As a reminder, the acidic pH of the skin preserves the integrity and cohesion of the stratum corneum and protects the skin from microbial infections. Indeed, when the skin's pH increases, the enzymes responsible for the production of ceramides, which have an optimal acidic pH, are inactivated, compromising the structure and function of the stratum corneum. Moreover, while the enzymes synthesising lipids decrease, other enzymes increase at acidic pH: the serine proteases. These lead to a reduction in corneodesmosomes, by degradation of desmoglein-1, which have the role of ensuring the cohesion of the elements of the stratum corneum.
When the pH of the skin is disrupted, the skin no longer resists external aggressions and no longer retains its hydration.
This is favourable to the development of dermatoses, such as atopic dermatitis. Indeed, it has been demonstrated that the higher the pH, the more intense the itching and the greater the dryness. Besides the fact that in atopic dermatitis the structure of the horny layer is altered, colonisation by the Staphylococcus aureus is a common characteristic of affected patients and is considered a major pathogenic factor of atopic dermatitis. However, the adhesion of Staphylococcus aureus to human keratinocytes increases with the rise in pH.
The growth of protective bacteria in the skin microbiota is optimal at acidic pH levels, whereas pathogenic bacteria thrive at neutral and basic pH levels (Staphylococcus aureus, Cutibacterium acnes). Thus, a disruption in skin pH disturbs the skin's microbiota and hinders anti-microbial protection. The risk of infection is therefore higher. Moreover, serine proteases, which are activated by a high pH, trigger the synthesis of cytokines that cause inflammation.
Maintaining the skin's pH balance is therefore essential in order to preserve the integrity of the stratum corneum and the protective role of the skin.