The Cosmetic Microbiome: Terms, Testing & Regulatory Nuances Every Formulator Should Know

By Lambros Kromidas, MS, PhD, Independent Consultant, and Sabrina Behnke, PhD, Arxada

Microbiome-based cosmetic and personal care products are becoming more visible as consumers and brands take a closer interest in the relationship between skin and the microorganisms that live on it. For many readers, however, the cosmetic microbiome space can still feel difficult to navigate because it brings together evolving terminology, microbiology, skin science and product safety considerations. 

This article is intended to give readers who are new to the topic a practical introduction to the cosmetic microbiome concept and the language needed to move through these waters with greater confidence. It begins with relevant terminology, then provides microbiology background and explains how the skin ecosystem may be influenced by internal factors, external exposures, and cosmetic product use. We also explain how the skin microbiome can be assessed, and why it is important to understand the difference between resident, transient, commensal, symbiotic, and potentially pathogenic microorganisms when discussing microbiome-related cosmetic products. 

The article also highlights safety and regulatory considerations related to microbial limits, adulteration, and the need to distinguish clearly between intentionally added probiotic microorganisms and microbial contamination. Key terms such as microbiome, microbiota, probiotic, prebiotic, postbiotic, and adulteration are defined in the sections that follow. First, however, we provide background on the cosmetic microbiome concept and the skin ecosystem it seeks to address.

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Background

Your skin is home to a vast variety and number of microorganisms^1,2 that are considered by some to act as a “second skin.” The different microorganisms create unique ecosystems that interact with each other and our skin. This diverse microbial skin ecology is influenced by internal and external factors. Factors such as skin site, age and gender, play a role in determining the normal microbiota of the skin. As we age, the variability of microorganisms we gain at birth changes.^3,6-8 External factors that may influence one’s skin microbiota include environment (such as ultraviolet exposure from the sun and urban pollution), occupation (such as farmer vs. city office worker), sanitary practices, what we ingest (such as antibiotics) and choice of clothing.³ 

Another external factor, more pertinent for our discussion, is the application of cosmetics and personal care products. Some cosmetic products, especially highly preserved ones, may have a temporary detrimental effect on skin microbiome but not all do. Cosmetics may be designed to minimize the impact on skin bacteria. Some may even be designed to improve the recovery of the microbiome after insult with disrupting products such as cleansers, soap and other, generally surfactant-based, products.^9,10 As such, the unique microbial ecosystem of the skin has become the focal point of research to understand their role in skin disease and health. Recent research is contributing to the development of advanced cosmetic products that utilize microbial communities present on the skin to enhance skin condition. We anticipate that cosmetic products may be able to support the growth of targeted beneficial microorganisms on the skin and restore or maintain microbial community equilibrium to support skin’s healthy outlook. 

We also believe that the new development of cosmetic products will also need to take into consideration the effects on skin’s microbial communities. As a better understanding of the intricate interplay between microflora (definition given below) and skin emerges, cosmetic ingredients and products must address our “second skin” and drive innovation to meet current and future consumer needs. Cosmetic manufacturers, therefore, are encouraged to start thinking about products that not only cater to our skin directly, but also to the skin’s microbial inhabitants. There will be many waves when navigating the cosmetic microbiome waters, but the challenges will be worth it. We hope this article will help one to stay afloat by surveying the complex and evolving waters of regulations governing, or not, cosmetic microbiome-based products. One must learn the ropes first—to speak the language of a cosmetic microbiome sailor.

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Key Definitions

Although several microbiome related terms are in use today, including prebiotic, probiotic, and postbiotic, there is currently no universally recognized glossary that defines these terms as it pertains to personal care products. To address this gap, the International Cooperation on Cosmetic Regulations (ICCR)¹⁴ developed a set of definitions that were also included in the 2024 microbiome guidance document for the personal care industry authored by a group of microbiome enthusiasts and Personal Care Products Council (PCPC) member companies.³ The following definitions are aligned with ICCR terminology and the 2024 PCPC publication.

The term microbiome refers to a characteristic microbial community that occupies a well-defined habitat with distinct physicochemical properties and encompasses not only the microorganisms present, but also their genetic material, their metabolic activities, and the ecological niches they create. In contrast, the microbiota describes the assemblage of microorganisms—such as bacteria, fungi, archaea, microalgae and protists—found in a specific environment, including the human body or individual sites like the skin. Although these terms are sometimes used interchangeably, the microbiome places emphasis on genomic content and functional activity, while microbiota refers to the organisms themselves. 

