From Fermented Food to Precision Fermentation for Cosmetic Active Ingredients: An Overview

History of fermentation

Fermentation is obviously not a new technology. For thousands of years bacteria and yeasts have been used to preserve foodstuffs, making them safe from spoilage (1). This originated in the prehistoric era, when fermentation likely began unintentionally as humans stored food in warm environments in which microbes thrived, transforming the food into a fermented version of its original (2). Later, beverages like beer and wine, and dairy products such as cheese and yoghurt started to emerge, as well as, in Asia, foodstuffs like kimchi, soy sauce and miso.In the 1850s, Louis Pasteur identified the role of microorganisms in fermentation, paving the way for what we now call modern microbiology and biotechnology (3). Today, fermentation is a (bio)technology which enjoys a great amount of attention from the academic world and the industry alike. It is considered one of the main technologies which pave the road to a more secure, sustainable and healthy future for humankind (4).

Fermentation in the cosmetics industry

Nowadays, fermentation is a well-established technology in the cosmetics industry. Much-loved ingredients, such as hyaluronic acid (5), beta-glucan (6), panthenol (7) and polyglutamic acid (8), can be produced biotechnologically. Functional cosmetic ingredients, such as surfactants and emulsifiers, can also be obtained through fermentation (9), underlining the great potency of this technology for the cosmetics industry.Currently an important emphasis lies on what many people call “probiotic active ingredients.” These are active ingredients, i.e., ingredients which have advantageous outcomes when applied on skin, scalp or hair. The term “probiotic” is recognizable for the consumer and has clear positive connotations. Building on the trust from the food sector and the associated known health benefits, many consumers perceive “probiotics” as safe and healthy. Taken quite literally, however, these active ingredients are not probiotic. The term implies that these ingredients contain live and viable probiotic cells. Most of them, however, are based on lysates or filtrates containing cellular components of, and/or metabolites produced by, these beneficial microbes. They are properly called “postbiotics” or “paraprobiotics.”

Definitions

At the time of writing, two different definitions of “postbiotics” are circulating. An important source of definitions for pre-, pro- and postbiotics is ISAPP (International Scientific Association for Probiotics and Prebiotics, https://isappscience.org/), which defines postbiotics as follows (10): “preparation of inanimate microorganisms and/or their components that confers a health benefit on the host.” In its publication from 2021, this definition further specified that postbiotic ingredients are derived from microorganisms, deliberately inactivated microbial cells with or without metabolites or cell components. In this publication ISAPP also explained what it does not consider to be postbiotic: viruses and bacteriophages, vaccines, filtrates without cell components, purified microbial components (e.g., proteins, peptides, exopolysaccharides), purified microbial metabolites (e.g., organic acids).There is also another definition. The ICCR (International Cooperation on Cosmetics Regulation, https://www.iccr-cosmetics.org/) is a voluntary international group of cosmetics regulatory authorities from Brazil, Canada, Chinese Taipei, the European Union, Israel, Japan, Republic of Korea, and the United States that meets on an annual basis to discuss cosmetics safety and regulation.

The ICCR defines postbiotics as “inanimate ingredients of microbial origin added to a cosmetic product with an intended cosmetic benefit. These components can either be cells or cell fractions, a filtrate of a fermentation or a metabolite of a microorganism” (11). ICCR sees “paraprobiotics” as a subgroup of “postbiotics.” It can be argued that its definition of “paraprobiotics” equals the ISAPP’s definition of “postbiotics.” ICCR’s definition of “paraprobiotics”: “Ingredients derived from inactivated probiotic microorganisms added to a cosmetic product with an intended cosmetic benefit. Paraprobiotics can either be inactivated microbial cells or components of cellular structures (e.g., cell walls), with or without metabolites” (11).

History of postbiotics in the cosmetics industry

For the sake of clarity, when we talk about postbiotics, we apply ISAPP’s definition. Postbiotics are part of what can be interpreted as a movement, and not a trend, in the cosmetics industry. Over the last years, many new postbiotic cosmetic active ingredients have been launched onto the market by many different suppliers. Some of these launches have been successful and others not. The use of postbiotic is, however, not new in the cosmetics industry. Already in the 1980s, the first skincare products which contained these ingredients appeared on the market. A number of these products are still available, some of which enjoy great attention by consumers worldwide.A first scientific publication on the use of postbiotics in topical cosmetic applications appeared back in 1982. Presented at the yearly congress of the German Dermatology Society (Deutsche Dermatologische Gesellschaft), this paper (12) describes the effect of “inactivated cultures of the species Bifidobacterium” (Bifida Ferment Lysate) on UV-induced immunosuppression and DNA damage in the context of this ingredient’s potential skin anti-aging effect. This activity was later reported in another paper from 2008. (13) as well. Interestingly, this ingredient was also the topic of scientific publications related to sensitive skin in 2010 (14) and skin barrier in 2023 (15). During the 40-odd years that postbiotic cosmetic active ingredients have been on the market, an ingredient such as Bifida Ferment Lysate clearly has left its mark on the skincare industry as a well-established ingredient with a proven track record.

