Skin is the largest organ of the body, providing a vital defense layer between an organism and the surrounding environment. The outer layer of skin -the epidermis- is home to a diverse microbial community. However, the epidermis is not a particularly easy environment for microbes to live in. It is dry, acidic, extremely salty and has significant concentrations of antimicrobial compounds. Despite these harsh conditions, each square centimeter of skin contains between one million and one billion microorganisms (Kong et al, 2012, Grice et al., 2008). On average people have 1.5-2 square meter of skin so just by looking at the numbers, you can see that the skin hosts an abundant community of microbes. These microbes come from all domains of life including the single-celled bacteria, archaea, as well as multicellular microbes like fungi, protozoans, and arthropods. In this post we discuss the entire community of microbes that live on skin, a.k.a. skin microbiome. More specifically, we are going to talk about similarities between skin microbiomes of people living together, and how different factors including hygiene and lifestyle influence which microbes are present.
What determines your skin microbiome?
Different areas of the skin host different amounts and types of microbes depending on the physical characteristics of that area -e.g. salinity, humidity, etc. So on a single person there is a distinct ‘geography’ of skin microbes. It is hard to say precisely what defines a healthy microbiome, but overall researchers tend to agree that a more diverse microbiome is generally healthier. Research suggests that having a wide range of microbes in and on our body makes our microbiome more capable and versatile at defending us. Palms and feet have the highest diversity of microbes, probably because of their direct contact with microbiota from the surrounding environment (Figure 1). However there are also differences between people in terms of the number and diversity of microbes found on the skin.
Figure 1. Comparison of Shannon diversity indices (y axis) between different body locations
This is because the skin is exposed to so many different things that could impact which microbes can survive there. For example, studies have found that your skin care routine (or lack of), can majorly impact the diversity of microbes that are living on you (Holland at el., 2002; Staudinger at el., 2011). Other biological factors such as biological sex, age, ethnicity, and health status also play a major role in determining the diversity of the skin microbiome. Some studies have even determined that “intimate cohabitation” (your live-in partner), can impact your skin microbiome, although to a lesser extent than the other factors listed above.
Biological factors impacting the skin microbiome
The female skin microbiome is significantly more diverse than the male one, meaning that the diversity of microbes living on the skin of an average female is higher compared to an average male (Figure 2). This may be because of inherent physiological differences between female and male skin environments, including different pH levels, perspiration rate, hormone metabolism, and skin care product application.
Interestingly, researchers have shown that the thigh skin microbiome can predict biological sex with 100% accuracy. When researchers used the data from all body locations their predictions were only accurate 80% of the time. This means that to determine the biological sex of the individual it is sufficient to study their thigh microbial diversity.
Figure 2. Skin microbiome diversity influenced by biological sex, environmental factors, and hygiene routine represented by the Shannon diversity index (y axis).
Lifestyle impacts the skin microbiome
How you eat, what you use for your skin care routine, and who you live with plays an important role in deciding the diversity of microbes on your skin. It is common knowledge that what you eat changes the microbes that live in your gut. But how do these factors influence the microbiota found on human skin? For example, alcohol consumption significantly reduces microbial community diversity. Researchers demonstrated that consuming multiple servings of alcohol each day decreases the number of microbial taxa present on human skin.
Also, the people you live with have an impact on your skin microbiota. One study found that the microbes present on the palms and in the mouth were more similar among people from the same family than to people not related to each other (Song et al., 2013). Further, cohabiting partners can be predicted in 86% of the cases using just the skin microbiomes. This means that if we have multiple skin microbiome samples and we want to predict which of them belong to cohabiting partners, we can do it with a rather high level of confidence by comparing their skin microbiomes. Interestingly, the skin microbial community changes when one of the cohabitants is absent, even if only for a few days. Think about it, if one of the partners goes on a short business trip, microbial communities of both partners start to charge almost immediately, isn’t that fascinating?
Finally, having pets and spending time outside is associated with higher microbial skin community diversity. For example, all participants who had at least one dog in their household had a significantly more diverse skin microbiome compared to their dogless peers. Speaking of dogs, from a microbial perspective, the skin communities of people are more similar to those of their own dogs than to other dogs. Not only does having a dog increase microbial diversity on our skin, but it also makes our microbial communities more similar to that of the dog. Spending just a few hours a day outdoors also significantly increases diversity of skin microbes. Probably because you have more direct contact with natural microbial communities found in the environment.
Limitations
These studies give us a broad idea of how human skin microbiome depends on person’s sex, their hygiene, and lifestyle. However, the authors of these studies acknowledge that a number of limitations were present in the methodological set-up and that further work should be done in this area.
For example, most of the participants in these studies were of Caucasian descent. Other microbiome studies have concluded that race is a significant factor influencing the vaginal (Hyman et al., 2014) and gut microbiome (Human Microbiome Project Consortium, 2012). So obviously, further research should be done using skin samples from people of as many ethnicities and descents as possible.
Finally, more skin microbiome studies should include same-sex couples because to date not much is known about how skin microbiome is affected by living with a partner of the same sex. So as always in science, there are lots of things left to do!
Take Home message
Recent studies investigating human skin microbiome overviewed main factors affecting what microbes live on our skin. They demonstrated that cohabiting partners share enough microbes to be clustered together using machine learning approaches in >86% of the cases. They also showed that women have a more diverse skin microbiome than men. Speaking of diversity. Getting a dog, spending more time outdoors, and occasionally skipping a happy hour is the way to go to increase microbial diversity of your skin microbiome. This research is far from being over and there are definitely more things to learn in this domain of knowledge so stay tuned if having a healthy diverse microbiome is one of your goals.
References:
Kong HH, Segre JA. 2012. Skin microbiome: looking back to move forward. J Invest Dermatol 132:933–939. https://doi.org/10.1038/jid.2011.417.
Grice EA, Kong HH, Renaud G, Young AC, NISC Comparative Sequencing Program, Bouffard GG, Blakesley RW, Wolfsberg TG, Turner ML, Segre JA. 2008. A diversity profile of the human skin microbiota. Genome Res 18:1043–1050. https://doi.org/10.1101/gr.075549.107.
Fierer N, Hamady M, Lauber CL, Knight R. 2008. The influence of sex, handedness, and washing on the diversity of hand surface bacteria. Proc Natl Acad Sci U S A 105:17994 –17999. https://doi.org/10.1073/pnas.0807920105
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Umbach, A.K., Stegelmeier, A.A. and Neufeld, J.D., 2021. Archaea Are Rare and Uncommon Members of the Mammalian Skin Microbiome. Msystems, 6(4), pp.e00642-21. DOI: https://doi.org/10.1128/mSystems.00642-21
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