Bacteria are so intricately enmeshed with the human body that, for some purposes, they could be considered part of it. This relationship starts as soon as a baby makes its way through the birth canal, collecting bacteria, virus and mold as he/she comes out.
Because the microbiome is so important to our health, I want to discuss the bacteria associated with the skin.
The skin microbiome is the complex community of bacteria, viruses, and fungus that live in and on our skin. Most of the microbiome, most of the time, is useful and necessary (no, I don’t like the after-gym odor either).
This microbiome is best left alone unless its disruption becomes apparent, like with acne.
A probiotic is a mix of live bacteria and/or yeast that are ingested to improve the health of the digestive system. A common use of probiotics is when we use them after taking an antibiotic, to compensate for the disruption of the natural digestive flora. Why can’t you buy probiotics for the skin? The FDA (Food and Drug Administration in the USA) sets strict limits for the number of live bacteria present in cosmetics. This makes sense: we must avoid excess or dangerous bacteria from entering our body. Our digestive system, with a stomach maintained at a very low pH, is well equipped to kill bacteria. Conversely, our skin is not well equipped to kill dangerous bacteria and, in the old times (as I always say, the old times were not better) without preservatives, contaminating bacteria present in cosmetics caused terrible damage, including blindness
What is a prebiotic? Prebiotics are compounds in food that induce the growth or activity of beneficial microorganisms such as bacteria and fungi. The most common example is in the gastrointestinal tract, where prebiotics like natural food fibers can alter the composition of organisms in the gut microbiome.
Which organisms make up the natural skin microbiome?
In healthy skin there is Cutibacterium acnes (formerly Propionibacterium acnes), Staphylococcus epidermidis and Corynebacterium plus others present at lower concentrations.
What do these bacteria do for us? In a strategy used by most living beings, they discourage other (potentially dangerous for us, competition for them) microorganisms from invading our skin by making the environment unsuitable for the invaders.
It used to be that the only bacteria that could be identified were those that could be cultured in a Petri dish. This method may not reflect the natural bacterial population accurately because some bacteria will not be easily cultured in artificial conditions. But now it is possible to have a more precise panorama of the actual natural bacterial population using genetics and DNA sequencing so that “all bacteria will be counted”. With this new methodology it is possible to explain, at least tentatively, why, in the case of acne, the causing bacteria may be present when there is no actual visible acne lesions. It seems that there is no single genotype for C. acnes; detailed genetic maps show hreat heterogeneity and some subtypes present in acne have some genes that other C. acnes not associated with acne lesions don’t contain.
There is also a growing consensus that a change in the skin environment is essential for the development of acne, and that it is this interaction between environment and some subtypes of C. acnes that start the process leading to acne. For example, the culture of C. acnes in a sebum-rich, low oxygen environment, that mimics that of a blocked hair follicle, promotes anaerobic fermentation and the production of short-chain fatty acids by the bacteria which, in turn, activates a mechanism that leads to the production of inflammatory factors by the skin cells (not by the bacteria).
We need to change the way we look at acne. Acne seems to be the consequence of changes in our own skin metabolism that select some bacterial genotypes, a change that reflect back to our skin metabolism. In the end, it is our own skin that determines how much inflammation and destruction there will be in response to proliferation of the acne bacteria that have been selected by changes in our own metabolism.
As we enter adolescence, our skin makes more androgens, produces more sebum and promotes the subtypes of acne bacteria that will prosper. The response of our own keratinocytes, as determined by our genetics and environment (including the food we eat) will determine whether acne will happen or not. Complicated, right? In other words, the disruption of the microbiome can be attributed to puberty and the resulting androgen-mediated overproduction of sebum that select for distinct strains genetically better adapted to exploit such an environmental change. For example, the so called C. acnes type II strains have less lipase activity (compared to type I strains) and this lower ability to use fats is a disadvantage when competing for space and resources. In addition, C. acnes has been shown to produce several proteins and peptides, toxic to other bacteria strains, which may help in the successful colonization in the follicle and on the skin surface.
Conversely, some strains of S. epidermidis can ferment carbohydrates to produce succinic acid, a chemical that has potent anti-S. aureus and anti-C. acnes activity. This is relevant to acne treatment because in theory we could improve skin health by using prebiotics like succinic acid and glycerol that will discourage C acnes from spreading.
As I said before, we can’t use probiotics on our skin for very good reasons, but we can use prebiotics, and we already do! The ingredients in Skin Actives Scientific acne control products contain several prebiotics that modify the skin environment to make it less hospitable to the acne bacteria. We can do even more and we will continue to improve our products as we learn more about the skin microbiome.
Take home lesson: don’t attack what you don’t know: if you start using strong oxidants or anti-bacterial products, or remove the external layers of the epidermis that constitute the natural skin barrier, you may get into trouble because you will be disrupting the natural equilibrium achieved by the skin and its microbiome.