Redness, skin that turns white on contact with water, excessive sweating and above all bad foot odor make up a very disabling picture of Nagashima type palmoplantar keratoderma.
For a long time, this very particular smell remained mysterious, even for dermatologists, but a team from Kobe University, in Japan, shows that it is the result of a very specific imbalance in the skin microbiota.
A genetic disease that affects 10,000 people in Japan
Affecting approximately 10,000 people in Japan and several hundred thousand more in East Asia, palmoplantar keratosis Nagashima type (NPK-N) is a condition that causes a variety of symptoms, including redness on the palms of the hands and soles of the feet, as well as a characteristic odor. However, the cause of this odor remained poorly understood.
Dermatologist Kubo Akiharu recalls the work of his team: “In 2013, our group identified SERPINB7 as the gene responsible for NPPK. Since then, more patients have come to our clinic and we have learned that many of them are very bothered by the odor of their feet. This pushed us to look for a way to help them“, he explained, quoted by Kobe University.
Kubo noticed that when the hands and feet of affected patients were soaked in water, the surface layer of the skin quickly absorbed the water and turned white. He hypothesized that this low skin impermeability led to bacterial proliferation, and that this proliferation could be the cause of the odor.
A key bacteria behind bad foot odor
To understand this smell, researchers compared 32 people with NPPK and 20 controls without skin disease, by taking bacteria from seven areas of the hands and feet, particularly between the toes. In the study published in the
Journal of Investigative Dermatologythey describe a significantly higher bacterial load on the affected areas, especially in the interdigital spaces, with a proliferation of
Staphylococcus epidermidis and above all
Corynebacterium tuberculostearicum. This imbalance, called dysbiosis, means more microbes, but also depleted bacterial diversity.
© Kobe University
Objective measurements show that foot odor is much more intense in NPPK patients than in controls, without any link to an associated mycosis. The authors suspect that
Corynebacterium tuberculostearicum
transforms sweat and skin debris into volatile molecules such as isovaleric acid, already involved in classic foot odor, but perhaps also into still unknown compounds which give this keratoderma its so characteristic odor.
To test this bacterial lead, the team applied 2.5% benzoyl peroxide gel, once daily, to the hands and feet of NPPK patients for 3 to 21 weeks. This treatment significantly reduced the bacterial load of Corynebacterium tuberculostearicumas well as the intensity of the measured odor, while the load in
Staphylococcus epidermidis the redness and thickening of the skin remained unchanged – thus revealing that the cause of these odors was the proliferation of C. tuberculostearicum .
“If the true smelly metabolites can be identified, it may become possible to develop more targeted treatments, for example by inhibiting the production of these metabolites.“added Kubo Akiharu.
Benzoyl peroxide and keratoderma: what are the prospects for foot odor?
The researchers emphasize that this approach remains exploratory, but that the identification of this bacteria and the search for the odorous molecules that it produces open up a concrete avenue for treating bad foot odor in palmoplantar keratoderma or even in other forms of keratoconjunctivitis sicca.