formulations have been specifically developed to manage in- ammation and preserve or restore both the skin barrier and the skin microbiota diversity. Water Moisturizers can be formulated with deionized water or thermal water. The physicochemical characteristics of thermal water de- pend on the nature of the geologic materials through which the groundwater has moved. Common soluble minerals include calcium (Ca2+), bicarbonate (CO3H-), silicates, iron compounds, sodium and magnesium salts, sulphur compounds, and met- als.61 Trace elements, including selenium or strontium, as well as purity and pH are also important parameters that may in- uence the speci c biological activities of thermal waters. For example, presence of selenium has free-radical scavenging and anti-in ammatory properties and also provides protection against toxic heavy metals.61-65 Thermal waters have a unique microbial signature related to their specific mineral content. In comparison to deionized wa- ter, thermal water can be viewed as containing prebiotic active ingredients (ie, non-viable food components that confer health bene ts associated with a modulation of the microbiota).56 The presence of specific trace elements in thermal water can drive the growth of bene cial bacterial species particularly if they are already found in its natural microbial content.54 The importance of thermal water is supported by results which showed that an emollient containing 50% selenium-rich ther- mal spring water (TSW) or the use of selenium-rich TSW alone during balneotherapy reduced disease severity and increased the diversity of skin microbiota in patients with either atopic dermatitis or psoriasis.66,54 In both groups of patients, there was an increase in keratolytic bacteria of the Xanthomonadaceae family that are naturally present at low levels on the skin and in TSW and a decrease in Staphylococcus spp.66,54 Prebiotics Prebiotics were initially de ned as non-digestible food ingredi- ents that bene cially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bac- teria.67 Prebiotics that might be included in skin products also have the potential to support maintenance of the normal skin microbiome.60 Relatively little is known about the bene ts of this approach, but it has been shown that application of a bio- mass lysate of the non-pathogenic gram-negative bacterium, Vitreoscilla liformis, helped to restore the skin microbiota in patients with atopic or seborrheic dermatitis.68-70 It is interesting to note that V. liformis biomass (VFB) prepared from organisms grown in a medium enriched with TSW resulted in more potent stimulation of mRNA expression for and levels of antimicrobial peptides in reconstructed epidermis.71 Treatment of patients with atopic dermatitis using an emollient containing VFB prepared with selenium-rich TSW vs another recommended emollient yielded greater clinical improvements with the VFB emollient that were associated with significantly increased genus Xanthomonas. In contrast, the comparator product was associated with increases in Staphylococci.41 This has been evaluated via high-throughput sequencing approach that targets the V1-V3 region of the bacterial 16S rRNA gene as recommended by Meisel JS et al, 2016.72 While inclusion of prebiotics in skin preparations appears promising, much more research is required to learn their bene ts and limitations. Other Components Occlusive agents, such as ceramides, included in moisturizers may be good carbon and nitrogen sources for bacteria. Ceramidase activity has been detected in bacterial skin ora and it has also been noted that skin ceramide levels are reduced in patients with atopic dermatitis.47 These results suggest that increasing levels of skin ceramides may be important for maintenance of skin health. Niacinamide (vitamin B3) is combined with emollients in some skin products and it is also employed in culture media for some bacteria. It may promote skin health as it has been shown to inhibit the growth of methicillin-resistant S. aureus.73
CONCLUSION
Understanding the complex relationship between normal skin barrier function and the skin microbiome is critical for the rational development of new skin care products.42,59,60,53,58,54 Appropriately developed formulations have the potential to selectively increase the activity and growth of bene cial microbiota, prevent skin dysbiosis, and restore or maintain efficient skin barrier function.41 This is particularly important for conditions in which barrier dysfunction may occur, such as with dry, sensitive, and reactive skin; exposure to aggressive cosmetic or hygienic routines; after aesthetic procedures; or when taking therapeutics including antibiotics and corticosteroids.The studies reviewed in this paper suggest that inclusion of prebiotics eg, ceramides, niacinamide, selenium-rich thermal spring water may all increase the efficacy of moisturizers and that some of this bene t may be due to positive effects on skin microbiota.
ACKNOWLEDGMENTS
The authors would like to thank Tom Prunty and Bob Rhoades of AraMed Strategies for medical writing assistance. Their support was funded by La Roche-Posay Dermatological Laboratories, USA.
DISCLOSURES
S. Seite? is an employee of La Roche-Posay, France. This review was supported by La Roche-Posay Dermatological Laboratories, USA.
