Upon birth, the previously sterile skin becomes colonized by various microorganisms. This ecosystem shapes the skin microbiome. The skin, due to its close contact with the surrounding environment (which changes the skin condition), has the most diverse microbiome of all organs. Different parts of one’s skin however are structured differently. So, is microbiome diversity equally distributed at all the different skin sites, and are there any aging-associated alterations on this community of microorganisms? Howard and his team have investigated the effect that age has on site-specific skin microbiome. In this study, 158 Caucasian females aged 20‒24, 30‒34, 40‒44, 50‒54, 60‒64, and 70‒74 years were studied.
Face, forearm, and buttocks! The three studied hosts
This study focused on face, forearm, and buttocks to better understand age-associated changes on skin microbiome. The results concluded that “there was considerable fluctuation in bacteria levels at specific ages with many genera.” Individual bacterial genera with an average relative abundance >1% were taken into consideration in the study. Some species of the detected bacterial genus, such as Corynebacterium, Staphylococcus, and Anaerococcus, are pathogenic and could cause skin infections. Some other genera such as Cutibacterium, Finegoldia, and Acinetobacter included species that are opportunistic pathogens. Lactobacillus bacteria are considered non-pathogenic organisms and are also known as probiotic. Interestingly, all three body sites demonstrated a decrease in relative abundance of Cutibacterium (the most abundant bacterium living in healthy skin) and Lactobacillus. and there were some genera with no change.
Is sebum on the menu?!
Skin is hosting this bacterial party, and as a host it provides food sources and antimicrobial defences. Sebaceous glands produce sebum lipids which are a food source for bacteria such as Cutibacterium. The study showed “a significant decrease in facial sebaceous gland area after age 40 years.” This means the lack of sebum may cause a decrease in the relative abundance of Cutibacterium (positive correlation). However, the results show this correlation has taken a negative manner with some other existing bacterial genera on skin (table 2). This inverted correlation could be due to the loss of antibacterial components in the sebum or because of Cutibacterium reduction which opens the space for others.
Natural moisturizing factors, lipid, and antimicrobial peptides
“Evaluation of natural moisturizing factors (NMFs) in the skin stratum corneum at the three body sites showed a significant age-related increase in both total and individual NMFs”. This change has shown to increase most bacteria proliferation and adherence in the skin, which indicates a positive correlation in most bacteria abundance and increased NMFs levels with age.
Besides sebum, major skin lipid classes such as fatty acids, ceramides, cholesterol, and sphingolipids were also examined, and the results showed an increase with age. “Sphingolipids have antibacterial activities and in aggregate were negatively correlated with Lactobacillus, indicating a potential role for aggregate sphingolipids in modulating Lactobacillus abundance”. The increase of skin ceramides, which serve as a potential food source for bacteria, was positively correlated with an increase in Streptococcus abundance with age.
And finally, the evaluation of antimicrobial peptides (AMPs) in the skin of individuals showed that only two AMPs significantly increased with age. Most bacterial genera negatively correlated with this increase.
What is driving these changes, the host, the microbiome, or both?
There are various interactions between the members of any given ecosystem. Thus, it is reasonable to question whether skin condition effects microbiome diversity or if these bacteria are affecting skin condition. Another study performed by Nodake et al. evaluated the effect of topically applying Staphylococcus epidermidis, and the results showed “an increase in skin lipid content and NMFs and improved skin barrier function”. So, we could say that the effect of host and microbiome on each other is actually a two-way street.
- Howard, C. C. Bascom, P. Hu, R. L. Binder, G. Fadayel, T. G. Huggins, B. B. Jarrold, R. Osborne, H. L. Rocchetta, D. Swift, J. P. Tiesman, Y. Song, Y, Wang, K. Wehmeyer, A. B. Kimball, R. J. Isfort. Aging-Associated Changes in the Adult Human Skin Microbiome and the Host Factors that Affect Skin Microbiome Composition. J of Investigative Dermatology, 142 (2022), pp. 1934-1946.e21
- Nodake, S. Matsumoto, R. Miura, H. Honda, G. Ishibashi, S. Matsumoto, et al. Pilot study on novel skin care method by augmentation with Staphylococcus epidermidis, an autologous skin microbe–a blinded randomized clinical trial. J Dermatol Sci, 79 (2015), pp. 119-126