Let's start this off by acknowledging that, whether we like it or not, the human body is home to billions of tiny micro-organisms - aka, microbes.
Some people shudder at the thought - but it's important to understand that bacteria does not equal bad.
It's also not as simple as "this bacteria is bad, and that bacteria is good". Many bacteria have the potential to be harmful or helpful - depending on the context.
Each area of the body has its own unique population of microbes - distinctly different populations with their own microbiome.
The microbiome of your armpit for example, will be home to a considerably different population of microbes than the microbiome of your face. This is because the microbe population in any given area will depend on the environment - the temperature, the pH, and the food sources. So, not all bacteria can survive in/on all parts of the human body. But more than that, not all bacteria have a beneficial effect when in any given area.
For example, there are microbes that live happily in our digestive tract and cause no problems - but if they were to populate the skin on our face, we might have a problem.
What I'm getting at here, is that when it comes to microbes, context is everything. Some bacteria can be helpful when in one part of the body, but harmful if they are living elsewhere. Some bacteria are helpful or neutral at a certain population level, but if they over-populate, they could cause problems. The environment, is crucial.
Now with that all in mind, lets talk about the skin biome.
We have a diverse population of microbes on our skin, but there are some dominant players: roughly 65% of skin microbes are cutibacterium, corynebacteria, and staphylococci (yes, staph). Around 89% though, are cutibacterium acnes (AKA c.acnes) - at least, in a healthy skin biome.
Some of you may find this surprising - for a long time, c.acnes has been regarded as problematic, inflammatory, and linked to acne development. It turns out though, c.acnes is not exactly the microbial villain of the biome that we once believed it to be.
Introducing: c.acnes
Formerly known as p.acnes (propionibacterium acnes), c.acnes is a species of bacteria that is relatively slow-growing, anaerobic (doesn't like oxygen but can tolerate it if it has to), and consumes the oils of the skin as its primary food source - making our follicles (the areas our pores lead to) the perfect habitat for c.acnes.
The bacteria was named c.acnes after the skin disease, acne - which the bacteria has been associated with for a long time. What we didn't know (up until recently), is that everybody (yes, even you) has c.acnes ALL over and inside their skin - including people who have zero acne. Which begs the question - is c.acnes really the culprit for acne development after all?
Lets' dive deeper.
Cutibacterium is a genus of bacteria, and within that genus there are multiple different species - including cutibacterium acnes.
Within each species, there are multiple strains (subspecies, if you will) - some of which are acne-causing, and some of which are critical for skin health.
The differences between these strains are so significant, that scientists have revised the naming for the following few major groups of strains within the c.acnes species:
1: c.acnes, subspecies: acnes (strains most associated with skin disease)
2: c.acnes, subspecies: defendens (strains most associated with skin health)
3: c. acnes, subspecies: elongatum (don't worry about this one)
C.Acnes Defendens
Of all the microbes on our skin, C.Acnes Defendens may be the most important for the health of our skin. It affects our skin biome from birth to death, it can keep us looking younger for longer, it can keep our skin clear, and it is critical for skin health.
C.acnes defendens consumes the oils our skin produces, and during its metabolic process, it secretes several substances which are critical to skin health:
Propionic acid
C.acnes creates propionic acid in copious amounts (which is actually why it used to be called propionibacterium). Propionic acid is a short-chain fatty acid that has a plethora of benefits for the skin, including:
actively suppresses growth of staph aureus (including MRSA)
broad spectrum antimicrobial activity against e.coli and candida albicans
lowers skin pH (our symbiotic, "good" bacteria love the low pH of our skin, while pathogenic (disease-causing) bacteria like staph aureus, do not like low pH environments
serves as an antioxidant
inhibits tyrosinase (a key enzyme involved in pigmentation)
inhibits biofilm formation of staph (formation of staph biofilms can contribute to skin disease)
regulates cell turnover
Antioxidants
C.Acnes produces copious amounts of antioxidants which are critical to reducing oxidative stress (which contributes to ageing, inflammation, and skin cancer) to a minimum. One of these antioxidants is the salt form of propionic acid. Another is a molecule that is unique to the cutibacterium species, called RoxP. This antioxidant is stable and is as powerful as vitamin C, and supports skin cell and immune cell health. However the advantage of RoxP over traditional vitamin C, is that RoxP is produced on your skin already if you have a healthy microbiome and healthy populations of c.acnes defendens. Reductions in c.acnes defendens, and subsequently RoxP, have been associated with oxidative diseases such as skin cancer.
