Cosmetic activity on the microbiota can be assessed through three complementary approaches (in vitro, ex vivo, in vivo), but the in vitro approach is the fastest, most reproducible and most cost-effective entry point for any screening effort.
A rapid and reproducible screening tool
In vitro assays, performed on isolated bacterial strains or on defined mixtures, provide a controlled and reproducible snapshot of how a product interacts with target skin bacteria. This reproducibility makes them the tool of choice for screening raw materials upstream of formulation. They also make it possible to establish a common technical language between laboratories, manufacturers and regulatory authorities, much as was achieved with conventional preservative efficacy tests (challenge test, ISO 21147:2018). Some authors indeed regard a well-designed in vitro test as the first building block of a standardization foundation for assessing “microbiome friendly” claims, a condition considered necessary before any harmonization of the sector.
The limitation of this approach is ecological: only a small fraction of skin microbes can be cultured using standard techniques, and a monoculture inherently disregards interspecies interactions, which are nonetheless decisive for the balance of the real microbiota.
Co-culture systems to move closer to the ecosystem
To narrow this gap, co-culture systems have been developed. A medium created in 2024 by Tokyo University of Science supports the concurrent growth of four key skin bacteria: Staphylococcus epidermidis, Staphylococcus capitis, Cutibacterium acnes and Corynebacterium. These defined consortia bring the in vitro test closer to the microbial reality of the skin, without, however, reproducing its full complexity.
Full-thickness, three-dimensional reconstructed skin models represent an even more advanced extension of the in vitro approach. They make it possible to observe how skin tissue responds to bacterial presence (barrier function, inflammatory markers, antimicrobial peptides) without the inter-individual variability inherent to human volunteers. A model published in 2022 (Rademacher et al.) proposes precisely this type of setup to study in vitro the effects of cosmetics on skin microorganisms.
Toward standardized microbial panels by skin zone
To gain in ecological relevance, these in vitro tests must reflect the biogeography specific to the skin: sebaceous zones (face, chest) are dominated by Cutibacterium acnes; moist zones (armpits, groin) by Corynebacterium and Staphylococcus; dry zones (forearms, legs) host more diverse and aerotolerant taxa. The development of standardized microbial panels specific to each zone (face, feet, vulvar area) is presented as an essential step toward a harmonized in vitro assessment of a product’s “microbiome friendly” character.
An ex vivo extension, based on microbial consortia collected by swabbing real skin, adds a further level of complexity by assessing the viability and selectivity of skin-relevant microbial communities exposed to the product under standardized conditions. This hybrid in vitro/ex vivo model is presented as a scientifically robust equivalent to the classic challenge test, provided a consensus is reached on the microbial panels and experimental conditions to be used.
What in vitro does not yet show
The in vitro approach nonetheless remains a controlled snapshot rather than a complete reflection of the real skin ecosystem: it accounts for neither the host context, nor interactions over time, nor the individual factors (genetics, lifestyle, environment) that shape the microbiota in vivo. This is why it is generally positioned as the first step in a tiered approach: the standardized in vitro test lays the foundation, in vivo sequencing studies add ecological depth, metatranscriptomics adds functional precision, and clinical evaluation confirms safety and performance under real conditions of use.
Sources
- Skinobs / Cosmetics Testing News (2026). Clinical Evaluation of the Skin Microbiome: From Analysis to Claim Substantiation.
- Markos, S. (2026). Defining ‘Microbiome-Friendly’: Why Standardization Is the Foundation for Skin-Respecting Innovation. NYSCC Cosmetiscope, vol. 32, no. 1, January 2026.
- Rademacher, M.; Zinn, M.K.; Beinio, R.; Bockmühl, D.P. (2022). A new model to investigate the effects of cosmetics on skin microorganisms in vitro. Cosmetics, 9(4), 88.
- van Belkum, A.; Lisotto, P.; Pirovano, W. et al. (2023). Being friendly to the skin microbiome: experimental assessment. Frontiers in Microbiomes, 1, 1077151.
- Tokyo University of Science (2024). TUS Skin Bacteria Co-culture (TSBC) medium.
- Parra, I. (2023). Market and marketing trends of the skin microbiome.
