition.
“Fresh sweat secreted onto the skin does not have an odour, as these particles are too big and clumpy so don’t become volatile,” Callewaert explained. “But the underarm is very moist and nutritionally rich so the perfect habitat for billions of bacteria. The bacteria convert the large molecules into smaller molecules that can become volatile and have an odour.”
"But depending on what bacteria you have, that odour can be good or bad," he added.
His research assessed odours by smelling underarms to evaluate the hedonic value and intensity of odour to distinguish the types of bacteria associated with certain smells.
"The human nose is the best way to do this," Callewaert said. "There is no machine that can really say if something smells good or bad. So we then have a odour panel with trained panellists to achieve a measurable value."
He referred to a previous study involving 200 participants with self-proclaimed body odour that revealed that 80% of the underarm microbiome was dominated by Staphylococcus or Cyanobacterium species. Staphylococci species, as well as a lower diversity of species, was associated with a better odour.
"We also found significant sex differences," he said. "Women have more of the Staphylococci, and men have more of the Cyanobacterium because of greater quantities of hair and secretion of lipids, which is a preference for this bacteria."
This led to more recent investigations into the efficacy of underarm microbiome transplants for altering body odours, which were found to successfully modulate the microbiome to increase staphylococci species and subsequently improve odour.
Based on these findings, the research team developed a spray containing the ‘good’ species, which has demonstrated potential to alter the microbiome and improve long-term body odours in trials.
Impact of deodorant and lifestyle
Callewaert explained that deodorants are a relatively new concept, and thus their effect on the microbiome has not previously been studied.
"So we studied this and found that if you used antiperspirants, there will be a wider diversity and increased amounts of the less beneficial cyanobacterium. If none is used, there is higher dominance of staphylococci.
"If you use anti-perspirants, it kills bacteria, which will result in less odour. But the favourable underarm conditions will mean that they always come back, and these ones will be stress tolerant, pathogenic and smellier," he said.
This ultimately leads to the ‘bad’ bacteria becoming resistant to anti-perspirants.
He noted the potential additional influence of the skin-gut axis on body odour, with research suggesting that frequent meat and fast food consumption is associated with more unpleasant smelling underarms and that plants are associated with an improved microbiome and odour.
"Whilst this is the case with body odour, in a research paper we published a few years ago we found that this was the case for acne, soriosis, eczema, basically every skin disease has some sort of link to the gut microbiome—so it’s all interlinked," he said.