508 JOURNAL OF COSMETIC SCIENCE
Mycoplasmatota) is a phylum of bacteria that lack a cell wall around their cell membrane
and which includes Mycoplasma spp. The mother-child relationship in the first few months
after birth allows transmission of microorganisms, and a study of one-year old children
revealed predominance of bacteria belonging to Firmicutes (about 50%), Actinobacteria
(about 20%), and Bacteroidetes (about 20%) phyla. Microbial diversity increases until about
age 8, with the number of staphylococcal and streptococcal species decreasing and the
amount of Actinobacteria and Proteobacteria species increasing.137,138 The skin microbiome
experiences a substantial shift at puberty when sex hormones drive maturation of sebaceous
glands that start sebum production.86 In adolescents, numbers of C. acnes increase due to
the increased availability of sebum.68 Adult skin has higher levels of Cutibacterium spp. and
Corynebacterium spp., in contrast to young children who have Gammaproteobacteria (which
includes Pseudomonadales and Enterobacterales) and Streptococcaceae at multiple sites.72 The
microbiome of adult skin remains fairly stable, but the composition at different sites is
dictated by different conditions (e.g., dry, moist, lipid-rich).66
Unlike other organs, skin aging is affected by both intrinsic and extrinsic factors. Sun
protected sites, such as the buttocks, largely undergo intrinsic aging processes influenced
by genetic, metabolic, and hormonal changes.98 Intrinsically aged skin is characterized by
reduced sebaceous gland functioning and lipid content, reduced activity of AMPs, decreased
blood flow, and degradation of collagenous and fibrous extracellular matrixes which leads
to atrophy, xerosis with fine lines and wrinkles.139,140 Lower sebum levels may result in
decreased numbers of Firmicutes (including S. aureus and C. acnes) and more Corynebacterium
spp. in the microbiome.68,80,138 Extrinsic aging is influenced by environmental factors,
primarily due to exposure to ultraviolet radiation.141 Both intrinsically and extrinsically
aged skin have a higher pH, less hydration, and reduced expression of tight junction
proteins compared to young skin however, photoaged skin exhibits increased rates of
proliferation and increased sebum.139,142 Changes in skin lipids with age leads to changes in
substrates and pH in the skin microenvironment, which results in alterations in the skin
microflora. The overall abundance of bacteria increases with age, but there are changes in
the bacterial populations within the skin microbiome so that Corynebacterium spp. increases
while Cutibacterium spp. and Lactobacillus spp. decrease in number.143-146
Dimitriu et al. studied the contributions of extrinsic (i.e., lifestyle) and intrinsic (i.e.,
age) factors to skin microbiome variation by profiling bacterial microbiomes of 495
North American subjects (ages, 9 to 78 years) at four skin sites using 16S rRNA gene
amplicon sequencing.68 These workers found the strongest associations involved aging,
demographics, and lifestyle factors, with bacteria in the genera Anaerococcus, Peptoniphilus,
Prevotella, and Corynebacterium dominating these associations. Corynebacterium spp. were
associated with chronological age and skin aging (e.g., hyperpigmented spots and wrinkles)
on the forehead, and a Propionibacterium (Cutibacterium) taxon was associated with forehead
porphyrins. Interestingly, they found that age was associated with two corynebacterial
taxa that mutually co-excluded each other and that the relationship between age and the
two corynebacteria taxa was similar at different skin sites. These workers also observed
that bacterial interactions at skin sites exposed to the environment showed higher
connectedness than those at occluded sites, which led them to suggest that exposed skin
sites have diverse microflora that are potentially more resilient to community disruption by
changes in the environment. Readers are directed to reviews on the skin microbiome for
additional information on gender, ethnicity and living/working environment on the skin
microbiome.66,74,77,82,86,98

509 Evolution and Challenges of Sustainability
THE EFFECT OF COSMETICS ON THE SKIN MICROBIOME
Extrinsic factors, including use of cosmetics, and drugs, lifestyle, and work environment
are associated with changes in microflora of the skin microbiome. Moisturizing products
used regularly for extended periods may hydrate the skin, and detergents in cleansers may
remove skin lipids and other compounds that may serve as nutrients for microorganisms—
both actions may lead to alterations in the skin microflora.66,147,148 Tu et al. demonstrated
that exogenous N-acetylglucosamine promoted hyaluronic acid (HA) production in human
dermal fibroblasts.149 Dimitriu et al. observed that N-acetylglucosamine, an ingredient
often used in skin care products that may be converted to HA in skin, has been reported
to stimulate skin microflora.68 It is believed HA hydration of the skin environment enables
diversification of the skin microflora.
Skowron et al.66 indicated that hand and face lotions did not have any major impact on
microbial diversity however, this may have been due to experimental design including the
type of products used, the test sites on skin, the amount applied, length of application,
and other factors. Moisturizers reduce the water loss from skin, support microbial growth,
and decrease skin cell exfoliation.69 The lipid components in moisturizers may be able
to promote growth of lipophilic bacteria, such as the staphylococci and cutibacteria.147
However, the increased level of skin hydration decreases sebum content and actually may
decrease the number of Cutibacterium spp.150 Lee et al. reported that application of a set
of moisturizing products increased the bacterial diversity, but reduced the number of
Cutibacterium spp.151
Bouslimani et al. used a multi-omics approach integrating metabolomics and microbiome
data from skin samples of 11 healthy human individuals in a 9 week study and found that
modifying the hygiene routine can alter the skin metabolome and microbiome, but this
alteration depended on product use and location on the body.147 Study participants applied
a moisturizer on the arm, a sunscreen on the face, an antiperspirant deodorant on the
armpits, and a soothing foot powder on the feet. Significant increases in abundance of Gram-
negative bacteria including Acinetobacter and Paracoccus genera were noted for subjects using
the antiperspirant and foot powder, but little variations were observed for the face, and no
variations were detected for the arms. These findings suggest that product use may modify
the chemical environment of skin to influence the skin microbiome in some skin sites.
The skin microbiome was site-specific, but it varied more between individuals and this
inter-individual variability was maintained over time despite same changes in product use.
Bouslimani and coworkers also found that many compounds from the cosmetic products
tested persisted on skin for weeks following product use (e.g., half-life for propylene glycol
ranged from 0.7–1.9 weeks, depending on panelist). They also noted that inappropriate
cosmetics or unsuitable application of cosmetics may negatively affect the skin microbiome
to reduce diversity and lead to dysbiosis.
Cleansing products may remove or kill microorganisms on skin. Soaps, combined with
washing in water, effectively reduced the number of microorganisms on hands.152 Body
wash formulations containing zinc pyrithione (ZPT) reduced S. aureus levels, which
favorably influenced AD.150 ZPT is a fairly broad spectrum antimicrobial that is commonly
used in products that treat seborrheic dermatitis and dandruff, and it is likely that regular
use of products containing ZPT for extended periods will alter the skin microbiome.
Antiseptics generally are used to reduce surgical infections. The active ingredients
used include iodine povidone, CHG, and ethanol.66 CHG is substantive to skin and its