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

510 JOURNAL OF COSMETIC SCIENCE
effectiveness lasts for several hours after application.153 Heckmann et al. reported that CHG
had comparable efficacy with isopropyl alcohol against C. acnes on skin, but it was found
that C. acnes colonizing deeper layers of the skin may be more resistant to the killing action
of CHG.154 These workers noted that benzoyl peroxide (BPO), which is allowed for use at
use 2.5–10% in OTC topical acne treatment products, penetrates the sebaceous glands,
and effectively inhibits growth of C. acnes. Long-term use studies revealed that antiseptics
with ethanol and isopropyl alcohol do not affect skin integrity or influence the diversity
of the skin microbiome.152 Interestingly, soap was found to be better at reducing the titer
of human intestinal and respiratory noroviruses than preparations based on ethanol or
isopropanol. Nevertheless, 78–95% ethanol or 70–100% isopropanol effectively eliminated
coronaviruses.155,156
These examples demonstrate that extrinsic factors such as cosmetic and drug product use
may alter the skin microbiome. It is recommended that studies be done to determine the
effect of topical products on the skin microbiome to ensure that use on target skin sites
causes no adverse effects on the microbiome composition or diversity. This should enable
selection of products that work with the skin microbiome to help prevent dysbiosis that
may result in skin disorders.
PREBIOTICS, PROBIOTICS, AND POSTBIOTICS
The GI tract and skin have epithelial surfaces in direct or indirect contact with the
environment, and these surfaces are colonized by the intestinal microbiome and skin
microbiome, respectively. Recently, there has been a great deal of interest in studying
the effects of probiotics because studies have demonstrated that oral probiotics may exert
beneficial effects in the GI tract and at distant sites in the body—to ameliorate AD and
eczema and to reduce inflammation following exposure to UV radiation.157 Furthermore,
there is growing awareness of a gut-brain-skin axis in which microorganisms in the GI
tract play a role in mediating skin inflammation and emotional behavior.158
Probiotics are live microorganisms which when administered in adequate amounts confer a
health benefit on the host.159 Prebiotics are food ingredients that are neither hydrolyzed nor
adsorbed in the stomach or GI tract but confer beneficial effects on the host by selectively
stimulating the growth of desirable bacteria in the colon. Thus, prebiotics support the
growth of probiotic microorganisms. Prebiotics include lactulose, fructooligosaccharides,
and fructose-containing polysaccharides, such as inulin. Postbiotics are dead microorganisms
or their metabolic byproducts which, when ingested, may have the ability to exert positive
biological responses to maintain intestinal homeostasis in a manner like probiotics.
These definitions are appropriate for oral products, but they are not suitable for products
applied topically. Thus, it may be very difficult to deliver viable microorganisms via
customary creams, lotions, and serums because the preservative systems in these products
would inactivate the probiotic microorganisms, and such products would no longer contain
“live microorganisms.” Probiotic microorganisms that benefit the GI tract include lactic
acid bacteria (e.g., Lactobacillus acidophilus, L. longum, L. rhamnosus, L, johnsonii, L. reuterii,
and Bifidobacterium infantis, B. longum, B. bifidum), Bacillus coagulans and Saccharomyces
boulardii (a yeast), but these microorganisms may or may not function as probiotics on the
skin. Similarly, the prebiotics that stimulate growth of probiotics in the GI tract may not
be suitable for stimulating microorganisms determined to be probiotics on skin.