Within the context of cosmetic products, probiotics are viable microorganisms, whether active or dormant, intentionally added to deliver a cosmetic benefit to the host either directly or through interactions with the host’s microbiome. Prebiotics, on the other hand, are substrates such as carbohydrates or other ingredients incorporated into a cosmetic product specifically to be utilized as nutrients by microbiome to support its metabolic activity and deliver a cosmetic benefit. Postbiotics are inanimate ingredients of microbial origin—such as ferments, lysates or metabolites—added to cosmetic formulations with an intended cosmetic benefit; these ingredients contain no viable microorganisms but retain functional properties derived from microbial processes. 

“Your skin is home to a vast variety and number of microorganisms that are considered by some to act as a ‘second skin.’”

Although not captured in the ICCR definitions we’d like to shed light on the term “adulteration,” as it is crucial to consider when intentionally adding organisms to cosmetic products. In regulatory terms, a cosmetic product is considered adulterated when it contains microbial contamination that may render the product injurious to users under customary or labeled conditions of use. This includes the presence of pathogenic organisms or contamination levels exceeding established microbial limits, as such contamination is regarded as a poisonous or deleterious substance. As we prepare to see more cosmetic and personal care products with living organisms on the store shelf, a clear regulatory differentiation between adulteration and intentional addition of probiotics is needed. A detailed discussion of this is given by Kromidas et al.¹⁵ and briefly highlighted in the next section.

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Probiotics Are Not Adulterated

Cosmetics claiming to be prebiotic and/or postbiotic should not contain live organisms. They are regulated as conventional cosmetics. If they are found to contain organisms above the established microbial limits, then they are contaminated.

Probiotic claiming cosmetics on the other hand require careful consideration since they contain deliberately added live microorganisms that may exceed established microbial limits as per U.S. FDA’s Bacteriological Analytical Manual (BAM)¹⁶ and ISO Standard 17516:2014¹⁷. The acceptable established limits are 100 CFU (Colony Forming Units) and 1000 CFU/g or mL for total non-pathogenic microorganisms for the eye/eye area products and body area products respectively. The intent for these microbiological limits was to address microbiological contamination; i.e., unwanted presence of microorganisms and not for intentionally added non-pathogenic microorganisms. In other words, the presence of microorganisms exceeding the established limits in probiotic products is not due to contamination.

Contamination of a conventional cosmetic product as well as those claiming to be prebiotic or postbiotic that exceed the established limits is an indication of cGMP failure because they are not meant to contain live microorganisms. This is not the case for probiotic cosmetic products since microorganisms are deliberately added under cGMP and therefore, their presence does not indicate contamination or unsanitary conditions. Probiotic cosmetic products, however, must still be free of deleterious substances that may render them injurious to consumers. Manufacturers still need to ensure that probiotic products are still free of contaminating microorganisms and particularly pathogens. Manufacturers must be able to prove that the microorganism(s) detected were deliberately added and harmless throughout the product’s shelf life.

“Manufacturers must be able to prove that the microorganism(s) detected were deliberately added and harmless throughout the product’s shelf life.”

cGMP manufactured cosmetic microbiome products are becoming more prominent as consumers, thanks to the limited brands in this space, are becoming aware of them and the role they play in their beauty routine. Therefore, understanding the role such products play and the interrelationship between the skin and microorganisms is key as we briefly explore below.

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Skin Microbiome Relations

An important goal of cosmetic skin microbiome research is to understand the intricate roles microorganisms play in skin homeostasis by taking into account that different areas of the human body support different and varied sets of microorganisms depending on the specific skin microenvironment, like dry, moist, or oily skin.^3-5 Besides bacteria, skin microbiome also includes fungi such as the yeast Malassezia spp., that is dependent on skin lipids.³ Therefore, an understanding of the skin microbial inhabitants, including commensals, symbionts, and pathogens, is a must for future research regarding the interactions between humans, microbes, and inter-microbial communities.³

Skin microorganisms can be categorized as either resident or transient. They can be considered as either commensal, symbiotic, or pathogenic. The resident microbiota is what makes the “second skin.” It is a unique signature of one’s microbiome that evolves as we age and/or prolonged exposure to environmental conditions. The resident microbiota primarily made of Cutibacterium acnes, Staphylococcus epidermidis, Corynebacterium diphtheria, Corynebacterium jeikeium, Pseudomonas aeruginosa, typically has a competitive advantage over the introduction of new microorganisms or transient microorganisms.³ Transient microorganisms only colonize when conditions are made favorable to their survival. Commensal microbiota uses the material and nutrients supplied by one’s skin without causing harmful effects. Symbiotic microbiota on the other hand is mutualistic by taking from the skin and giving something back that may benefit the host skin.³ Finally, pathogenic microbiota can cause harm if the opportunity arises under the right conditions.³

