This month we have collaborated with Sreeja Shrestha, SkinOreal, to bring you the first instalment of our Diversity Awareness Month series – ethnicity and the skin microbiome.
Sreeja introduced the skin microbiome concept for her audience, and shared her perspectives on the influence of ethnicity on the skin microbiome – see here.
Now we dive deeper into the topic, and explore how and why ethnicity affects the skin microbiome – including the influence of genetics, lifestyle and geography. As we show in this article, the direct influence of ethnicity on the skin microbiome in terms of race and genetics is not yet clear, as the effects of geography and lifestyle appear to take a more dominant role.
However, further research to tease out any potential contributions from genetics and to fully understand how these factors interact with one another is key, as such information may have significance for skin health and care.
Skin microbiome variation in different ethnic populations
A ‘normal’ or ‘healthy’ skin microbiome is not easy to define, as skin microbiome composition and structure vary from person to person and depend on several factors – including ethnicity.
To illustrate, skin microbiome differences have now been observed across many different populations. Bacteria known as Cutibacterium (formerly known as Propionibacterium) were shown to dominate the skin of people from the US (New York and Colorado), for example, whereas Staphylococcus dominate Amerindian populations from the Venezuelan Amazon – with an Amerindian population also found to have higher overall diversity . Research comparing the hand skin microbiome of women from the US and Tanzania also revealed higher abundances of certain bacteria in the US population (Propionibacteriaceae, Staphylococcaceae and Streptococceacae), and higher abundances of soil-associated bacteria in Tanzanian women (Rhodobacteraceae and Nocardioidaceae) – already highlighting the influence of different lifestyles .
Another study comparing Chinese and Pakistani individuals from a single college in China found that nationality (along with body site) significantly affected skin microbiome community structure, with different bacteria dominating the skin of each group . East Asians have also been shown to have a unique microbial composition compared to Caucasians and Hispanics, with higher levels of Proteobacteria and different strains of Corynebacterium [4,5].
Taking a broader look, an analysis of the total number of microbial species in specific populations has found that the species diversity steadily increases as you add more populations – for example, from US, Tanzania and China – to the dataset, indicating that each country brings its own set of species .
Nature versus nurture
Although clear differences have been found in different ethnic groups, it remains an open question whether race and genetics are at play or if your skin microbiome is simply a consequence of geography and lifestyle.
Nature: race and genetics
Ethnicity is a key genetic trait when it comes to skin structure, as racial differences lead to variation in skin physiology. The composition of our stratum corneum and dermis (the outer and inner layers of our skin), and the elasticity, collagen and pigment levels of our skin, are all examples of physiological trains that differ across different racial groups [6,7].
For instance, structural and functional differences have been found in the skin of African American, Caucasian and East Asian populations. East Asian skin was characterized by a relatively weak skin barrier, as was Caucasian skin but to a lesser degree, whereas African American skin had low ceramide levels but high protein cohesion in the uppermost layers of the stratum corneum – differences that are reflective of the high prevalence of xerosis (dryness) in black skin and increased skin sensitivity in East Asian skin .
Variation was seen across different Asian populations. Darker skin tones in Asian populations were shown to correlate with a thicker and more structurally compact dermis compared to lighter skin, with a high lipid content . Japanese populations appear to high skin surface moisture, and those from Seoul, South Korea, and Calicut, India, have high sebum (oil) concentrations .
More research is needed to understand the extent to which ethnic differences in skin structure influence skin microbiome variation, but there is evidence showing that skin structure variation can impact the microbiome. For example, we know that skin microbial composition varies highly depending on body site due to differences in skin structure and physiology – such as sebaceous gland activity . Taking skin-resident fungi as an example, ethnic variations in the concentration of sebaceous (oily) lipids are thought to influence the success of Malassezia yeast in colonizing our skin . Skin microbiome variations, such as lower abundances of Staphylococcus bacteria and greater abundances of Corynebacterium, have also been linked to physical differences that are related to different genotypes in people from East Asia and Europe or Africa when looking at the axillary microbiota .
