82 SKIN MICROBIOME
a natural competition for resources by these bacteria within such locations. It should be anticipated that there would be reverberations among neighbouring bacteria when one species is diminished, providing an opportunity for the others to flourish at its expense.8
This is
important to note given that many microbiome- based products promote their activity with the notion of ‘balancing’ the skin microbiome. While many marketing trends suggest that
increased biodiversity of species is beneficial, this contention should be qualified in the context of which biogeographical site being evaluated. It is considered normal for C. acnes to dominate the sebaceous sites of the skin,5 and any increase in the species diversity by incursion of organisms not usually present could have dramatic ramifications on the health of this biogeographical region. Furthermore, there can also be a temporal
component to the balance of the skin microbiota. This is best exemplified by the archaeal microbiome members, where they exhibit peak growth on human skin from early childhood to roughly the age of 12; at which point, there is a steep decline in their numbers. Later, around the age of 60, the archaea
surge back to the population levels observed earlier in life.9 From this perspective, there should be
an appreciation that topical microbiome modulators can produce different effects on the microbial constituents given the age of the individual. Finally, increased species diversity may not
necessarily translate to improved skin health. A recent report found that as skin goes through chronological ageing, there is an increased diversity of microbial groups relative to younger skin.10
Thus, generalized claims of ‘increased
biodiversity’ might need to be tempered, and discussed within a specific context.
Nature of microbiome-focused ingredients The majority of microbiome modulatory products can be slotted into one of three broad categories: prebiotics, probiotics, and postbiotics. The first involves materials that either promote or interfere with the growth of particular groups of microorganisms found on the skin.11 The probiotic approach seeks to topically
apply live microbes that will colonize a specific region of the skin biogeography in the hopes of restoring microbial homeostasis.12
In contrast to
the prior two strategies, postbiotics abandon the effort to coax the growth of microbes in one direction or the other. Instead, the beneficial bio-molecules produced by a specific microbial species are harvested, purified, and applied to the skin in a finished formulation.13 Each of these microbiome-focused
endeavours has been met with mixed success. Unfortunately, it is not uncommon for these terms to be conflated or inaccurately described. A common example of this is when a
company describes their product as a probiotic that promotes the growth of beneficial organisms, which, based upon that definition, would technically be classified as a prebiotic.
PERSONAL CARE May 2024
Figure 2: Definitions of prebiotics, probiotics and postbiotics Further complicating matters, other groups
have suggested combined claims of prebiotic and probiotic in the same substance (what is now considered a ‘symbiotic’), where the active material contains a live organism supplemented with a substance that will promote its growth.12
Prebiotics This is by far the largest group of materials that have purported impacts on the skin microbiota. Again, the intention behind these ingredients is to shift the growth patterns of certain microbial populations. In theory, bolster the beneficial ones, while hampering expansion of the detrimental ones. Yet attributing human concepts of ‘good’
and ‘bad’ to microorganisms is rarely so clean- cut as seen with C. acnes that can have a role in the development of acne vulgaris yet also exhibit activity supportive of skin health.5 Adding to the confusion, many of these
prebiotic solutions contain a horde of different ingredients that are potentially capable both individually and combinatorially of driving microbial expansion or diminishment. Furthermore, it is important to consider that something as innocuous as showering with water alone has a significant impact on the relative levels of the members of the skin microbiota. The component list of the many
commercialized prebiotic solutions can be quite varied. These include but are not limited to: botanical extracts, yeast ferments, amino acids and peptides, simple and complex
carbohydrates, just to name a few. Even common ingredients found in finished
formulations like propanediol, glycerin, and squalane can impact whether or not some microbes flourish. Given all of these complications, it can be a difficult task to discern which or how many molecules in a bio- active material are responsible for the observed prebiotic effect.
Probiotics The notion of introducing a live microorganism to the skin in the hopes that it will achieve a foothold and flourish is no small task. Functionally, the skin microbiota acts as a protective barrier preventing incursions from other microbial species, making it an extension of the immune system. The existing commensals are entrenched,
and leverage anti-microbial measures to competitively exclude potential encroachment by other species. Naturally, the consumer should be curious
as to what types of living organisms are being suggested as good topical probiotic measures. The most popular microorganisms that have been used to date are arguably various Lactobacillus species and to a lesser extent Lactococcus and Bifidobacterium.14-16 These particular bacteria have been
employed extensively in the food industry as there is considerable evidence for their benefit to gut health. When visiting the dairy section of a local supermarket, consider how many bacterial species are highlighted as important
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