Acid mantle: What we need to know

Acid mantle and permeability barrier

Homoeostasis: Lipids secreted by lamellar bodies are processed by enzymes, namely β-glucocerebrosidase and acidic sphingomyelinase, to generate ceramides and other lipophilic components. These molecules are then arranged into a lamellar bilayer structure around the corneocytes,and are responsible for the permeability barrier function of the stratum corneum. The lipid processing enzymes are active at acidic pH and are inhibited at higher pH. Thus, the acid mantle helps in the formation and maintenance of the permeability barrier. At higher pH, this barrier function is compromised making the skin susceptible to various allergens. ^{5 , 6}

Desquamation: Physiological desquamation is an essential component of homeostasis of the stratum corneum and is brought about by the activities of serine proteases (kallikrein 5 and kallikrein 7) which cleave desmoglein 1 and alkaline ceramidase. These enzymes are active at neutral or higher pH and their continued activity inhibits the secretion of lamellar bodies. When the acid mantle is disturbed, persistent and appreciable exfoliation is noted, as seen in a variety of skin conditions. ^{5 , 6}

Acid mantle as an antimicrobial barrier

Acidic pH favours the growth of normal flora and inhibits the pathogenic bacteria. Thus, the acid mantle helps in the formation of a non-specific antimicrobial barrier. Additionally, certain antimicrobial peptides like dermcidin are best effective at acidic pH. In a study, it was shown that dermcidin offered greater than 90% bactericidal effect against Staphylococcus aureus at pH 5.5; however, at pH 6.5 the bactericidal effect was reduced to 60 percent. Besides, an acidic pH favours the conversion of nitrate (secreted in sweat) to nitrite (a non-specific antimicrobial compound) by bacteria; which re-emphasises the antimicrobial role of the acid mantle. ¹

Acid mantle and inflammation

Corneocytes have pools of precursor forms of inflammatory cytokines (pro-IL-1α and pro-IL-1β). With the disturbance of barrier function and rise of pH, kallikreins are activated which in turn causes a release of active cytokines IL-1α and IL-1β. These cytokines through a cytokine cascade restores the stratum corneum barrier. While a transient increase in skin pH helps in the restoration of the stratum corneum barrier, a persistent change in pH contributes to epidermal inflammation mediated by IL-1α and IL-1β. ⁵

Skin pH and dermatoses

Increased skin pH leads to impaired barrier function (as measured by increased trans-epidermal water loss and increased sensitivity to sodium lauryl sulphate), defective exfoliation and increased susceptibility to infections by pathogens including Staphylococcus aureus and Candida albicans. These altered biological functions are central to the pathogenesis of several dermatoses. Understanding the role of altered skin pH in the development of different dermatoses is helpful in designing novel treatment strategies as summarised in Table 3. For example, defective barrier function is the pathophysiological hallmark of both atopic dermatitis (because of endogenous filaggrin deficiency) and irritant contact dermatitis (because of exposure to irritants). Attempts at restoring the acid mantle and thereby facilitating repair of stratum corneum barrier functions by topical acidic preparations have led to significant clinical improvements. In a study, twice daily application of 4% l-lactic acid formulations (pH 3.7–4.0) led to significant improvement in the stratum corneum ceramides concentration, barrier function and reduced sensitivity to sodium lauryl sulphate after four weeks of application. ^{1 , 4 – 6} Another instance, where the defective stratum corneum barrier function due to altered skin pH might play a role, is women complaining of increased pruritus in the genital area during menstruation. Blood with a pH of 7.4 raises the vaginal pH (3.5–4.5) and leads to a transient defect in the stratum corneum barrier. As a result, women complain of dryness and pruritus. To tackle this issue, tampons with micro–ribbons™ containing citric acid and l–lactide have been developed and are commercially available in certain countries. ¹

Skincare products and cosmetics

As the role of skin pH in health and disease is being studied, it is imperative to apply the concepts in formulating various topical formulations like soaps, cleansers, moisturisers, other skin care products and cosmetics. Such products are meant for daily use and hence, they can influence the skin pH to a great extent. ^{11 , 12} It has been demonstrated that hand washing with alkaline soaps can increase the skin pH (mean of three units) for a time period of up to 90 minutes after washing. Homeostatic mechanisms discussed earlier continue to work to restore skin pH. However, frequent use of alkaline soaps ultimately causes sustained increase in surface pH leading to impaired barrier repair functions. ¹ Hence, daily hand washing with soaps has an etiological role in the initiation, maintenance, aggravation and progression of hand eczema. The widespread recognition of the skin pH-altering potential of soaps led to the development of syndets with a neutral or acidic pH. In addition, cleansers may have active acidic ingredients like lactic and citric acid, sodium acetate, sodium lactate, sodium citrate, diammonium citrate, etc., to match the physiological skin pH. Cleansers with acidic pH alter the acid mantle to a lesser extent and for a lesser time, and hence, are considered suitable for daily use, frequent use, patients with sensitive skin syndrome and other conditions related to an increased skin pH. ^{1 , 4 , 13}

Unfortunately, there is a dearth of pH-balanced skin care products and cosmetics. Also, information about pH-balanced products is not readily available. Scientists, pharmaceutical industries, physicians, patients, the general population and legislators need to recognise the importance of pH-balanced skin care products and cosmetics in skin health and disease.