A vast number of microorganisms inhabit the human body, outnumbering cells by 10-to-1, with skin being one of the largest habitats.1–4 However, science is just beginning to explain the complex relationships between microbes and the human body, in addition to cosmetic and drug products applied to skin. Until recently, methods used to identify and study the skin’s microbiota relied on the ability to culture individual species in the laboratory. Now, sequence-based metagenomic techniques allow for the analysis of entire environmental niches and have demonstrated that previous culture techniques substantially under- estimated the skin’s microbial population and diversity.5 Using these technological advances, the Human Microbiome Project has already begun to dramatically change the understanding of skin’s microbial ecology.6 Goals of the present project aim to identify the role of this microbiome not only in disease, but also in the maintenance of health.3, 7
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A vast number of microorganisms inhabit the human body, outnumbering cells by 10-to-1, with skin being one of the largest habitats.1–4 However, science is just beginning to explain the complex relationships between microbes and the human body, in addition to cosmetic and drug products applied to skin. Until recently, methods used to identify and study the skin’s microbiota relied on the ability to culture individual species in the laboratory. Now, sequence-based metagenomic techniques allow for the analysis of entire environmental niches and have demonstrated that previous culture techniques substantially under- estimated the skin’s microbial population and diversity.5 Using these technological advances, the Human Microbiome Project has already begun to dramatically change the understanding of skin’s microbial ecology.6 Goals of the present project aim to identify the role of this microbiome not only in disease, but also in the maintenance of health.3, 7
As science continues to elucidate the nature of interactions with microbiota, the question arises as to whether it may be possible to selectively harness the benefits of some organisms while protecting against the potential dangers of others.3, 8–10 Specifically, understanding the relationship between the skin and its microbial inhabitants presents an interesting approach to cosmetic formulation for maintaining or improving skin health.
Normal Human Skin Microflora
The establishment of one’s normal skin flora typically occurs during delivery as a newborn.11 It consists of several major groups of bacteria, comprised of staphylococci, coryneform bacteria, micrococci and Gram-negative bacilli, in addition to the Malassezia genus of yeasts.12 Populations vary by body site and factors such as skin hydration and availability of nutrients.13 Clinical data on the number and types of organisms inhabiting the skin from culture-based techniques is abundant, but there is little understanding of the mutualistic and commensal relationships of these organisms with their habitat.10
Microbiome Contribution to Health
The skin performs many important functions, including the prevention of water loss, regulation of body temperature, protection from environmental insults and immunological functions.14, 15
The skin’s microbiome contributes to this protective barrier through a variety of mechanisms, such as excluding transient and pathogenic bacteria by competing for nutrients and habitat.10, 16, 17
The normal microbiome also may protect and contribute to skin health through sophisticated mechanisms like educating the immune system to recognize commensal microflora and secreting antimicrobial compounds to prevent colonization by pathogens.17–20
Cells in epithelium contain pattern recognition proteins called Toll-like receptors (TLRs) that are able to distinguish bacterial cell wall components like lipopolysaccharides in the outer membrane of Gram-negative bacteria, and lipoteichoic acid components in the cell wall of Gram-positive bacteria.21 Engagement with TLRs stimulates the release of nuclear factor kB (NF-kB), a transcription factor that regulates genes responsible for innate and adaptive immune response. NF-kB proteins exist in cellular cytoplasm in an inactive state due to association with inhibitors of kB (IkBs). When IkB is degraded, NF-kB is free to travel into the nucleus and up-regulate genes that trigger inflammatory or immune responses. 21 Interestingly, a commensal-derived intestinal probiotic, Bifidobacterium animalis strain BB12, has demonstrated inhibition of immune responses in cultured colon cells by interfering with IkB degradation.22 Additionally, it has been proposed that low expression of bacterial-specific TLRs (TLR2, TLR4 and TLR5) in Langerhans cells, the specialized immune cells of the skin, contribute to tolerance of commensal bacteria.23, 24
Commensal and probiotic bacteria also prevent colonization of pathogens by creating unfavorable conditions and by producing antimicrobial substances. Lactic acid bacteria and propionic acid bacteria secrete organic acids creating a low-pH environment that discourages growth of other species.17, 19, 25–27 Staphylococcus epidermidis can also stimulate keratinocytes to produce antimicrobial compounds like β-defensins.20, 25, 28 Probiotic bacteria from the Lactobacillus species secrete their own antimicrobial compounds, referred to as bacteriocins, which may also contribute to host benefits.27
Pre- and Probiotics
The goal of pre- and probiotic treatments is to re-balance commensal microbiota to promote the health of the host. In the early 1900s, the work of Nobel Prize-winning immunologist Elie Metchnikoff suggested that consumption of fermented foods led to longevity of human life but it was not until the 1950s that the term probiotic became more widely used in medical and scientific publications.18
