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  • Alex Cloherty

What's on my skin?

Today's Microbial Monday is inspired by a question asked by my friend Jesse. He knew that everybody had a different skin microbiota "fingerprint" – but why is this true, and what are the consequences of this?

It's true: each of us humans have an impressively unique "fingerprint" of different microbes living on us. You can think of it like a microbial salad, and each of us has different salad ingredients. This is true to the extent that scientists are already able to identify people only by collecting the DNA of bacteria on surfaces they have touched – no human DNA needed. This technique can even be used to differentiate between identical twins, who have the same human DNA, but host different "salads" of bacterial DNA. Because most research on this topic is concerning bacteria, I'll continue to focus on bacteria throughout this post, although mites, fungi, and viruses also populate our skin.

So why is every human's skin microbiota so unique? As usual in microbiota research, the answer is by no means simple. Let's start with a survey of what is actually growing on your skin.

You have probably already realized that not all areas of your skin are the same. Some parts are more dry, like your forearms and legs, and others are more oily, like your face and between your shoulder blades. Accordingly, there are different predominant bacterial species growing in these different areas. On the dry skin areas, Staphylococci predominate. This includes bugs like Staphylococcus epidermis and Staphylococcus aureus, which can cause some nasty infections if they accidentally get into a cut in your skin. In the oilier areas, there are more Corynebacterium and Propionibacterium. Aside from their role in Swiss cheese production, Propionibacteria have also been implicated in acne – specifically Propionibacterium acnes.

Already, you might start to understand how different people's bacterial fingerprints could be so different. For instance, maybe you know somebody who always has sweaty hands. This person probably has more of the moisture-loving bacteria on their hands than somebody with drier skin would. Or perhaps your best friend growing up had totally acne-free teen years, while you were plagued by it. That friend may have had less oily skin than you, which could translate into fewer Propionibacterium. To add a layer of complexity to bacterial skin microbiota, skin oiliness, and as a result bacterial colonization, also changes with age (thank goodness, for those who had to deal with those pesky Propionibacterium).

The story gets even more complicated from here. There is a huge range of other factors that can affect the bacteria living on your skin, aside from relative oiliness and age. Factors like sex, level of immune functioning, the clothing you wear, occupation, and even type of birth (vaginal versus caesarean section) can impact your bacterial skin salad.

On top of having different species living there, there can also be variation in the total number of bacteria growing. For instance, these researchers found that people with albinism tend to have a higher total number of bacteria growing on their skin than people with normal pigmentation, perhaps due to increased sun damage of the skin.

Ok, so we all have different bacterial skin salads. That's been established. But what does all this mean for the health of our skin?

As is often the case with microbiota, the key to health is balance between different species, and the relative amounts of those species. For instance, these researchers found that it's not the general presence of Propionibacterium acnes that makes some people more susceptible to acne. It's actually the absence of one strain (subspecies) of Propionibacterium acnes that can increase the likelihood of acne. This is a common theme in microbiota research findings: microbiota is key to health, often because the "good" bugs simply crowd out the "bad" bugs.

Small changes in this balance can result in feedback loops. For instance, the presence of certain bacteria on human skin can make humans seem more tasty for mosquitos. In turn, this could make humans with a particular profile of bacteria more prone to being infected with malaria or other mosquito-transmitted diseases, lowering their overall immunity and resulting in further changes in their skin microbiota. As another example, people with acne often scrub their skin extra hard and treat it with an arsenal of chemicals and creams, thinking that they will clean the bacteria from their pores. Perversely, this can destroy their good microbiota and induce the secretion of more oil (rough scrubbing dries skin out), leading to more acne.

In conclusion, with microbiota the story is often the same. A little bit of (almost) everything is a good thing, as in many other areas of life.

Until next week,

- Alex

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