Mikrobiom

Gloves, masks, helmets, shoes - all have a common denominator, they are meant to protect our skin from damage. Similarly, our body - forms a barrier to be responsible for our comfort and safety - this is the skin microbiome. As in real life, sometimes over-protection can be harmful, so in order to keep our skin in good condition we need to take care of the correct balance of bacteria that live on it. The term 'microbiome' was first used by Joshua Lederberg, - an American geneticist and microbiologist, Nobel Prize winner, who introduced a single common term for the collection of genomes of all commensal, symbiotic and pathogenic microorganisms that reside in the human body. However, the real beginnings of defining the influence of microorganisms on the human body themselves date much earlier, to the early 19th century. Today, everyone who is interested in taking care of their own skin has heard of the microbiome. But do we know how to do it and, above all, why?

 

The skin - a space where the microbiome is formed

The structure of the skin resembles a three-tier iceberg. At the very top is the epidermis, below it is the dermis and at the very bottom is the subcutaneous tissue. The epidermis is the most visible, top layer of the skin, 0.05 to 0.1 mm thick. It is the area most readily colonised by bacteria. Within the epidermis itself, 4 layers can be distinguished: basal, spinous, granular and stratum corneum. In the epidermis, a process called keratinisation takes place, which involves the migration and transformation of skin cells, starting from the basal layer to the outermost layer - the stratum corneum. As a result, dead cells are pushed to the surface of the skin, requiring exfoliation. The dermis is the layer that supports the epidermis. It measures between 2 and 3 mm. It constitutes the extracellular matrix, which is made up of numerous collagen and elastin fibres and a basal substance rich in hyaluronic acid. This is where the sebaceous glands, hair, sweat glands, blood and lymphatic vessels and numerous sensory nerves are located. The subcutaneous tissue is the thickest layer of the skin, measuring from a few millimetres to a few centimetres. It is connected to the dermis by elastin and collagen fibres. It is mainly made up of adipocytes (fat cells) and connective tissue. Blood and lymphatic vessels pass through it on their way to the dermis.

Skin microbiome

The skin is an organ that is in constant contact with both the external and internal environment. It is the human body's interface with the external environment. It is a barrier that separates the inside of the body from the outside environment, protecting it from potential attack by foreign organisms or toxic substances. The skin is colonised by a diverse collection of micro-organisms - bacteria, fungi, viruses and mites - referred to as the skin microbiome. Most of the micro-organisms that inhabit the skin are not harmful and function in symbiosis with the skin cells.

Skin environment

The physical and chemical characteristics of the skin are the main factors that determine the organisms that inhabit it. Generally speaking, the skin is an inhospitable environment for microbial growth, as it is cool, acidic, dry and tends to scrub constantly. For this reason, micro-organisms are regularly removed from its surface, limiting their opportunities for growth and biofilm formation. In addition, microbial growth is inhibited by the presence of the hydrolipidic mantle, which acidifies the pH and contains compounds that have an antimicrobial effect, such as dermicidin, lysozyme and sebum. Antimicrobial agents are also secreted by keratinocytes, sebocytes, mast cells and sweat gland cells present in the skin. More than 20 peptides with such effects have been identified on the skin surface, including cathelicidin (LL-37), defensins: defensin β1 (HBD1), HBD2, HBD3, psoriasins, antimicrobial protein RNase 7 (SAP-2) and SLPI protein. Due to these living conditions, the skin can only be colonised by specific microbial species. Moreover, these conditions have a significant impact on the number of microorganisms. Consequently, the skin microbiome is resistant to any changes and is characterised by stability in terms of composition and abundance.

Composition of the skin microbiome

Actinobacteria (36-51%), Firmicutes (24-34%), Proteobacteria (11-16%), Bacteroidetes (6-9%) are the four main types of bacteria found on the skin. Results from human skin samples showed that gram-positive bacteria belonging to the genera Staphylococcus spp., Corynebacterium spp., Enhydrobacter spp., Micrococcus spp., Cutibacterium spp, and Veillonella spp. predominate. In moist sites, the most abundant bacteria are Staphylococcus (Firmicutes) and Corynebacterium (Actinobacteria). Fat-covered sites represent the least diverse population, with Cutibacterium species (Actinobacteria) being the most frequent isolates. Fungal species include Malassezia spp., Cryptococcus spp., Rhodotorula spp., Aspergillus spp., and Epicoccum spp. Malassezia spp. - account for approximately 80% of the total fungal flora. In addition to bacteria and fungi, the skin microbiota also includes nematodes (Demodex spp.), which are small mites living in hair follicles and sebaceous glands. The last group of the skin microbiota are viruses. They are the least studied part of it. It is believed that viruses are not only pathogenic agents, but also factors contributing to the maintenance of skin homoeostasis. Viruses have been shown to be an unstable but important component of the skin microbiome, especially viruses containing double-stranded DNA (dsDNA), such as Polyomaviridae and Papillomaviridae.

