Skin acidity and its microflora

Svetlana Tkachenko , Ph.D., Associate Professor of the Department of Dermatology, Venereology and Medical Cosmetology, Kharkov National Medical University (Kharkiv)

Victoria Sycheva , dermatovenerologist (Kharkov)

Currently, it is firmly established that the human body and the microorganisms inhabiting it are a single ecosystem. From a modern point of view, normal microflora should be considered as a collection of many microbiocenoses, characterized by a certain species composition and occupying one or another biotype in the body.

All normal human microflora is divided into several classes:

• resident (permanent), which accounts for up to 90% of microbes present in the body;
• optional - less than 9.5%;
• transient (random) - up to 0.5%.

About 20% of the total number of microorganisms live in the oral cavity (more than 200 species), 18–20% occur on the skin, 15–16% in the pharynx, 2–4% in the urogenital tract in men, and approximately 10% in the vaginal biotope in women, and most microorganisms (up to 40%) are found in the gastrointestinal tract.

Acidity and antimicrobial protection

The acidic pH of the skin is the result of its physiology, through which exogenous skin flora is regulated. At the same time, conditions are created for the resident microbiota, which maintains biological stability, purity and protection from pathogenic microorganisms.

Normal flora grows better at an acidic pH, while pathogenic bacteria (Staphylococcus aureus, etc.) grow better at a neutral pH. A more acidic pH helps protect the skin against colonization by non-resident and pathogenic bacteria, since many of them survive better in a narrow pH range around neutral. The acidic condition of the skin is caused by sweat gland secretions, sebum and the breakdown of fatty acids by S. epidermidis. Therefore, resident microflora is partly responsible for the acidic pH of the skin.

In addition, the standard skin temperature is slightly lower than normal body temperature, its surface is slightly acidic and predominantly dry, while most bacteria require a neutral pH, a temperature of +38 ° C and high humidity for optimal growth. Thus, the skin microenvironment dictates the microbial spectrum and population density.

Despite the variability of external conditions, the skin maintains a stable microbiological ecosystem. One of the prevailing hypotheses regarding the role of skin pH is its supposed importance in antimicrobial protection. A possible explanation for this theory is that the top layer of skin is very dry and tightly packed, making the first line of defense unfavorable for many bacteria. In addition, the salty secretion of the sweat glands makes the microenvironment hyperosmolar, which is also unfavorable for bacteria.

The multicentric study also found that acidic skin surface pH (4.0–4.5) keeps resident bacteria attached to the skin, whereas alkaline pH (8.9) increases bacterial dispersal across the skin. The importance of pH for antimicrobial function has been proven in neonatal eczema and atopic skin, which have a neutral pH.

Voluntary colonization

The development of bacterial flora on the skin from birth to adulthood has not yet been systematically studied. During the prenatal stage, the skin remains sterile, but soon after birth it becomes colonized with bacteria. At the same time, the skin allows the colonization and growth of only those bacteria that directly or indirectly protect the host from pathogenic microorganisms.

Colonizing bacteria can:

• produce antibiotics (bacteriocins) and toxic metabolites;
• preventively join competing bacteria, inducing a low redox potential;
• deplete essential nutrients;
• inhibit translocation;
• destroyed by toxins.

To prevent bacterial skin adhesion, hand washing with a skin cleanser containing microbial anti-adhesive ingredients that work through electrostatic interactions may be indicated. Recent studies have demonstrated that the use of lactic acid (1%, pH 3.0) and sodium carbonate decahydrate (1%, pH 11.0) under acidic conditions caused much less dispersal of resident flora from the inner surface of the forearm than under alkaline conditions. This confirms the role of electrostatic interaction between bacteria and the positive charges of the skin in an acidic environment.

