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From the moment you’re born your body becomes a veritable metropolis for microbes, as a host of bacteria, fungi and others take up residence on your skin.1 This fascinating collection of organisms living on you is known as your skin microbiome.
By numbers alone, microbes outnumber your own body cells ten to one!2 In fact, with a population numbering in the trillions,2 and a thousand or more different species calling your body home, the skin’s microbiome is more populous and diverse than all of Earth’s cities put together.
We’re conditioned to think of all microbes on our skin as ‘germs’, pathogens, harmful bacteria, fungi and viruses seeking to do us harm, things we’d be better off without. After all, they can cause food poisoning, disease, infection. But actually, harmful microbes make up only a very small percentage of the population. In human cities there are only ever a small number of rule breakers and trouble-makers, and the same is true for your microbiome. The majority of your tenants take very good care of the place, content to just go about their business without bothering you. In fact, many are not just harmless, they’re actually beneficial.3
Just as a city is made up of many different boroughs, suburbs or neighbourhoods, your skin is similarly divided into different regions, each home to distinct microbial communities. You wouldn’t think microbes are terribly picky when it comes to choosing a home, but different species can be very selective about the sort of neighbourhood they’ll settle in. Ecologically speaking, these regions may be as different as a rainforest and a desert.4
For example, roaming the dry, open plains of your forearms and buttocks you’ll find actinobacteria, Firmicutes, Proteobacteria and Bacteroides. Head south to the humid regions between your toes and pay a visit to the gram-negative bacilli, coryneform and pathogenic Staphylococcus aureus. Voyage north to the chest, back and face, where the sebaceous glands create an oily landscape, and you might encounter lipophilic organisms, the Propionibacterium and Malassezia fungi.4,5
That’s not to say that all of us essentially have the same skin microbiome. The microbiome is affected by factors like gender, left or right handedness, clothing choice, environmental humidity, UV exposure, and use of cosmetics or antibiotics, so your skin microbiome could be as individual as your fingerprint.
The microbiome is not something new. It’s likely been with us, evolving with us, for millions of years, ever since the first complex multicellular organisms dragged themselves out of the primordial soup. This relationship has been so successful because it benefits both parties. In exchange for providing housing and nutrients to the microbes, we receive benefits for our metabolism (in the case of gut bacteria) and immunity.6 On our skin, P. acnes breaks down triglycerides from sebum into free fatty acids, a vital component of the skin barrier.7 The presence of these fatty acids also lowers the surface pH of the skin (making it more acidic), which inhibits the growth of harmful bacteria like S. aureus and S. pyogenes.1,8 One of the champions of your skin, S. epidermis, goes out of its way to protect you from its troublesome cousin, S. aureus and other nasties like E. coli, which can cause infections and other issues. It produces phenol soluble modulins that selectively kill opportunistic or pathogenic ‘squatters’ that try to take up residence on your skin.
Our skin microbiome is inextricably linked to our skin health. With eczema, for example, S. aureus moves in and takes over the neighbourhood resulting in lower diversity in the area and increased chance of S. aureus infection for eczema sufferers.9,10 Seborrhoeic dermatitis similarly appears to be linked to the fungi Malassezia spp. and a wide range of fungicides can help combat this condition.11 Other conditions such as psoriasis and acne are also associated with changes in the skin microbiome, although whether these changes are the cause of such conditions, or the effect of them, remains to be seen.2
In the past, the only way to determine what was living on our skin was to take a swab of the skin, try to grow the microbes on a suitable culture, then identify them under the microscope.12 The problem with this method was that only a small percentage of microbes would grow, so the rest remained invisible. That’s like trying to take a census of the population based on just a single household on each block. Recent advances in technology now allow researchers to sequence the genetic material of a sample and directly identify all the microbes present.12 This revelation has thrown open the doors to the microbial world, a world we’d previously only glimpsed through the keyhole. With thousands of new strains of microbes discovered so far, we may be on the cusp of a new era in skin health.
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