Bacteriocins
Bacteriocins are antimicrobials which can inhibit certain bacteria. We first realised that microbes produce substances that are antimicrobial to other microbes in 1928 - when a scientist by the name of Alexander Fleming left a staph culture uncovered by accident. The culture was contaminated by a fungus called penicillum notatum, and Alexander noticed that where the fungus was growing, the staph could not. The result of this accidental and fortuitous discovery was of course: penicillin, our first true antibiotic.
An example is of a bacteriocin, is cutimycin - produced specifically by c.acnes and targeted at the staph species, including staph aureus and staph epidermis. Cutimycin keeps staph epidermis isolated to the skins surface where it can be beneficial, and away from the follicles where it can dysregulate the biome. Most of the bacteriocins produced by c.acnes are directed at keeping staph species controlled and preventing them from over-populating.
ALL of these substances support a healthy microbiome, and subsequently, a healthy skin.
But here's the thing - a lot of our skincare habits have been designed around killing bacteria - primarily c.acnes, including the good kind. We have over-emphasised the role of microbes in acne, and therefore over-emphasised the use of antimicrobials to treat acne. In the process, we may well be compromising and dysregulating our skin biomes, and doing more harm than good.
Of course we now know that not only are some strains of c.acnes incredibly important for skin health (c.acnes defendens) - we also know that c.acnes alone is not necessarily the culprit for acne development. Interestingly, some studies have even shown that acne-prone skins actually have LESS c.acnes overall - which would make sense, now that we understand what some species of c.acnes do.
So, how can we protect our c.acnes defendens? How can we look after our skin biome, and provide our symbiotic microbes with the environment they need, to take care of our skin in return?
Pathogenic strains of c.acnes tend to be more robust and fast-growing than protective strains - but as long as we foster a good environment for our symbiotic skin microbes, the protective strains should prevail and train our immune system to slowly but systematically target the pathogenic strains until balance is achieved. We can care for our skin biome in several ways:
Embrace the natural oils of your skin
Most of us don't think of oil on our skin as being a good thing - but the oil your skin produces, provides a perfect food source and environment for our good microbes to thrive. The old adage of "oily skins age slower" may well have something to do with c.acnes having the food that it needs. Oil is vital to skin health, and to the health of our skin biome - learn to love your oil for that.
Be mindful of how you cleanse
This is a very simple yet hugely impactful way you can support the health of your skin biome. We need to be mindful of how often we cleanse*:
Cleanse twice at night if you wearing make-up and/or sunscreen, and once if you weren't.
In the morning, you do not need to use cleanser - you can simply wipe over your skin with water to freshen up, but you do not need to use surfactants on your skin.
*note, this is a general guide and may vary for some skins.
We also need to think about what we cleanse with. Gentle is best, and if your skin feels tight after cleansing - your cleanser is most likely not gentle enough. We want our cleanser to remove superficial sweat, make-up, sunscreen, debris from the day - but thats it. It should clean our skin but leave our barrier and most of our oils intact.
Be mindful of your use of antimicrobial skincare
This is not to say that you shouldn't use ingredients that are anti-microbial - but we need to weigh up the pros and cons. For example: benzoyl peroxide (BPO) can be helpful in reducing acne, but it also acts like an A bomb for our microbes. A solution is to limit use of BPO to BPO washes, rather than leave-on BPO products. That way we are limiting contact with the skin, but still able reap the benefits of the ingredient.
Another example is salicylic acid (SA) - SA is excellent in reducing acne and congestion, and reducing inflammation, but we need to be mindful of our use. One SA serum for acne? Absolutely. Two SA serums, an SA cleanser, an SA exfoliant and an SA toner, all on a daily basis? Perhaps not.
Anti-microbial ingredients still have a valuable place in skincare - but moderation is the key here.
The bottom line
"Do the best you can, until you know better. Then when you know better, do better" - Maya Angelou
Skin biome care is an evolving science, so we are still learning - but one thing is for certain: skin health and skin biome health are inextricably linked, so we must care for both. When caring for your skin, use the lens of "will this benefit my skin, AND my skin biome?"