Bacterial strains isolated from normal skin include Staphylococcus, Micrococcus, Corynebacterium, Brevibacteria, Propionibacteria, Acinetobacter. S. Aureus, Streptococcus pyogenes, Escherichia coli, Pseudomonas aeruginosa and are transient colonizers. Gram-negative bacteria are a lesser component of the flora of normal skin: Acinetobacter is one of the few Gram-negative bacteria commonly found on the skin. The presence of E. coli on the surface of the skin is an indicator of fecal contamination. The yeast is rare on the surface of the skin, but the lipophilic yeast Pityrosporum ovalis is occasionally found on the scalp.

Species and sexual characteristics

Racial and gender differentiation of skin microflora is not fully understood. More recent studies using molecular techniques have provided a better understanding of the microbial ecology of the skin. 182 strains of bacteria were identified on the skin of the human forearm, of which 8% were unknown strains and had never been described before.

In addition, some gender-specific features of the skin microbiota have been identified. Half of the bacteria identified in the samples were represented by the genus Propionibacteria, Corynebacteria, Staphylococcus, Streptococcus, which are usually considered to be resident flora of human skin. Among 6 individual samples, only 4 bacterial strains were common: Propionibacterium acnes, Corynebacterium tuberculostearicum, Streptococcus mitis, Finegoldia AB109769. Interestingly, 3 bacterial strains were found only in men: Propionibacterium granulosum, Corynebacterium singulare, Corynebacterium appendixes.

Higher acidity means more bacteria

The acidity of the skin surface influences many factors for the growth of resident flora and pathological microorganisms. Acidic pH is one of the main factors that makes the skin less hospitable to bacteria.

High densities of bacteria are found in regions of the skin with a less acidic pH, such as the genitofemoral areas, anal area, submammary folds, and axillary folds. Relatively dry and exposed skin areas have a lower pH and lower microbiopopulation density. For example, the inner surface of the forearms has a bacterial population of 102−103 cfu/cm2 (colony forming units), compared with 105 cfu/cm2, respectively, present in the axillary cavity area.

Artificial occlusion of the forearm leads to a significant change in skin pH, composition and density of bacterial strains. For example, in one study, before occlusion, the pH of the skin was 4.38, and after 5 days of occlusion it increased to 7.05. Similarly, if the number of bacteria before occlusion was 1.8 * 102 cfe/cm2, then after 5 days of occlusion it increased to 4.5 * 106 cfe/cm2.

It follows that a moist skin environment promotes bacterial growth and colonization. In skin folds, where the pH is slightly higher, there is an increased bacterial density. The composition of bacterial strains on the skin varies depending on the skin site (Table 1).

Protective barrier of bacteria

Normal microflora also acts as a barrier and serves to prevent invasion and growth of pathogenic bacteria. Healthy growth and retention of resident flora effectively prevents colonization by transient bacteria (E. coli, Pseudomonas, S. aureus, C. albicans). Skin antimicrobial protection includes mechanical rigidity 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 plays a beneficial role in relation to skin microflora (Table 2).

Acidic skin pH (pH 4.0–4.5) helps resident bacterial flora remain attached to the skin and prevents skin invasion by pathogens, whereas alkaline pH (8.0–9.0) promotes bacterial dispersion. Acids produced by bacteria contribute to local protective mechanisms - for example, S. epidermidis, P. acnes, Pityrosporum ovale, 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 forming unfavorable conditions for skin pathogens.

The acidic pH of the skin facilitates the production of natural antimicrobial peptides, promotes wound healing and regulates keratinization and desquamation.

The skin flora also produces protein and lipid antibacterial components called “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 improving interaction with the bacterial membrane.

Maintaining balance

The acidic pH of the skin helps to balance the environment for resident bacteria. Changes in skin pH and other organic factors play a role in the pathogenesis of certain skin pathologies, their prevention and treatment. P. acnes is a classic example of how a slight increase in skin pH 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 makes the environment more acceptable, resulting in rapid growth of this bacterium.

