Figure 1. The skin is a natural habitat for a rich variety of microorganisms.

Figure 2. Traditional microbiologic testing methods such as bacterial cultures on agar plates can show the germ-killing activity of substances.

Skin Barrier Function

The skin’s primary role is to act as a protective barrier between the internal body and the external environment. The strength of this barrier is a key determinant of skin health. One of the most widely used assessments is the measurement of the TransEpidermal Water Loss (TEWL) (Figure 3), which quantifies the amount of water evaporating through the skin into the environment (13). Elevated TEWL indicates impaired barrier integrity, a hallmark of many skin disorders (14), (15), (16). With this straightforward method, even subtle barrier damage can be easily detected.

Figure 3. Tewameter^® measurement with open measurement chamber does not influence the natural evaporation from the skin.

Skin Surface Hydration

Stratum corneum hydration refers to the water content of the uppermost layer of skin, which is crucial for the skin barrier and the regulation of important functions. Its water holding capacity depends on the presence of natural moisturizing factors and its barrier function. Since this skin layer interacts strongly with the environment, it is easily affected by external influences such as dry air or frequent washing, but can also be influenced by intrinsic conditions (17). Low hydration results in dry, scaly skin and a compromised barrier through which pathogens may enter (18), (19). Excess hydration, for example from continuous wet exposure, leads to a maceration of corneocytes, again weakening the skin’s structural integrity (20), (21).

Balanced hydration is therefore critical for skin health. Various instruments measure stratum corneum hydration, employing methods such as impedance, capacitance, infrared, near-infrared, or Raman spectroscopy.

Skin pH Value

The skin surface pH plays a fundamental role in buffering acids or alkaline substances and shaping microbial communities. Many beneficial bacteria thrive in the slightly acidic conditions of the healthy skin surface. In atopic dermatitis, elevated pH levels are often observed alongside increased colonization by Staphylococcus aureus (22).

By measuring skin surface pH, researchers can determine whether products support or destabilize the microbiome’s preferred acidic environment.

Figure 4. The occurrance of acne is correlated to excess sebum production.

Sebum Levels

Sebum, secreted in sebaceous gland-rich areas such as the face and scalp, is another crucial determinant of microbial ecology. The role of sebum in atopic dermatitis was often dismissed in the past. More recent studies however, have uncovered that in diseased skin conditions such as atopic dermatitis, sebum levels are considerably lower than in healthy individuals (23). On the other hand, excess sebum production, providing a nutrient source for Cutibacterium acnes, is correlated to the occurrence of acne (Figure 4) (24), (25).

Sebum production can be measured with a Sebumeter^® or collected and analysed by microporous tapes absorbing the sebum from the skin surface. Additionally, C. acnes activity can be monitored using specialized imaging: the bacterium produces porphyrins that fluoresce under certain UV wavelengths (Figure 5).

Monitoring both sebum production and bacterial activity provides insights into the effectiveness of anti-acne treatments (26).

Blemish Evaluation

Blemishes, including pimples and blackheads, signal deviations from an optimal skin condition. They can be assessed visually or with high-resolution standardized photography (27). When photography is used, special attention should be paid to ambient light, positioning of the face, and background colours such as clothes and hair that influence camera settings and affect comparability. Dedicated imaging systems with controlled illumination and fixed positioning solve this issue and improve reproducibility, enabling both subjective scoring and automated analysis.

Figure 5. Strong fluorescence of the Cutibacterium acnes porphyrins under UV-illumination of the Visiopor® camera.

Healing Capacity

The skin’s healing ability reflects its resilience and overall health. Healing can be assessed directly, for example by monitoring wound closure after controlled biopsies or excisions. Luckily, also less painful alternatives exist for assessing the healing ability of the skin, especially the barrier regeneration. One method involves standardized repetitive tape stripping (e.g. with Corneofix^® or D-Squame^®) (Figure 6). This controlled damage increases TEWL, which is then monitored until values return to baseline. The speed of recovery provides an objective measure of healing capacity without creating open wounds.

Figure 6. Stripping skin layers by adhesive tapes is a well-recognized way to artificially damage the skin barrier in a standardized way.

Irritation Response

Skin irritation often manifests as redness (Figure 7) (28). Traditional assessment relies on expert visual grading, but human perception struggles with subtle differences, day-to-day comparisons, and the perception of redness for different Fitzpatrick types. Standardized photography provides more consistent evaluation, and added image analysis software enables automated colour quantification (29).

Besides photography, handheld colour measuring probes offer a quick and objective alternative for redness measurement (30).

Other parameters, such as surface topography and elasticity, can add information to provide a broader picture of skin health.

Figure 7. A compromised skin barrier is prone to irritation.

Perspectives and Ongoing Questions

Despite the progress in microbiome research, major challenges remain. No single microbial profile defines healthy skin, and the wide inter- and intra-individual variation makes universal conclusions unlikely. Microbiome analysis alone is therefore insufficient to demonstrate the benefit of microbiome-targeting products.

Traditional skin testing methods, by contrast, remain robust and reliable. They provide measurable outcomes—hydration, TEWL, pH, sebum levels, irritation response—directly linked to skin health. For this reason, they continue to serve as the foundation for substantiating product claims, even when those claims are framed in microbiome-related language.

Although everyone is talking about the microbiome, there are still many unsolved questions. How can we promote the diversity of the microbiome and prevent dysbiosis? What role does the microbiome play in certain diseases, and how can we use it therapeutically? What are the effects of post-, pro- and prebiotics, and cosmetic procedures on microbial communities? How do environmental influences affect the microbiome?

These topics need to be addressed in future tests and might even lead to the definition of new skin types, taking into account their predominant microbial communities. This would fundamentally change how dermatology and cosmetology categorize and treat skin and could even reinforce the already booming trend towards the individualisation of cosmetic products (Figure 8).

Figure 8. The definition of new skin types considering also the predominant microbial communities, requires a stronger specialisation of off-the-shelf products.

The skin microbiome is a fascinating and rapidly evolving research field. However, despite considerable scientific activity, no consensus exists on an “ideal” microbiome. The complexity and variability of microbial communities mean that microbiome analysis, while informative, rarely provides conclusive evidence of product efficacy.

For now, the most practical and scientifically reliable approach remains the use of traditional biophysical skin testing methods and imaging techniques that allow researchers to prove claims about skin health independently of microbiome analysis.The future of microbiome skin care research lies in bridging these two approaches: combining microbiome profiling with established skin health assessments. This integrated perspective will clarify how products influence both the skin’s physical state and its microbial inhabitants, helping to ensure that interventions genuinely support long-term health rather than disrupt delicate balances.