Rosacea: A potential link between gut and skin microbiota raised?

The term "microbiota" refers to the collection of microorganisms living in a specific environment. This includes viruses, bacteria, and yeasts. There are several types of microbiota in the human body, but the gut microbiota is the most populated among them. Indeed, there are 10 times more bacteria in the gut than cells making up the human body and 100 times more bacterial genes in the gut than genes in the human genome. The gut microbiota is a complex ecosystem. Its diversity is generally correlated with human health: the richer it is, the better the individual's health will be. Various factors have been identified as potentially influencing the composition of the gut microbiota.

• The mode of birth.

  The way a child is born partially influences their gut microbiota. Indeed, studies have shown that a child born vaginally will have a gut microbiota similar to that of their mother's vagina, primarily composed of bacteria such as the Lactobacillus and the Prevotella. Conversely, a child born by caesarean section will have a gut microbiota similar to their mother's skin microbiota, with more bacteria of the type Streptococcus, Corynebacterium and Propionibacterium.

• The intake of antibiotics.

  The intake of antibiotics by the mother during pregnancy influences the composition of her child's microbiota. Indeed, by definition, antibiotics are molecules that target microorganisms, either killing them or inhibiting their growth. By modulating the mother's microbiota, antibiotics also have an effect on the child's intestinal microbiota.

• Infant nutrition.

  Various studies have highlighted that there is a relationship between the diet of a newborn and their gut microbiota. Indeed, babies who are breastfed will have a lower concentration of bacteria of the type Gammaproteobacteria compared to those fed with formula milk. On the other hand, their microbiota will have a higher presence of bacteria of the type Bifidobacteria.

The skin is one of the primary interfaces between our internal and external environments and, as such, is exposed to various assaults and changes to which it must adapt. It thus possesses a barrier function that is crucial, supported not only by its structure and composition, but also by the numerous micro-organisms it harbours, constituting its natural microbiota.

The gut microbiota can impact the composition of this skin microbiota. Indeed, the bacteria present in the gut produce various types of short-chain fatty acids, or SCFAs, which then influence the microorganisms of the skin. One can particularly take the example of Propionibacterium, an intestinal bacterium that synthesises acetate and propionate. The latter has a antimicrobial effect against certain skin pathogens, such as Staphylococcus aureus, even though it has no effect on Cutibacterium acnes, a bacterium involved in acne.

Rosacea is a chronic skin disease causing redness, small pimples as well as tingling and itching. Although initially benign, it can have a significant impact on the quality of life of those who suffer from it, due to the physical and aesthetic discomfort it causes. Several factors have been identified as potentially triggering rosacea flare-ups , among which are diet, stress, and ambient temperature...

Some studies also suggest that there is a link between an imbalance of the gut or skin microbiota, also known as dysbiosis, and the development of rosacea. Indeed, it has been observed that the TLR-2 receptor of the epidermis, particularly activated by the mites Demodex, was upregulated in patients with rosacea. This activation by parasites triggers inflammatory mechanisms, including the synthesis of inflammatory prostaglandins IL-1β and IL-8. Furthermore, the chitin from the exoskeleton of the Demodex can also stimulate the pro-inflammatory response via the TLR-2.

Furthermore, a metagenomic study conducted on 12 patients suffering from rosacea and 251 individuals not affected by this condition showed that the gut bacteria Peptococcaceae and Methanobrevibacter were only present in individuals suffering from rosacea whereas the bacteria Acidaminococcus and Megasphaera were significantly more abundant in patients with rosacea. Conversely, another study carried out on 11 patients and 110 controls observed a lower proportion of Acidaminococcus and Megasphaera in individuals affected by rosacea, contradicting the previous results.

Thus, the role of these various microorganisms in the pathophysiology of the disease remains to be precisely determined as the studies contradict each other. Moreover, at present, the scientific community does not know whether alterations in the skin and gut microbiota are potentiators of inflammation or secondary effects in response to changes in the skin or intestinal flora. That's why, although a link between the gut and skin microbiota and the development of rosacea has been observed, further studies are still needed to precisely determine the mechanisms at work.

• KIM H. S. Microbiota in Rosacea. American Journal of Clinical Dermatology (2020).

• Thesis by Jérôme GASPÉRINI. The interrelation between the gut microbiota and the skin microbiota: the balance or imbalance of these ecosystems and their impact on skin pathophysiology (2022).

• LOPEZ V. N. & co. Rosacea, Microbiome and Probiotics: The Gut-Skin Axis. Frontiers in Microbiology (2024).