I always obtain a sprig of mistletoe at Christmas time. I don’t know why but we’ve been doing it for years. We hang it near the front door so that anyone tall gets hit on the head on entry (I’m short, or getting shorter as the years go on, so I sail gleefully under it). Why we kiss under the mistletoe or even associate mistletoe with Christmas is not quite clear either. It is however a parasite, a plant that relies on the tree it lives on to provide it with nutrients and water. As with all parasites, their success depends on not killing their host, so they form are sort of symbiotic relationship with the tree.
Human skin also has a symbiotic relationship with the many organisms that live on it. We are not sterile creatures and the more diverse our skin microbiome is, the more healthy our skin is too. Our microbiome is made up of bacteria, yeasts, fungi and viruses and these all live in symbiosis with each other to support the skin barrier and can help with immunological tolerance too.
In certain skin conditions, and atopic dermatitis is the most studied, in addition to the filaggrin deficiency, increase in pH of the skin and Th2 skewing of the immune response, there is microbial dysbiosis.
What is microbial dysbiosis?
In atopic dermatitis (AD), the skin is predominantly colonised with Staphylococcus aureus (Staph aureus) and other Staphylococcal species. The association of the abundance of Staph aureus with AD severity is well known.
[Hulpusch C, Rohayem R, Reiger M et al. Exploring the skin microbiome in atopic dermatitis pathogenesis and disease modification. J Allergy Clin Immunol 2024;154(1):31-41.]
How does S aureus do that?
Staph aureus makes toxins which disrupt the skin barrier, it can form biofilms that make it resistant to antimicrobial treatment, cause a pro-inflammatory response and downregulate filaggrin production.
So what can be done about microbial dysbiosis?
There is some evidence that some of the mainstays of eczema management ie emollients, dilute bleach baths, and phototherapy can improve skin microbial diversity.
[Seite S, Flores GE, Henley JB et al. Microbiome of affected and unaffected skin of patients with atopic dermatitis before and after emollient treatment. J Drugs Dermatol 2014;13:1365-72.]
[Khadka VD, Key FM, Romo-Gonazalez C et al. The skin microbiome of patients with atopic dermatitis normalizes gradually during treatment. Front Cell Infect Microbiol 2021;11:720674.]
[Lossius AH, Sundnes O, Ingham AC et al. Shifts in the skin microbiota after UVB treatment in adult atopic dermatitis. Dermatology 2022;238:109-20.]
What about prebiotics, probiotics and postbiotics?
A prebiotic is a non-digestible food ingredient that selectively promotes the growth of beneficial bacteria. It is most commonly used in the gut. These are mainly fructo and galacto-oligosaccharides which are metabolised in the gut to short-chain fatty acids (SCFAs). It is these SCFAs which are thought to have some skin benefits such as reduced transepidermal water loss and improved barrier function.
[Rios-Carlos M, Cervantes-Garcia D, Cordova-Davalos LE et al. Unraveling the gut-skin axis in atopic dermatitis : exploiting insights for therapeutic strategies. Gut Microbes 2024 ;16(1) :2430420.]
Topical prebiotics are still somewhat unexplored and remain controversial.
Postbiotics are preparations of inactivated micro-organisms and their components. They have also been mostly studied in the gut. Topical formulations containing Vitreoscilla filiformis combined with prebiotic ingredients have been shown to reduce AD severity.
[Gueniche A, Valois A, Kerob D et al. A combination of Vitreoscilla filiformis extract and Vichy volcanic mineralizing water strengthens the skin defenses and skin barrier. J Eur Acad Dermatol Venereol 2022;36(suppl 2):16-25.]
[Gueniche A, Knaudt B, Schuck E et al. Effects of nonpathogenic gram-negative bacterium Vitreoscilla filiformis lysate on atopic dermatitis: a prospective, randomized, double-blind, placebo-controlled clinical study. Br J Dermatol 2008;159(6):1357-63.]
What about probiotics?
These are living, non-pathogenic micro-organisms. The use of these is garnering attention and could be promising with the theory that these micro-organisms could reverse the microbial dysbiosis on atopic dermatitis skin.
Topical probiotics containing Roseomonas mucosa have been shown to reduce the burden of Staph aureus on the skin and have also shown a significant improvement in AD disease severity and reduced topical steroid use.
Staphylococcus hominis A9 (ShA9) has also been shown to inhibit Staph aureus growth while sparing normal skin commensals with subsequent improvements in eczema.
[Myles IA, Earland NJ, Anderson ED et al. First in-human topical microbiome transplantation with Roseomonas mucosa for atopic dermatitis. JCI Insight 2018;3:e120608.]
[Nakatsuji T, Hata TR, Tong Y et al. Development of a human skin commensal microbe for bacteriotherapy of atopic dermatitis and use in a phase 1 randomized clinical trial. Nat Med 2021;27(4):700-9.]
However, treatment with living bacteria is not without concerns so genetically engineered probiotics are now being looked at.
[van Hout, NE, Nevot G, Jansen PAM et al. From prebiotics to engineered microbes: microbe-inspired therapies for atopic dermatitis. Br J Dermatol Nov 2025; https://doi.org/10.1093/bjd/ljaf451]
However, many of these trials are still in the in vitro phase, with topical application of genetically engineered probiotics still being some way off. There are, understandably, concerns over safety, toxicity and biocontainment.
Kind regards,
Sandy
Dr Sandy Flann, Consultant Dermatologist.
