642 JOURNAL OF COSMETIC SCIENCE
needed to better understand the role of gut metabolites in AD, with future studies focusing
on optimizing the production of anti-inflammatory bacterial metabolites by promoting the
growth of beneficial microbes.36
THE GUT-SKIN AXIS AND ROSACEA
Rosacea is an inflammatory skin condition characterized by facial flushing, visible blood
vessels on the skin, acne-like eruptions and thickening of skin due to fibrosis. It is most
prevalent in white females.85 The skin microbiome in individuals with rosacea shows
increased levels of Serratia marcescens and C. acnes.86 Additionally, research has noted
a higher incidence of GI disorders among rosacea patients, suggesting a potential link
between gut microbial dysbiosis and rosacea.87 There are associations between rosacea
and various GI issues including Helicobacter pylori infection (HPI), small intestinal
bacterial overgrowth (SIBO), celiac disease, Crohn’s disease, irritable bowel syndrome
(IBS) and ulcerative colitis.87–90 Furthermore, rosacea patients’ gut microbiomes differed
in composition when compared to non-affected patients, exhibiting increased abundance
of Acidaminococcus, Megasphaera, Fusobacterium and Bacteroides and a lower abundance of
Peptococcaceae, Methanobrevibacter, Prevotella and Sutterella (Table III).85,91 These distinct traits
are hypothesized to be linked to elevated levels of certain enzymes involved in intestinal
cobalamin transport, sulphide metabolism and glucose transport.85,86 Additionally, various
dietary triggers have been associated with rosacea, including products related to heat,
alcohol, capsaicin and cinnamaldehyde, although further studies are required to clarify this
specific gut-skin axis of communication.87
CURRENT APPLICATIONS OF THE GUT-SKIN AXIS
Targeting the gut microbiome to improve skin health has become an increasingly popular
strategy in both research and clinical practice. This approach leverages the gut-skin axis
Table III
Comparison of Different Inflammatory Skin Conditions (Acne, AD, Rosacea) and Their Respective
Associations With Gut Microbiome Dysbiosis
Skin
condition
Symptoms Association to gut microbiome References
Acne Papules, pustules,
whiteheads,
blackheads, scarring
of the skin
Increased: Bacteroides
Decreased: Firmicutes, Proteus,
Clostridium, Bifidobacterium,
Butyricicoccus, Coprobacillus,
Lactobacillus, Allobaculum,
Lachnospiraceae, Ruminococcaceae
(25, 61, 67, 68, 70)
AD Itchiness, dry skin,
rashes, raw skin,
sensitive skin
Increased: F. prausnitzii, Clostridium,
Enterobacteriaceae, Staphylococcus, E. coli
Decreased: Akkermansia, Bacteroidetes,
Bifidobacterium
(25, 71, 72, 73, 74,
76, 79, 80, 81, 82)
Rosacea Facial redness, flushing,
visible blood vessels
on the skin, acne-like
eruptions, thickening
of skin due to fibrosis
Increased: Acidaminococcus, Megasphaera,
Fusobacterium, Bacteroides
Decreased: Peptococcaceae,
Methanobrevibacter, Prevotella, Sutterella
(85, 86, 87, 90, 91)

643 Bidirectional Gut-Skin Axis
to achieve systemic health benefits, including enhanced skin health, by modifying the
gut microbiome.92 The most notable therapeutic application of this strategy involves the
development of live biotherapeutic products (LBPs). LBPs, which can be either naturally
occurring or genetically modified, are oral or topical treatments containing live bacteria
or fungi designed to treat, prevent, or cure diseases.93,94 Because LBPs are therapeutic
in nature, they are subject to stringent regulatory and clinical standards. In contrast,
nutricosmetic products are non-living oral supplements intended for cosmetic benefits and,
as such, are generally subject to less rigorous regulation.95 Despite this, nutricosmetics have
the potential to offer immediate commercial opportunities for improving health and well-
being by leveraging their specific beneficial effects.
THERAPEUTIC APPLICATIONS
Various clinical trials are investigating microbiome engineering techniques to address
different health conditions. These include LBPs, fecal microbiome transplants (FMTs),
phage therapy, dietary interventions, personalized nutrition and synbiotics (a combination
of probiotics and prebiotics).96 In the realm of microbiome research, probiotics refer to live
microorganisms that benefit the host by enhancing immune function and outcompeting
harmful bacteria. Complementary to these, prebiotics are non-digestible compounds that
selectively promote the growth of beneficial microbes, contributing to improved host health.
Postbiotics, or paraprobiotics, consist of non-viable microbial cells and their metabolic
by-products, which have significant immunomodulatory effects. Lastly, synbiotics combine
both prebiotics and probiotics in a single supplement and function to support immune
health and restore balance to the microbiome.97
Unlike conventional probiotics, LBPs are specifically designed to treat certain diseases or
conditions and must undergo clinical trials to evaluate their safety and effectiveness.93,94
Therefore, in the context of skin conditions, LBPs offer an alternative to topical and
oral antibiotics by selectively targeting pathogenic microorganisms while preserving
beneficial microbiota, thereby aiding in the reestablishment of microbiome equilibrium.93
Consequently, research and clinical trials exploring LBPs have gained momentum in the
past decade.98
In the case of AD, numerous studies have explored the use of LBPs as an effective treatment.
These often include Lactobacillus-containing LBPs, specifically L. fermentum, L. paracasei, L.
plantarum, L. rhamnosus, L. salivarius, L. sakei and various mixtures thereof.99–102 However,
meta-analyses conclude inconsistent results, as many studies show that LBPs improve
Scoring Atopic Dermatitis (SCORAD), though not always to a clinically significant degree.
This variability highlights the need for further research.102 Most studies in this area have
focused on infants and children with AD. Some research has also targeted pregnant women
at high risk of having infants who develop AD for example, one study administered
L. rhamnosus strain GG (ATCC 53103) supplements daily from 4 weeks before delivery
until 6 months postpartum.103 In the intervention group, 26% of children developed AD
compared to 46% in the placebo group, demonstrating the treatment’s effectiveness.103
Additional research into adult-type AD exhibited that B. animalis subspecies lactis LKM512
was antipruritic (effective in alleviating itch) through the mechanism of increasing the
expression of the antipruritic and antinociceptive metabolite kynurenic acid (KYNA).84 An
additional study demonstrated that supplementation with a mixture of B. longum CECT