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.

511 Evolution and Challenges of Sustainability
Currently, there is a great deal of research directed at studying the skin microbiome,
and some products involved with this technology have been introduced in recent years.
Sullivan et al. patented a skin treatment using Lactobacillus extract (i.e., a postbiotic) for
stimulation of β-defensins in skin cells to increase the skin’s natural defenses against
infection or to make products less irritating by decreasing the microbial density on
sensitive areas of the skin.160 A recent report by Spragge et al. indicated that the intestinal
microbiome protects against pathogens by “nutrient blocking” that is promoted by
diversity of the microflora and by the presence of key species that increase the overlap
between the nutrient use of the GI commensals and pathogens.161 It is believed that a
similar situation may occur on skin.
The intrinsic and extrinsic factors affecting the diverse microflora at the different skin
sites present a very complex system to study. Although single culture studies may be
informative, Boxberger et al. observed that there are clinical implications of the gut-brain-
skin connection in acne so that researchers may need to test with multiple microorganisms
under different conditions to better understand their full range of functions and how they
can be modified to lessen the severity of skin disorders and skin inflammation.162
The CNS may be candidates to be considered as probiotics on skin to lessen the severity of
acne, for wound management, and for down-regulation of the SIS. S. epidermidis, S. hominis,
and S. capitis are prominent members of the skin microbiome. They are able to produce
an array of antimicrobial substances (i.e., short-chain fatty acids from lipase hydrolysis of
sebum triglycerides, AMPs, and PSMs), and their ability to stimulate skin cells to produce
AMPs, including cathelicidins and defensins (to which they are refractory), make them
possible candidates to consider for use as probiotics. We are learning from studies of the
intestinal microbiome and the use of prebiotics. It is likely that selected ingredients will
act similarly and benefit desirable commensal microorganisms in the skin microbiome.
Finding a prebiotic (i.e., substrate) to facilitate growth or colonization resistance by selected
CNS in situ may be a better approach than trying to add living CNS for modulating the
skin microbiome to achieve desirable outcomes.
GAPS IN OUR KNOWLEDGE THAT NEED TO BE ADDRESSED IN FUTURE
PRODUCTS
The evolution of cosmetic preservation has involved transitioning from the use of traditional
preservatives to use of multifunctional ingredients to replace some or all the preservatives
used in some aqueous products. However, there are several gaps in our knowledge that
need to be addressed. These issues are discussed next.
ADEQUATE PRODUCT PRESERVATION
The goal of preservative efficacy testing is to determine the minimum concentration and
types of preservatives or multifunctional ingredients required for the adequate preservation
of aqueous cosmetic products. The acceptance criteria used for preservative efficacy testing
are critical, and more rigorous criteria provide a greater measure of protection than relaxed
criteria.
Unfortunately, there is no consensus on what is necessary or sufficient for preservative
efficacy test acceptance criteria. These criteria range from the rigorous (e.g., linear