We alluded above that internal and external factors may have a detrimental effect on skin bacteria. Why is it after repeated washing and scrubbing, applying topicals, being exposed to the sun, sea, and chlorinated pools we don’t completely sterilize our skin? That’s because microorganisms also reside in pores, sebaceous glands, and skin folds giving them an extra level of protection. Another reason is because bacteria tend to form biofilms on the skin that can help them tolerate external stressors. A biofilm is a cluster of microorganisms adhering to each other and a substrate—in this case skin. Plaques on one’s teeth are a good example. They adhere by producing a matrix consisting of extracellular polymeric substances (EPS) consisting of proteins, DNA, lipids, and polysaccharides that optimize nutrient uptake, coordinate communication between cells, and protect against mechanical and chemical forces.^11,12 That is, resist decontamination. There is a negative side to this as biofilms may play a role in some skin disorders such as the development of acne.^3,13

To understand the role the skin microbiome plays one must be able to assess it as we briefly describe next.

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“Second Skin Assessment”

To determine the microbiome on the skin one must collect it, enumerate it, and identify it. The typical sampling methods used in sampling are swabbing, tape stripping, and biopsies. Swabbing is easy to perform and least invasive. That’s done by rubbing a pre-moistened (e.g., saline) swab across the skin to pick up microorganisms.³ Tape stripping is also rather simple to do but a little more invasive since it removes stratum corneum cell layers from the skin. An adhesive tape is applied to the skin with uniform pressure and then pulled from the skin. This action removes a layer from the skin with its associated denizens. This method provides more reproducible results and a higher biomass than the swab. This method is useful if one wants to access deeper layers of the skin.³ The least frequently used technique is skin biopsy because it is more involved, requiring special instruments, and being the most invasive of the three methods. It actually punches or cuts out a skin section. Biopsies provide microbiome profiles at even greater depths than tape striping could.³

Once a sample is collected, it needs to be processed for analysis immediately or stored frozen at liquid nitrogen temperatures for a later time. Stored samples must be in an appropriate solution or medium as not to affect the integrity of the microorganisms especially the nucleic acids.³ One may want to identify the diversity of microorganisms and numbers present at a particular skin site. The preferred process is through the extraction and identification of the genomic content through sequencing and bioinformatics.³ In the identification of the genomic content, a targeted gene or region is chosen and isolated through polymerase chain reaction (PCR) amplification. For bacteria the 16S ribosomal RNA is isolated as it is only found in bacteria and it is used for identification.³ In fungi, it is the internal transcribed spacer regions between the ribosomal RNA genes that is used for identification.³

In examining the skin diversity of microorganisms, one looks at two specific metrics—alpha and beta biodiversity. Alpha diversity measures the variety of species within a specific ecosystem or habitat. It focuses on the number of different species and distribution of individuals among those species within a defined skin area. Beta diversity measures the variation in species composition between different skin areas or habitats or ecosystems. In other words, it quantifies how many species are unique to each skin habitat and how they change as one moves across various skin locations or environments.³

In conclusion, the cosmetic microbiome concept requires a working understanding of both the language and the biology behind it. Clear definitions, basic microbiology, knowledge of the skin ecosystem, and appropriate assessment methods are all important for developing and communicating microbiome-related cosmetic products responsibly. As this area continues to evolve, careful attention to safety, claims substantiation, and the distinction between intentional probiotic use and microbial contamination will be essential for building trust with consumers, regulators, and the broader personal care industry.

About The Authors

Lambros Kromidas, MS/PhD is an independent regulatory, toxicology and scientific affairs professional active in the personal care industry. He held upper management positions at Avon, Coty, Beiersdorf, RIFM and Shiseido. He received a MS in microbiology and PhD in toxicology from St. John’s University (New York) and conducted post-doctorate research at Cornell University Medical College, Department of Physiology. He is a member of SCC, SOT and was an active PCPC participant for 30 years and chaired the Microbiome Task Force of the Microbiology Committee.

Sabrina Behnke, PhD is director of marketing and growth, personal care at Arxada, where she leads strategy for the company’s preservation and microbiome-friendly ingredient portfolio. She holds a PhD in Microbiology from Montana State University and a BSc in Water Sciences from the University of Duisburg-Essen. Dr. Behnke’s industry career spans R&D microbiology at Reckitt Benckiser and strategic marketing and business development roles at Symrise and TRI-K Industries. She previously chaired the Personal Care Products Council’s Microbiology Committee, and is a frequent industry commentator on preservative trends, microbiome-friendly formulation, and evolving regulatory expectations in personal care.

The viewpoints expressed in this paper are solely those of the authors and do not necessarily reflect those of any competent authority or company. The purpose of this article is to guide and inform the reader. The reader is encouraged to verify any opinions and facts the author presents.

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