The search for cause and effect

As is generally the case with many ingredients on the marketplace, the chemical composition of postbiotic cosmetic active ingredients is, by definition, complex. Understanding their mechanism of action on skin is a critical aspect of ensuring these ingredients’ effectiveness as well as safety. On the other hand, the full elucidation of the interaction between microbial constituents and human cells is not just of interest for the cosmetics industry, but also for the food industry. The food industry has arguably been trying to understand the interplay between probiotic bacteria and human cells for much longer than the cosmetics industry. Up until now, the food industry has not been able to fully answer all its scientific questions. All in all, the bars are set high. Luckily, new scientific publications of high relevance for the cosmetics and food industries appear regularly. Importantly, one of the main messages of these papers is that probiotic bacterial cells do not have to be alive to exert most of their benefits for human cells and organs (16, 17). This does make sense, as interaction between cells, one of them dead (and lyzed) or both of them alive, is by definition of a biochemical and cell biological nature.Molecules originating from one cell type can have an influence on the other cell type and vice versa (18). It is of great interest to understand which molecules that originate from postbiotics have what cell biological influence on human skin cells and skin as a whole. This knowledge is not just of academic interest, but can also help modify and improve biotechnological fermentation processes toward new and further improved postbiotic cosmetic active ingredients. “Wild fermentation” can then become “precision fermentation.”

Probiotic metabolites

Most probiotic bacteria produce large amounts of lactic acid. They do so to gain and maintain a beneficial environment in which they can thrive. The topical application of lactic acid has multiple benefits, making lactic acid an interesting metabolite of probiotics and constituent of postbiotics (19). Probiotic bacteria produce many other metabolites with biological activity. Other acids like, for instance, short chain fatty acids (SCFA’s) are produced by probiotic bacteria and can be constituents of postbiotic cosmetic active ingredients (20). An important disadvantage of the SCFA’s butyric acid, propionic acid, acetic acid and isovaleric is their characteristic odor, but they can potently interact with human cells’ G-protein coupled receptors (21) and peroxisome proliferator-activated receptor-γ (PPARγ, 22) and even with olfactory receptors (23). Through these interactions they activate cellular MAPK (24) and JAK pathways (25), leading to cosmetic benefits. They themselves can have anti-oxidant properties as well (26).Derivatives of lactic acid, such as ethyl lactate and propyl lactate, are other biologically active molecules which can be produced by probiotic bacteria and be part of postbiotics. Like SCFA’s, they too can interact with G-protein coupled receptors (27). Another derivative of lactic acid, phenyllactic acid, can be produced when phenylalanine is made available to certain types of probiotic bacteria during the fermentation process (28). This molecule is of interest as it has some potent anti-inflammatory (29) and anti-oxidant properties (30). Phenyllactic acid is also described as a potent tyrosinase inhibitor (31), making it an interesting candidate for skin lightening active ingredients. When 4-hydroxyphenylacetic acid is made available to certain types of probiotic bacteria, it can be converted into ferulic acid (32). As a phenolic acid, ferulic acid is a somewhat sensitive molecule, but can act as a strong anti-oxidant (33).

An important group of biologically active metabolites in postbiotics are the indoles (34). Examples of interesting molecules are indole-3-lactic acid, indole-3-carboxaldehyde and 3-hydroxyindole. Indoles are derivatives of tryptophan, produced to the probiotic fermentation process with a multitude of potential benefits for human skin (35). A molecule such as indole-3-lactic acid alone can interact with human skin cells in many different ways. Like ethyl lactate, indole-3-lactic acid interacts with aryl hydrocarbon receptors (36, 37).

Yet another interesting group of probiotic metabolites and postbiotic constituents are the lactones (38). Lactones are cyclic esters formed by the intramolecular condensation of a hydroxyl group and a carboxylic acid group within the same molecule which can have specific antimicrobial activity (39). They are characterized by their ring structures, which can vary in size. Examples of lactones which can be metabolites obtained from fermentation of probiotic bacteria are 3-hydroxybutyryl-L-homoserine lactone and hexanoyl-L-homoserine lactone. A large group of amino acids, mainly glycine and its derivatives, such as threonine, alanine and serine, can also be a product of fermentation of probiotic bacteria (40). The diamine putrescine (41) and its polyamine derivatives spermidine and spermine (42) have also been recognized as biologically active metabolites in postbiotics. Other groups of metabolites with well-known beneficial effects on human cells and organs have been discovered in postbiotics as well. Vitamins are an important group of singular molecules which can be part of the final postbiotic ingredient after fermentation of certain probiotic microbes, for instance (43, 44).

Probiotic cellular constituents

Adding to the endless complexity and possibilities, not only single molecules can have an influence on human cells, constituents of the bacterial cell itself can have potent effects, too. Toll-like receptors (TLRs) are so-called pattern recognition receptors and, as such, are a crucial component of the innate immune system in human skin (45). Peptidoglycans are an important constituent of the probiotic bacterial cell (46). Among other effects, they provide structural support for the maintenance of cell shape and osmotic protection, but they also interact with TLR2 on skin cells (47). Another example is bacterial DNA, which can interact with TLR9 (48). There are, however, many further examples of probiotic cellular constituents which can interact with pattern recognition receptors on skin cells.Overall, much is known about probiotic bacteria, their metabolites and cellular components, as well as the constituents of postbiotics. Cosmetic postbiotic active ingredients are, and have been for a long time, established in the marketplace. They have shown to work and to be safe to use. Together with some peptides and vitamin derivatives, such as niacinamide and retinol, postbiotics can arguably be interpreted as one of the few categories of cosmetic active ingredients with a real and proven track record. There is, however, still much to discover.

Conclusion

Without any doubt, the future lies in precision fermentation. Imagine postbiotics as an orchestra of metabolites and cellular constituents which, together, generate the “symphony,” i.e., the biological activity of the postbiotic active ingredient. Understanding precision fermentation, i.e., understanding how to “make sure that the orchestra does justice to the piece to be played,” represents the search for the Holy Grail. This requires knowledge, experience and scientific creative thinking while never losing sight of the safety of the end product. With bifida ferment lysate in 1982, postbiotic cosmetic active ingredients had a flying start, and rightly so. Science has commenced and consumers have evolved into well-informed and critical users of cosmetic products. These changes also make up postbiotics, from the past to the present, with great potential for the future.