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

629 The Skin Microbiome and Aging
researchers identified a positive correlation between higher corneometer measurements,
reflecting increased skin moisturization, and a taxon that may be associated with a
Lactobacillus species. These observations suggest that the presence of these key microbiome
signatures could be biomarkers to profile a young skin appearance.
The presence and influence of lower-population species may also have a significant effect on
the appearance of youthful skin. Several publications have identified decreasing populations
or even absence of Lactobacillus populations in older skin.17,26,33 It has been suggested that
Lactobacilli might contribute to a balanced microbiome through acidification of skin
from lactic acid production as well as the secretion of anti-inflammatory components and
antimicrobial peptides. Lactobacillus species detected on skin include L iners, L crispatus, and
L gasseri, which are predominately found associated with the human vaginal microbiome,
which may be introduced to skin during birth. These species are in some contrast to
the more commonly found lactic acid bacteria used in the personal care industry such
as Bfidiobacterium, Lactococcus lactis, L plantarum, and L paracasei, which have primarily
been isolated from fermented foods. It could be that the use of Lactobacilli and their
derivative metabolites derived from skin might have more compatibility on skin due to
its evolutionary niche. In fact, some Lactobacilli, such as L crispatus, may also contribute to
physical biomechanical properties through binding to and associating with skin components
like collagen and fibronectin, through surface proteins (S-layer proteins).34 These structural
characteristics of various Lactobacillus species may suggest the capacity to interact with skin
and its components, and thus may favor a younger skin phenotype as part of its niche.
Other minority microbiome constituents could be conversely responsible for collagen
degradation in older skin, leading to less firm and more wrinkled skin. One study details the
association of Negativicutes populations, a relatively unknown class of bacteria that is known
to be an inhabitant of the skin and intestines, with Veillonella being the best characterized
genus member.35 Negativicutes had a significant presence on older skin that possess a
functional pathway for enzymic breakdown of collagen.27 It would be interesting to see if
these microbial associations would be similarly observed with a decreased measurement
in skin elasticity or increased observation of facial wrinkles. Another study authored by
Zhou et al. assessed the collagen diffusion coefficient (CDC) using an optical device that
emits infrared light and its scattering as an indicator of the relative quality and quantity of
collagen in the skin.16 As expected, CDC was found to be higher in younger skin, which
correlated with high populations of C acnes and low skin microbiome diversity. Conversely,
on skin with low CDC measurements, there was a positive correlation to a phylogenetic
clade of S epidermidis, clade L. This clade has been found to be associated with nosocomial
infections, indicating a more virulent population of this skin commensal organism.16 There
is more to learn on the influence of the skin microbiome and a mechanistic understanding
of the impact on collagen quality and quantity in skin and how this relates to skin elasticity
and firmness.
NEW INSIGHTS FOR SKIN MICROBIOME AGING
While there is much agreement in the scientific literature that C acnes is a dominant
microbiome population in youthful skin, a study conducted by Leem et al. has a different
observation of this species after normalizing C acnes communities to age.33 In this study,
different skin microbiome profiles were characterized based on clinical parameters in