Probiotics are defined as viable microorganisms that beneficially affect the flora of the human gut. However, the most recent definition of probiotics sanctioned by the World Health Organization and the Food and Agriculture Organization of the United Nations has been expanded to include applications beyond intestinal health. It now states that probiotics are live microbes that, when administered in adequate amounts, confer a health benefit on the host.29
Prebiotics are ingredients that selectively stimulate the growth and/or activity of beneficial organisms, and synbiotics are synergistic combinations of prebiotics and probiotics.18, 29 Dietary fibers and oligosaccharides are commonly used as prebiotics because they remain intact in the digestive tract and are selectively fermented by beneficial gut bacteria.29 In reference to skin treatment, it may be reasonable to expand the role of prebiotics to ingredients that influence environmental factors like pH to promote the growth of beneficial bacteria.30 The use of oral probiotics has been linked to improvements in atopic dermatitis in infants, and protection from UVB radiation in murine models due to activation of the immune system. However, at present, there are only a limited number of studies on topically applied pre- and probiotics.31–39
Considerations for Cosmetic Formulation
The application of cosmetics may directly influence the ecological balance of skin microbiota by introducing inhibiting factors such as antimicrobials and preservatives. These may act indiscriminately and produce changes in environmental conditions, i.e., pH and available water, or shift the nutrient balance. Short-term effects may be minimal but the long-term effects of consistent and repeated application are unknown,13 and concerns about antimicrobial tolerance and resistance continue to pervade literature and the media.40–42 Together, these elements make pre- and probiotic strategies to maintain healthy skin microbiota an interesting approach to treating skin.13, 33, 43
Probiotic foods are made either by the fermentation of raw ingredients or the addition of probiotic strains to finished products. The standard concentration range of viable probiotics to claim health benefits in functional foods is generally recognized as 106–108 CFU/g.29, 44, 45 Functional foods and supplements containing probiotics must stand up to manufacturing conditions and the harsh environment of the digestive system. They must also be shelf stable, often requiring refrigeration.29 For these reasons, the use of live bacteria in cosmetics may pose a major technical hurdle; however, increasing evidence has shown that cell-wall fragments and dead bacteria, referred to as probioactives, can also elicit an immune response.20, 27, 44, 46–48 Due to complications with formula preservation and an incomplete understanding of potential for pathogenic behavior, bacterial lysates and prebiotics may thus be more viable alternatives to live cultures for cosmetic preparations.
Potential Application: Acne
The role of Propionibacterium acnes is much debated but it is believed that hyperproliferation of this bacterium in comedones exacerbates inflammation in acne. This is due to the production of free fatty acids that irritate the follicular wall, and bacterial cell wall components including lipoteichoic acid that provoke immune response.10, 16, 17 49
Current metagenomic research has indicated P. acnes is the primary resident of sebaceous follicles, and that S. epidermidis is a secondary follicular colonizer in acne patients; the influence of other or unculturable organisms appears to be unlikely.49 Current topical and oral antibiotic treatment modalities for acne are often successful in reducing the number of P. acnes but can promote antibacterial resistance and lead to other undesirable consequences, such as photosensitivity.49, 50 Actives for treating acne also can be irritating and drying to the skin. Therefore a prebiotic strategy that reduces the hyperproliferation of P. acnes, which exacerbates acne, may prove to be a milder, more preferable option.16
Black currant and pine: Bockmuhl et al. investigated a prebiotic approach to acne treatment by first screening plant extracts and combinations of extracts for the ability to stimulate growth of S. epidermidis and moderate P. acnes, both members of the commensal microbiota. Screening was conducted by optical density using a spectrophotometric turbidity measurement to estimate bacterial counts in culture. Of the extracts tested, a synergistic combination of black currant and pine was selected for further study in vivo because it showed the highest inhibition level of P. acnes and stimulated S. epidermidis (see Figure 1).33 It should be noted that a 1-log change (tenfold) is required to demonstrate reduction or increase in the field of microbiology; the measurements shown here only indicate a directional change.
A subsequent in vivo study enlisted 11 female subjects between the ages of 22 and 34 to use a regimen of prebiotic products twice daily during a period of three weeks. Total bacterial counts were taken from samples obtained by cup scrub method and staining with 4'6-diamidino-2-phenylindole, which binds to cellular DNA and fluoresces under ultraviolet light. P. acnes counts were obtained with targeted oligonucleotide DNA probes and compared to total bacterial counts. The regimen included a gel cleanser, toner and fluid moisturizer containing 0.5% of the extract combination. Bacterial sampling occurred at baseline, day 7 and day 21.
