Skin microbial communities differ from one individual to another owing to the genetic and environmental factors. Skin conditions, such as pH, temperature, and moisture content, and a variety of factors, including antibiotics, cosmetics, soaps, personal hygiene products, lifestyle, and nutrients, affect the skin microbial community and abundance. Application of probiotics and prebiotics to the skin causes specific changes to the microbial diversity and colonization, leading to reconstitution of skin microbial communities. The re-balance of skin microbiota may have a significant effect on the functional differences between healthy and damaged skin. Therefore, we evaluated the potential of GOS, a prebiotic, as a cosmetic material by measuring the changes of skin factors and skin microorganisms.
Oral ingestion of GOS has been reported to be effective in moisturizing, maintaining skin barrier function, inhibiting pigmentation [17], and improving wrinkles [18]. Unlike previous studies, GOS-containing cosmetic serum were applied to the skin in this experiment, and found a significant difference in water-holding capacity and TEWL, corresponding to the skin barrier function, compared with those in the ND group (Figs. 1 and 2). The stratum corneum of the epidermis, the outermost layer of the skin, acts as a barrier to the harmful external environment and plays a vital role in preventing and retaining moisture in the skin [19]. The stratum corneum of the epidermis, the outermost layer of the skin, acts as a barrier to the harmful external environment and plays a vital role in preventing and retaining moisture in the skin [20], keeping the skin moist and shiny. When a lactic acid bacteria culture was applied to the skin, it maintained the pH at weakly acidic and contributed to skin hydration [19, 21]. In the present study, application of GOS-containing cosmetic serum led to the growth of lactic acid bacteria among the skin microorganisms, which may have a moisturizing effect. The lactic acid bacteria such as Streptococcus pneumoniae (Figure S4) were found to be increased in the NF group compared with that in the ND group. Additionally, oral administration of GOS increases the mRNA expression of tissue inhibitors of metalloproteinase (TIMP-1), which is an inhibitor of the collagen-degrading enzyme matrix metalloproteinase (MMP). In addition, GOS and Bifidobacterium co-administration reported a decrease in MMP-9 expression [22]. The GOS-containing serum treatment induced the proliferation of Lactobacillus, showing the possibility of suppressing MMP expression. Moreover, the results showed an increase in the specific species of lactic acid bacteria such as S. pneumoniae in the NF group (Figure S4). The most abundant phyla identified on the skin of the subjects were Proteobacteria, Firmicutes, and Actinobacteria (Figure S2), corroborating with previous studies. Staphylococcus is the most common among the Firmicutes phylum, while Cutibacterium is the most abundant species among the Actinobacteria phylum.
Among the skin microorganisms of the subjects participating in the experiment, Proteobacteria, Firmicutes, and Actinobacteria were identified as the most abundant phyla (Sup. 2). In other studies, these three phyla have been reported to be the most prevalent on the skin [23, 24]. Staphylococcus is the most common among Firmicutes phylum, while actinobacteria is the most abundant species among Cutibacterium. The facial skin samples from the pre-treatment groups harbored fewer species than those from the post-treatment groups. Moreover, no statistically significant difference was observed in the bacterial richness on the facial skin of the post-NF group compared with the post-ND group; however, the diversity of the bacterial communities was found to be caused by the application of the GOS-containing cosmetic serum (Fig. 4). GOS treatment was expected to induce higher diversity than the serum base treatment because GOS as a prebiotic enhances environmental diversity of the skin microbes. Prebiotics are “fertilizers” or “foods” with components that selectively promote the growth of these essential microorganisms, thereby potentially improving the host health [25]. S. aureus was the most common species found on the skin of the subjects, as shown in Figure S3 and 5, and has been recognized as a crucial cause of skin infection [26]. Staphylococcus is relatively common on healthy skin but in one-third of the population the presence of S. aureus is considered a significant risk factor for future infections [27]. S. aureus weakens the skin barrier and activates deleterious host immune responses. For instance, S. aureus produces proteases that are capable of penetrating the dermis of patients or mice with atopy disease (AD) and deleterious mutations in the filaggrin gene [28, 29]. In fact, in AD, abundance of S. aureus was associated with immune dysfunctions, including T helper cell 2 lymphocyte asymmetry, reduced antimicrobial peptides (AMPs), exacerbated allergic reactions, and destruction of the skin barrier [30]. S. aureus increases the production of type 2 cytokines, such as thymic stromal lymphopoietin, interleukin (IL)-4, and IL-13 [28]. Moreover, several other molecules produced by S. aureus also induce skin inflammation and exacerbate AD. In particular, α-toxin degrades keratinocytes in the presence of type 2 cytokines. All these factors contribute to local inflammation and further affect the skin barrier, leading to the exacerbation of skin diseases such as AD [31]. However, other bacterial species found on normal skin seem to help maintain immune homeostasis [32]. S. epidermidis present on the skin of healthy individuals may reduce inflammation [33], improve the development of skin T cells, and promote the expression of AMPs [34].
Skin sites can be classified based on their physiological properties as follows: sebaceous (oily), moist, or dry, and can affect the distribution of the skin microbiota [2]. Enhydrobacter, Corynebacterium, and Staphylococcus are predominant at the moist sites. In the ND group, water-holding capacity was positively correlated with these strains (Fig. 7). Moreover, TEWL was positively correlated with the relative abundance of S. aureus, but was not significantly different (Fig. 7). In the human AD epidermis, the relative abundance of S. aureus and S. epidermis is increased while that of Propionibacterium decreased relative to other genera (Streptococcus, Acinetobacter, Corynebacterium, and Prevotella). Furthermore, in this study, Bacillus and TEWL were significantly positively correlated in the ND group. Bacillus is a genus belonging to Firmicutes and is found on normal skin [35]. Further, increased colonization of Enterobacteriaceae is observed in adult and infant populations with AD [36, 37]. In fact, in the case of AD, TEWL is reduced, demonstrating a negative correlation between Enterobacteriaceae and TEWL.
In our previous study [17, 38], GOS were shown to inhibit the production of pro-inflammatory agents IL-6, IL-8, IL-12, tumor necrosis factor-α and prostaglandin E2 in UV-irradiated HaCaT cells and hairless mice. The increase in the population of lactic acid bacteria and inhibition of inflammation-producing substances by GOS may have reduced the population of S. aureus and production of pro-inflammatory substances. The application of cosmetics containing prebiotics from certain plant extracts is effective in inhibiting the growth of the inflammation-causing bacterium but not that of coagulase-negative Staphylococci [10]. The application of prebiotics is far superior to that of antimicrobial cosmetics, which use antibiotics or antibacterial agents to unselectively reduce bacterial growth [39]. Moreover, prebiotics such as GOS improve the host health by selectively stimulating the growth of beneficial bacterial species such as Bifidobacteria and Lactobacilli. Additionally, various lactic acid bacteria, including Lactobacillus, Lactococcus, Pediococcus, Propionibacterium, Leuconostoc, and Carnobacterium have been reported to use prebiotics for the production of bacteriocins [40, 41]. Oh et al. [41] reported a clinical test which revealed that bacteriocin HY 449 from Lactococcus spp. regulates skin inflammation and acne and inhibits the growth of inflammatory skin bacteria, such as S. aureus, Streptococcus pyogenes, and P. acnes. The production of bacteriocin, owing to GOS-induced lactic acid secretion, may also reduce the number of harmful microorganisms on the skin.