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Hair Loss: Is the Scalp Microbiome to Blame?

Hair Loss: Is the Scalp Microbiome to Blame?

Hair loss is a phenomenon that causes concern among both men and women. To find a suitable treatment to remedy this, the cause must be determined. The scalp microbiome could be responsible. Let's delve into this in this article.

Published January 31, 2024, by Manon, Head of Scientific Communication — 5 min read

What is the scalp microbiota?

Our skin hosts numerous bacteria and viruses that live on the surface and in the superficial layers of the epidermis, forming a complex ecosystem. The scalp is categorised among the thick skins as it has an average thickness of 8 mm, compared to 0.5 mm for the eyelids. It is rich in sebaceous glands which produce sebum released via the follicular ducts. Unlike other skin areas of the body, it is therefore constantly moist due to an increased secretion of sebum.

Just like the rest of the skin, the scalp is home to a multitude of microorganisms that make up the microbiota. This population that colonises this skin surface consists of bacteria, viruses or yeasts. Among the bacteria, you will find, among others, Cutibacterium acnes, Propionibacterium acnes or Staphylococcus epidermidis. There are also yeasts such as Malassezia globosa or Malassezia restricta. The Malassezia yeasts do not produce lipids themselves and can benefit from the lipids of the sebum. Other living beings acting to synthesise nutrients are also present to promote hair strand growth.

For optimal health, the entire composition of the microbiota works together to maintain the balance of the scalp's vital functions. It acts as a protective system against allergens, free radicals, pollutants and other external aggressors. It also soothes itching and redness that appear on the skin's surface.

Does the scalp microbiota promote hair loss?

The scalp microbiota is home to numerous bacteria, notably Propionibacterium acnes and Cutibacterium acnes which have been found to potentially play a role in hair loss.

What about Propionibacterium acnes ?

Indeed, in 2012 a study was conducted on 4 patients suffering from diffuse alopecia. After a targeted antibiotic treatment for Propionibacterium acnes, the alopecia had disappeared. Propionibacterium acnes, is an aerotolerant and anaerobic Gram-positive bacillus that produces propionic acid as a metabolic by-product. It is ubiquitous in the sebaceous glands of humans and derives its energy from the fatty acids present in sebum.

Propionibacterium acnes synthesises numerous enzymes that degrade the extracellular matrix (hyaluronate lyase) and the corneal envelope, (endoglycoceramidases, sialidase). It also synthesises enzymes that degrade heat shock proteins which activate the innate immune system (GroEL, DnaK), and other enzymes involved in the metabolism of porphyrins. Porphyrins are activated by ultraviolet light to produce reactive oxygen species, which can contribute to follicular inflammation via the complement cascade. These porphyrins are generally associated with cases of androgenetic alopecia and are thought to be involved in the hair loss process, although their mode of action is still not fully understood. This study suggests that the bacterium Propionibacterium acnes is directly involved in the pathogenesis of this condition due to the improvement in alopecia observed during treatment with antibiotics against Propionibacterium acnes.

And what about Cutibacterium acnes ?

In the case of androgenetic alopecia, the bacterium Cutibacterium acnes has been found to be present in greater quantities compared to patients not suffering from this condition. Cutibacterium acnes is capable of synthesising numerous enzymes involved in the metabolism of porphyrins which, once activated, can contribute to oxidation and follicular inflammation. The same conclusion as the previous study was made, assuming that porphyrins were involved in the hair loss process, without yet knowing its mode of action.

What should we remember?

Based on previous results, a connection has been established with a scalp colonised by one or more bacteria present in larger quantities than normal, leading to an imbalance in the scalp's microbiota. This will trigger a cascade of reactions resulting in hair loss, but further analysis is necessary to understand the progression of these reactions.


WANG E. & al. Is Propionibacterium Acnes Associated with Hair Casts and Alopecia? International Journal of Trichology (2012).

FILAIRE E. & al. Androgenetic Alopecia: Microbiota Landscape and Role of Lindera strychnifolia Roots Extract as a Natural Solution for Hair Loss. International Journal of Cosmetic Science (2020).


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