The ratio of P. acnes to total bacteria was found to decrease in eight subjects at day 7 and 10 subjects by day 21. Although the small size of the study restricts statistical significance in these findings, the results indicate a trend of P. acnes balancing with respect to total microbial colonization. The data also demonstrates the selective activity of the extract, a key aspect for prebiotic treatments, and that multiple applications over an extended period may be needed to demonstrate appreciable differences in bacterial counts. A large scale, placebo-controlled study would be necessary to demonstrate further efficacy of the prebiotic activity of a black currant and pine blend of extracts for amelioration of the P. acnes imbalance associated with acne.33
Lactic acid bacteria and Japanese konjac: Ghazzewi and Tester evaluated the ability of a synbiotic combination of lactic acid bacteria (LAB) and konjac glucomannan hydrosylates (KGH) to inhibit growth of P. acnes in vitro.36 LAB are commonly used as functional food ingredients in dairy products and have received formal generally recognized as safe (GRAS) status from the U.S. Food and Drug Administration (FDA) for food application.29 In addition to competition for adhesion, LAB have also been shown to inhibit the growth of pathogens by lowering pH and producing lactic acid and antimicrobial peptides.51, 52 Multiple strains of LAB, i.e., Lactobacillus casei, L. acidophilus, L. plantarum, L. gasseri and L. lactis, were assessed for potential synergistic activity with KGH.
Glucomannan is a water-soluble, high molecular weight polysaccharide consisting of glucose and mannose sugars in a ratio of 1-to-6 and derived from the roots of the the Japanese konjac plant.53 Glucomannans are a substrate for LAB and have been shown to selectively stimulate their growth by acting as a nutrient source, making the material an ideal ingredient to create a synbiotic preparation with LAB.34 The efficacy of LAB and the LAB/KGH combination were assessed by measuring the zone of inhibition around 8 mm wells of test solutions in nutrient agar plates. All strains of LAB showed inhibition of P. acnes and the addition of KGH enhanced performance. It appears that LAB may have the potential to moderate P. acnes and be used to treat acne, but more definitive data must be generated; the authors of this study indicated plans for further in vivo studies to validate efficacy.36
Potential Application: Sensitive Skin
Reactive or sensitive skin is a subjective hyperreactivity to physical or chemical factors. Itching, burning, stinging and a sensation of tightness are all common complaints associated with sensitive skin.54, 55 Increased permeability of the stratum corneum and heightened levels of the neurotransmitter Substance P, associated with pain and itching, have been implicated in the condition.56
Bifidobacterium longum lysate: Guéniche et al. demonstrated that a Bifidobacterium longum lysate applied at a concentration of 10% decreased various markers of immune response including vasodilatation, edema, mast cell degranulation and TNF-α in skin explants, compared with a control. These results led the group to further investigate B. longum lysate in vivo. Sixty-six subjects were randomly assigned to receive either a control cream or cream containing 10% B. longum lysate to apply during the course of two months. The skin of patients using the B. longum lysate cream showed reductions in skin sensitivity in lactic acid sting tests at the two-month examination (see Figure 2). The B. longum lysate cream also enhanced the skin barrier, as evidenced by an increase in the number of tape strippings needed to produce a transepidermal water loss greater than 15 g/cm2/hr after two months of treatment (see Figure 3).54
Conclusions
It is clear that the relationship between skin and normal skin microbiota is quite complex, and that knowledge about the composition and diversity of their habitats is growing with advances in analytical techniques.3–5, 7, 8, 57–62 New information about the role of the skin microbiome in disease and health will likely provide additional direction for cosmetic and topical drug development. Cosmetic and topical drugs have the ability to influence the skin microbiota, but to a large extent these treatments have not yet taken into account the role of the normal microbiota. In contrast, pre-, pro- and synbiotic strategies that selectively balance the skin’s microbiota present a novel approach to treating and maintaining skin health.10, 16, 20
This is a relatively new area of research in the cosmetic industry, with a select few publications on topical treatments to improve skin health appearing in the past several years. Studies have demonstrated it may be possible to reduce P. acnes, levels that may ameliorate acne.20, 33, 36 Additionally, treatment of sensitive skin with a bacterial lysate showed some improvements in skin barrier and sensitivity.54 These approaches may prove to be gentler, safer alternatives to treatments that can be irritating and have the potential for antimicrobial resistance, among other side effects.
The implication of microbiota in a multitude of other skin disorders may also lead to more research opportunities for cosmetic treatments. For example, psoriatic lesions also show significant alternations in microbial community, specifically increased Streptococcus and decreased P. acnes, compared with unaffected skin of the same subjects and that of subjects with normal skin.43, 63 Another organism, M. furfur, is associated with several chronic skin disorders including dandruff.64 However, there is still much to learn about the ways in which microbes play a role in disease and defense of human health so that the industry can more accurately understand the consequences of formulating topical cosmetics and drugs with pre- and probiotics.
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