 

The role of the skin microbiome

The skin microbiome plays an important role in maintaining skin homeostasis, protection against invading pathogens and regulation of the immune system. The secretion of protease enzymes by the skin microbiome is associated with the exfoliation and renewal of the stratum corneum, while the secretion of lipase enzymes is associated with the breakdown of the surface lipid layer. In addition, urease enzymes are involved in urea degradation. Other functions of the microbiota include the production of biofilm or bacteriocins (antimicrobial agents). Furthermore, the skin microbiota plays an important role in protection against potential pathogenic microorganisms, thanks to the phenomenon of competition (a healthy microbiome colonises the skin and thus occupies niches that could be an environment for pathogen growth) and the production of antimicrobial peptides (AMPs). The effect of the skin microbiome on skin immune function is poorly understood. Nevertheless, it is known that bacteria residing in the skin have an important impact on maintaining immune system homoeostasis. Additionally, they modulate the innate immune response and influence the development of the acquired immune response.

Normal skin microbiome versus dysbiosis

A normal, healthy skin microbiome is characterised by high species diversity and a balance in the quantitative ratios between the different microorganisms. Dysbiosis refers to an imbalance in the skin microbiome, which can lead to the onset or development of diseases such as acne vulgaris, rosacea, psoriasis, seborrhoeic and atopic dermatitis or even alopecia areata. The state of the skin microbiome is influenced by many factors. Colonisation of the microbiota begins at birth and its composition is influenced by the mode of delivery. The composition is then determined by several internal (ethnicity, gender and age) and external factors (lifestyle, hygiene habits, cosmetics used, antibiotics taken, geographical location, climate and season). Differences are observed in the skin microbiome of people living in different environments (rural or urban): the skin microflora of people living in industrialised countries is much less diverse. Antibiotics are also important factors that negatively affect the diversity of microorganisms on the skin surface, as they can alter the composition of the bacteria populating the skin, including the commensal bacteria Staphylococcus spp. known to compete for colonisation with the pathogenic Staphylococcus aureus. The state of the skin microbiota is also affected by hygienic and cosmetic procedures carried out with chemicals that remove the skin microflora, and the resulting changes in the physicochemical conditions on the skin surface hinder recolonisation of the commensal microflora, while creating favourable conditions for skin colonisation by pathogens.

How to take care of the microbiome?

The skin microbiome is a very diverse environment, influenced by many factors. The right products should be chosen based on an individual assessment of our skin condition. However, there are some universal characteristics that can guide us. In particular, we should opt for products that are gentle, that do not disturb the skin's natural pH and that take care of its hydration. There is a growing trend among cosmetic manufacturers to provide information on the product about its effect on the skin microbiome. At MPR Labs, we offer a service that is able to confirm such a declaration. This is made possible by performing the relevant microbiological analyses, during which participants apply the cosmetic in question for four weeks without changing their diet, lifestyle or habits. By comparing the skin microflora before and after applying the product for one month, it is possible to determine how the product has affected the microbiome of our skin. Products recommended as having a positive impact on the skin microbiome can provide real support for restoration and protection of the skin.

Literature:

1. Geloen Alain, Raillan Alexandra, translation: Alexandra Veksej, The Skin Microbiome. A guide to the world of natural and sustainable skin care, wyd. Koncept sign, 2021
2. Manon Boxberger, Valérie Cenizo, Nadim Cassir & Bernard La Scola, Challenges in exploring and manipulating the human skin microbiome, Microbiome volume 9, Article number: 125 (2021)
3. Silvia Carmona-Cruz, Luz Orozco-Covarrubias and Marimar Sáez-de-Ocariz, The Human Skin Microbiome in Selected Cutaneous Diseases, Cell. Infect. Microbiol., 07 March 2022, Sec. Microbiome in Health and Disease
4. Katarzyna Adamczyk, Agnieszka A. Garncarczyk, Paweł P. Antończak, The microbiome of the skin/ The microbiome of the skin, Dermatol Rev/Dermatol Division 2018, 105, 285-297
5. Magdalena Malinowska, Beata Tokarz-Deptuła, Wiesław Deptuła "The human microbiome" - Department of Immunology, Faculty of Biology, University of Szczecin, Department of Microbiology, Faculty of Biology, University of Szczecin 33 - 36

Authors:

• Monika Rek

MA in chemistry, co-founder and safety assessor at MPR Labs. She has several years of experience in the beauty industry, in the field of safety assessment of cosmetic products. She assists in the implementation of cosmetics in the European market.

• Łukasz Miernik, Co-Founder, MPR Labs - Testing and Registration of Cosmetics

Co-founder of MPR - Labs. Professionally, I am involved in bringing new cosmetic products to market by providing our clients with the necessary studies and safety assessment reports. Privately and from my acquired education, I look broadly at the economic aspects of the world around us.

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