As well as skin structure, research has also suggested that differing immune systems across ethnic populations could play a role. The complexity of our immune system is strongly linked to our evolutionary background, and has been shown to vary between ethnicities. For example, people of African ancestry generally show stronger immune responses and a stronger inflammatory response compared to Europeans, which can limit the growth of bacteria [13,14]. The differences in immune system activity appear to influence both internal and external microbiomes in terms of diversity and community structure .
Nurture: Geography and lifestyle
So it appears that genetics do help to shape an individual’s skin microbiome, but geography and lifestyle have been identified as significant contributors to ethnic-driven variation in skin microbiomes [3,16,17,18].
When it comes to the physical environment, factors such as differing altitudes have been shown to affect the microbiome – regardless of body site . Studying the human skin microbiota from populations across eight sites in western China with different elevations revealed that that skin microbial diversity first decreased and then increased with increasing altitude, and a positive correlation between altitude and bacteria such as Lactobacillus, Chryseobacterium and Acinetobacter has been found . High-elevation skin microbiota networks have also been found to be more fragile than those at low-elevation areas, which could possibly contribute to the higher incidence of microbiome-associated skin diseases in high-altitude regions .
Climatic factors could also lead to deviations in the skin microbiome – particularly in extreme environments. In terms of temperature, the optimal range for the survival and growth of key skin bacteria residents is 33.2–35.0° C, and hydration has been shown to influence the growth of these resident skin flora [20,21]. Increased UV exposure in areas with intense sun can also disrupt the balance of skin microbes by damaging microbial DNA, suppressing the immune system and triggering the release of antimicrobial peptides (see our article on ‘UV and the microbiome’ for more information).
It seems that it’s not just where you live that can determine your skin microbiome composition though, but also how you live. Living in an urban environment, for example, leads to a unique skin microbiome composition compared to those living in rural environments. Why? Urbanization reduces microbial abundance and diversity . In urban populations – and particularly among younger age groups – a less diverse microbial community has been observed [16,23,24]. This reduction in microbial diversity has been linked to disrupted skin structure and functioning, as well as higher rates of infection and the development of other skin disorders and diseases – including acne, eczema, rosacea, psoriasis and dandruff, as well as allergies and autoimmune disease (see our article on ‘Urban living and the skin microbiome’ for more information).
Many factors contribute to the unique skin microbiomes of rural and urban residents, such as reduced exposure to environmental microbes and differences in lifestyle, house architecture, number of inhabitants per house, air pollution, detergents and cosmetic use in urban populations . This was seen in play in the study investigating differences in the skin microbiome of US and Tanzanian women, as women in Tanzania had greater exposure to the soil-associated bacteria that dominated their skin microbiome .
Similarly, in the study investigating Malassezia fungi in different populations, it should be noted that the two ethnic groups used were also based in different geographical locations – with an Asian population in Singapore, and a Caucasian population in Zurich, Switzerland. As well as physiological factors, it is likely that environment is also a driver of variation due to factors such as phylogeography, climate and cultural and lifestyle variations .
A case of nature and nurture?
Although it remains challenging to analyse the direct impact of race on the skin microbiome, there is some evidence to suggest that genetics play a role due to the variation in skin structure and immunity [3,15]. However, it seems that an individual’s external environment and lifestyle may overshadow genetics in many cases.
One recent study has shown that the above factors, including geography, lifestyle and physiological factors of the skin together explain variability in microbial composition . Another study also found that contribution of lifestyle factors to microbiome variation tended to be particularly high in sites that were regularly exposed to the environment – such as hands – but interestingly the gut microbiome consistently varied with race (Asian, Black, Hispanic, White) and/or ethnicity and ancestry (Arapaho, Cheyenne, Dutch, Ghanaian, Moroccan) in both adults and children . So even if the effects of genetics on the skin microbiome are masked by lifestyle and geography, internal microbiomes may be more subject to genetic differences. Whether diet and nutrition also play a predominant role here also remains an open question [27,28,29].
Going forward, however, more work is needed to understand the extent and significance of skin microbiome variations in different ethnicities and the full array of contributing factors. Gaining more knowledge in this area is key as it may help understand the reason for variation in susceptibility to certain pathologies or skin disorders in diverse populations and feed into treatment and management approaches for different groups, as well as highlighting different skincare and cosmetic needs.
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