Skin acidity and infection

Svetlana Tkachenko, Candidate of Medical Sciences, Associate Professor of the Department of Dermatology, Venereology and Medical Cosmetology, Kharkov National Medical University (Kharkov).

The fact that the surface of the skin has an acidic pH became known at the end of the 19th century. Later, Alfred Marchionini proved the protective properties of the acidophilic shell and since then the term “Marchionini acid mantle” has been used in cosmetology. Scientific data obtained over a century and a half indicate that, with the exception of some areas, the surface of the skin has acidic properties. Today, thanks to the work of Otto Braun-Falco, the generally accepted skin surface pH is 5.5.

We have already told you what acidity is and what it depends on , as well as how it affects the skin microflora . Today we will talk about how it is related to skin infections.

Favorable environment

Skin antimicrobial protection includes mechanical rigidity (inelasticity) of the stratum corneum, reduced moisture content, lipids of the stratum corneum, lysozyme, pH 5. It is generally accepted today that the normal pH of the skin surface has a beneficial effect on the skin microflora.

Acidic skin pH (pH 4.0–4.5) helps resident bacterial flora remain attached to the skin and prevents pathogens from entering the skin, while alkaline pH (8.0–9.0) helps dispersal of bacteria. Acids produced by resident bacteria also contribute to local protection. For example, S. epidermidis, P. acnes, Pityrosporumovale, Corynebacteria produce lipases and esterases that break down triglycerides into free fatty acids, leading to a decrease in the pH of the skin surface and thus creating unfavorable conditions for skin pathogens.

An acidic skin pH also facilitates the production of natural antimicrobial peptides, helps wound healing, and regulates keratinization and desquamation. The skin flora produces protein and lipid antibacterial components - bacteriocins, which are involved in bacterial competition for survival in this microenvironment. For example, the bacteriocin Pep 5 produced by S. epidermidis is partially active against other staphylococci, especially S. aureus. Interestingly, acidic skin pH increases the activity of these antibacterial lipids and peptides by better interacting with the bacterial membrane.

Dermatoses and acidity

The acidic pH of the skin balances the environment for resident bacteria. Changes in skin pH and other factors play a role in the pathogenesis of certain dermatoses, their prevention and treatment.

P. acnes is a classic example of how slight alkalization of the skin surface can facilitate the transition of a resident bacterium to a pathogenic one. At a normal pH of 5.5, the growth of P. acnes is minimal, but a slight shift to the alkaline side helps this microorganism develop vigorously.

When using occlusive dressings in medical practice, it is necessary to take into account that prolonged occlusion of the skin significantly disrupts the skin pH and the growth of normal skin flora, accelerates transepidermal water loss and carbon dioxide emission. Recent studies have shown a relationship between changes in skin pH and its consequences in atopic dermatitis, especially impaired skin barrier function and increased colonization by S. aureus. In atopic eczema, not only the skin pH is significantly increased compared to normal healthy skin, but also the growth of S. aureus and exotoxin production are enhanced, which can induce eczema in healthy skin.

Risk of candida infection

Changes in skin pH from acidic to alkaline may also be a risk factor for the development of candida infection. An interesting study was in which a suspension of C. albicans was applied to the right and left forearms pre-buffered to pH 6.0 and 4.5 and occluded for 24 hours. More pronounced inflammatory phenomena were observed at high pH, ​​which confirmed the increase in yeast virulence by high skin acidity values, as well as the ability of pH to modulate anti-infective protective ability.

It was also found that the pH level in the skin folds of 50 patients suffering from type 2 diabetes mellitus (non-insulin-dependent) was significantly higher than in healthy volunteers. Based on this, skin acidity has been implicated as a risk factor for candida infection. Dialysis patients also showed a significant increase in skin surface acidity.

On the surface of skin folds, such as the armpits, the pH is physiologically higher than in other areas of the skin, which promotes the growth of local flora. In addition, it was found that axillary deodorants work due to the action of the secretion of the axillary apocrine glands on local bacteria. Application of deodorant showed a significant decrease in pH in the axillary region, which inhibited the growth of bacteria in the axillary region.

Water and soap as influence factors

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As previously thought, many external factors influence the pH of the skin surface. Some of them include the use of soaps, detergents, cosmetic products. Long-term use of these agents disrupts the acidity of the skin surface and damages the skin microflora to some extent, at least in the short term. There is evidence that washing damaged skin with soap and water is not effective in reducing bacterial colonization. It is known that small changes in skin surface pH from normal levels (pH 5.5) to more alkaline values ​​(eg, pH 6.0) can enhance the growth of P. acnes, but do not affect the proliferation of S. aureus. This finding is especially important since small increases in pH can occur as a result of soap use. The growth of Brevibacterium epidermidis, which is associated with body odor, can only be prevented if the pH is reduced to 5.0 or lower. Interestingly, washing with tap water with a pH of about 8.0 can increase the acidity of the skin and keep it in this state for up to 6 hours after the procedure.

At the same time, the microflora of healthy skin is quite resistant to external influences of the ecosystem. Studies have shown that washing daily for three weeks, or not washing for the same amount of time, will not lead to either an overgrowth of transient microflora or a significant shift in the composition of resident microorganisms.

However, repeated use of alkaline cleansers, detergents and even plain water (pH 8.0) can adversely affect the natural pH of the skin and disrupt normal flora. To maintain normal physiology and microflora of the skin, you need to use cosmetics and skin cleansers that do not damage the pH of the skin, or you should take into account the side effects on the skin flora and correct them in a timely manner. Also, to regulate healthy skin flora and maintain optimal skin biochemistry, topical pre- and probiotics, which are promising according to researchers, can be used.