627 The Skin Microbiome and Aging
Bacteroidetes.18 The relative proportions of these bacteria can vary depending on the part of
the body in which they are found, due to differences in environmental niches and nutrient
sources.19 Different body site chemistries are divided into dry, moist, and sebaceous type-
areas and support different skin microbiome populations.20 Each site harbors a distinct
microbiome profile consisting of organisms that are best suited to utilize the available
nutrients in that specific niche. Dry sites like the leg or forearm, for example, tend to
be dominated by phyla Proteobacteria and Firmicutes and have more diverse microbial
communities compared to moist or sebaceous sites.20 Alternatively, moist sites like the
axilla are populated by Staphylococcus and Corynebacterium species, while sebaceous sites like
the face, scalp, and back are dominated by members of the Cutibacterium genus (formerly
known as Propionibacterium genus).20
Understanding the significance of the skin microbiome is crucial in the context of skin
health and aging. While earlier studies focused on microbiome profile taxonomy, more
recent studies have attempted to clarify microbiome populations’ contributions to skin
phenotype.12,21 By gaining a deeper understanding of the skin microbiome and its role in
skin aging, we can develop innovative strategies for maintaining youthful and healthy skin.
TRADITIONAL UNDERSTANDING OF SKIN AGING
To fully comprehend the significance of the skin microbiome’s impact on the aging of skin,
one needs an understanding of the complex process of skin aging, which involves both
intrinsic and extrinsic factors. Intrinsic or chronologic aging is genetically determined and
is a natural and inevitable process. It is characterized by the gradual decline in skin function
and is typically associated with skin laxity, fragility, decreasing epidermal thickness,
decreased sebum production, and the development of expression lines.22 In contrast,
extrinsic aging is driven by environmental factors such as chronic exposure to solar UV
irradiation, pollution, and smoking. Extrinsic aging is exemplified by more pronounced
signs of aging, including deep wrinkles, hyperpigmentation, and a significant loss of skin
elasticity.23 These environmental influences can accelerate the aging process and lead to
more severe skin changes. In addition, aged skin is commonly believed to be less hydrated
and more permeable compared to younger skin. However, some studies have shown that the
functional barrier of aged skin is not necessarily impaired, and increased susceptibility to
irritation has not been consistently demonstrated. While aged skin may exhibit differences
in trans-epidermal water loss (TEWL), the impact on irritant and permeability testing
remains inconclusive.8,22,24 Nevertheless, age-related changes in the skin can be assessed
through a range of biophysical parameters, including measurements of skin moisturization
through capacitance measurements, TEWL, skin deformation resistance, echodensity, and
topographical analysis, as well as photographic and clinical grading assessments to evaluate
the presence of wrinkles, sagging, and skin color irregularities, providing valuable insights
into clinical changes in skin.25
SKIN MICROBIOME AND AGING
Incorporating measurements of the skin microbiome as an additional clinical parameter
in the study of aging skin can provide valuable insights into a deeper understanding
of underlying mechanisms through which the skin microbiome influences the aging

628 JOURNAL OF COSMETIC SCIENCE
process. This can lead to more innovative strategies for maintaining healthy and youthful-
looking skin. It has been observed that as the skin ages, there are general changes in
the skin microbiome composition. Multiple studies have found that skin microbiome
diversity increases in aged skin driven by a population shift in Proteobacteria and
Actinobacteria.5,7,8,9,11,14,17,26–29 This observed increase in skin microbiome diversity suggests
a fundamental change in physiological skin conditions, allowing population expansion
among opportunistic pathogens and transient microorganisms. This observation contrasts
with an overly simplistic assessment of high microbiome diversity resulting a “healthy”
microbiome. Rather, the specifics of microbial populations are important to understand the
overall health and balance to any population niche.
The largest shifts in these populations seem to be driven by two main species, Cutibacterium
acnes (formerly known as Propionibacterium acnes) and Corynebacterium kroppenstedtii. In young
facial skin, C acnes typically dominates with approximately 70–80% relative abundance
of the bacterial sequence. However, in older skin, there is a significant decrease in the
abundance of C acnes populations, accompanied by a notable increase in the relative
abundance of C kroppenstedtii, which is known to be an opportunistic pathogen.5,6,10,14,15,16,17
It is hypothesized that this large shift is driven by age-related changes in sebum, which is
the preferred nutrient source for C acnes.7,30 Triacylglycerides found in sebum are metabolized
into glycerol and free fatty acids, which might preserve a balanced skin microbiome in
youthful skin and favor lower diversity conditions, compared with older skin.31,32 Similarly,
a study on menopausal women confirmed that age-related changes in sebum amounts on
skin likely decreases C acnes populations while creating a niche for Streptococcus species.7,17,26
While these observations have been shown in several studies, understanding the mechanism
of these observational changes in tandem with clinical measurements is important to
understand some of the key interactions for skin aging.
CORRELATIONS BETWEEN MICROBIAL OBSERVATIONS AND SKIN
MEASUREMENTS
Myers et al., for example, performed a study assessing crow’s feet wrinkle clinical grading,
skin hydration measurements, and TEWL in conjunction with 16S rRNA profiling.8 They
confirm that skin microbiome diversity increases with age, but with a greater resolution
than previous studies. For instance, the authors found a correlation between a more
diverse skin microbiome and impaired skin barrier function, resulting in elevated levels
of TEWL. They also made a connection to several microorganisms found on aged skin,
such as Microbacterium and Brevibacterium. The Microbacterium genus had previously been
found on skin exposed to high levels of polycyclic aromatic hydrocarbon pollution, while
Brevibacterium has been reported to be predominant on psoriatic lesions and in localized
skin infections of bedridden elderly subjects.8 Although these genera might exist in
minimal quantities on healthy skin, an increase in skin microbiome diversity can lead
to their greater population density, potentially impairing skin barrier function. This is
evidenced by their correlation with increased TEWL in this clinical study. This study also
observed several key commensal bacteria, including Staphylococcus, Kocuria, Peptostreptococcus,
and Lysobacter, which were associated with lower clinical wrinkle grading of the crow’s feet
area. While Staphylococcus is commonly known to be a major genus associated with the
skin microbiome, Kocuria, Peptostreptococcus, and Lysobacter are lesser known, having been
shown to be associated with both skin and environmental microenvironments. Lastly, the