When using occlusive dressings in medical practice, it is necessary to take into account that prolonged occlusion of the skin significantly disrupts the pH of the skin and the growth of normal skin flora, accelerates transepidermal water loss and carbon dioxide emission. Recent studies have shown an interaction between changes in skin pH and its consequences in atopic dermatitis, particularly impairments in skin barrier function and increased S. aureus colonization.

However, other studies have confirmed that in atopic dermatitis, increased colonization with S. aureus and other bacteria is associated with a decrease in sphingosine and ceramide production. In atopic eczema, not only is the pH of the skin significantly increased compared to normal healthy skin, but the growth of S. aureus and exotoxin production are also increased, which can induce eczema on intact skin.

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 inoculated onto the right and left forearm, 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 non-insulin-dependent diabetes was significantly higher than in healthy volunteers. Based on this, skin acidity has been implicated as a risk factor for candidal infections. Dialysis patients also showed a significant increase in skin surface acidity. In most intertriginous areas (eg, axillae), the pH is physiologically higher than in other regions of the skin, which promotes the growth of local flora. 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 reduction in pH in the axillary region, which inhibited the growth of axillary bacteria.

Influence of external factors

As previously thought, many external factors influence the pH of the skin surface. Some of the external factors 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. Damage to the skin's pH can cause irritation or disruption of the keratinization process. Frequent hand washing with soap can damage the skin and facilitate colonization by many bacteria. 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 not 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 a fairly resistant ecosystem to external influences. Taking a daily bath for 3 weeks or stopping washing the forearms for the same amount of time did not result in excessive growth of transient microflora or a significant shift in the composition of resident microorganisms. The use of synthetic detergents with acidity similar to the skin surface resulted in an increase in skin surface pH for a short time, and these temporary changes were limited to the superficial layers of the stratum corneum.

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Thus, repeated use of alkaline cleansers, detergents, and even plain water (pH 8.0) can adversely affect the skin's natural pH and disrupt normal microflora. To maintain normal physiology and microflora of the skin, it is necessary to use cosmetics and skin cleansers that do not damage the pH of the skin or side effects on the skin flora should be taken into account and corrected in a timely manner. Research is needed on pre- and probiotics in regulating healthy skin flora and maintaining optimal skin biochemistry.

Table 1. Microflora of normal skin in areas with increased density (according to the Handbook of cosmetic science and technology / Edited by Andre´ O. Barel, Marc Paye, Howard I. Maibach. − 3rd ed. Informa Healthcare USA. −2009.− 887 p.)

Table 2. The effect of skin pH on its microflora (according to the Handbook of cosmetic science and technology / Edited by Andre´ O. Barel, Marc Paye, Howard I. Maibach. – 3rd ed. Informa Healthcare USA. – 2009. − 887 p. )

The view of modern science

The acidity of the skin surface is one of its physiological characteristics (along with hydration, structure, temperature), changes in which inevitably affect the functioning of this organ and the body as a whole. Skin acidity is a constant, but its indicator may be subject to some physiological fluctuations, depending on the time of day, climate, individual characteristics of the body, ensuring its functions in different conditions.

The pH value of healthy skin became a topic of debate in the late 19th century. In 1892, E. Heuss discovered that the entire surface of human skin is acidic. This statement is still an axiom today. A well-known term characterizing the acid mantle of the skin is “Marchionini’s acid mantle,” named after the scientist who confirmed acidophilicity as a protective ability of the skin.

The exact origin of cutaneous acidity is not yet fully established, but recent studies indicate that many endogenous factors are involved in its formation (including the presence of lactic acid, free fatty acids, urocanic acid, pyrrolidone carboxylic acid in sweat and sebaceous secretions).

Acidity affects the lipid composition of the stratum corneum, its hydration, the barrier function of the skin, and its microbiota. The acidic pH of the skin promotes an optimal microenvironment for resident bacteria and their enzymatic activity.

It is well known that an increase in skin pH can be associated with the pathogenesis and severity of many dermatoses, including acute eczema, simple contact dermatitis, atopic dermatitis, ichthyosis, acne vulgaris, and candidiasis.