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    Topic review

    Human Skin Microbiome

    Subjects: Dermatology
    View times: 13
    (This entry belongs to Entry Collection "Skin Pathologies ")


    The skin is the common coat of the body and is the largest organ of the human organism. Its role is to provide an optimal environment for deeper tissues, by separating them from the external environment, and at the same time ensuring contact with it by exchanging substances and receiving stimuli.

    1. Introduction

    The skin is the largest organ of the human body, with an average surface area of 30 m2 in adults[1]. As the outer layer of the body—with a thickness of 2–3 mm—it functions as both a physical barrier, protecting its interior against the negative influence of various environmental factors, and an immunological barrier, reducing the effects of injuries and infections. In addition to its protective role, the skin is also responsible for thermoregulation processes, preventing water loss from the body, enabling temperature sensations, and supporting vitamin D synthesis [2][3][4][5]. One factor that determines good skin functioning, is a properly working skin microbiome. This complex set of microorganisms consists of bacteria, fungi, viruses, micro-eukaryotes (mites), archaea, and phages.
    The species composition, abundance, and distribution on the skin surface may vary depending on internal and external factors (Figure 1). An optimally shaped skin microbiome is the foundation of the skin’s immune system. In turn, disturbances of species composition, internal interactions, and the relationships between the microbiome and other areas of the body result in pathological changes that are not exclusively limited to the skin [5].

    Figure 1. The intrinsic and extrinsic factors that influence the skin microbiome.

    2. Microorganisms Inhabiting the Skin

    The average number of microorganisms isolated using traditional culture methods from the skin surface ranges from 103 to 104 CFU/cm2. However, in the most humid places, such as the groin, armpits, and nostrils, this value exceeds 106 CFU/cm2. The number of microorganisms inhabiting the scalp, especially from the forehead and around the ears and head, is about 106 CFU/cm2. On the other hand, on the upper back, chest and arms their number ranges from 104 to 105 CFU/cm2. The high number of microorganisms in hair and beards is not insignificant for the skin [3][6]. Most (>90%) of bacteria of the human skin microbiome are classified into four types: Actinobacteria (52%), Firmicutes (24%), Proteobacteria (16%), and Bacteroidetes (6%)[7]. Among them, coagulase-negative staphylococci, especially Staphylococcus epidermidis (103–104 CFU/cm2), anaerobic Cutibacterium acnes (formerly: Propionibacterium acnes), Corynebacterium, Micrococcus, Streptococcus, and Acinetobacter are the dominant species [3][6][7][8][9].
    The skin structure, which determines the composition of the skin microbiome, is an individual trait that depends, among others, on the age, sex, and health of the host. Hygiene habits have a big influence on the skin microbiome. Additionally, the host lifestyle and environment affect the number and composition of microorganisms that inhabit the skin. The microbial and fungal diversity specific for different human body sites is presented in Figure 2 and Table 1.
    Figure 2. Distribution of bacteria on skin sites.

    Table 1. Composition of the human skin micobiota in various locations.

    3. Interactions between Skin Microorganisms

    The dominant resident species of skin bacteria are commensals. Together with immune cells and keratinized skin cells (replaced every four weeks), these are responsible for the appropriate skin immune barrier functioning [3][10]. There are diverse mechanisms of skin immune system support that are associated with the activity of the microorganisms. While a properly functioning microbiome of healthy skin supports the body’s immune barrier, its transition into a dysbiosis state may lead to numerous systemic disorders. Dysbiosis, which is a disturbance of the structural and functional balance of the normal microbiome, is caused by internal and external stressors. Factors used in the fight against dysbiosis and helping to restore the balance of the skin microbiota include the use of probiotics and prebiotics. Dysbiosis alters the proportions of organisms in the healthy skin microbiome and may trigger the pathogenic potential of the commensals. Examples of diseases associated with the skin microbiome composition disturbance include acne, atopic dermatitis (AD), and dandruff [11].

    4. Skin Disinfection and Its Influence on the Microbiome Condition

    In the COVID-19 pandemic (winter 2020–2021), skin disinfection, especially hand sanitation, is of great importance. Its effect depends on many environmental factors, e.g., temperature, humidity, as well as the type, concentration, and exposure time of the selected aseptic agent. The specificity and the number of microorganisms that inhabit the disinfected region, skin pH, humidity, and structure, the thickness of glands on the skin, and their secretions are also essential for successful disinfection processes [12].

    5. Summary

    The skin microbiome plays a relevant role in maintaining the proper functioning of the human organism. Currently, there is growing interest in skin microbiome research. Researchers have been concentrating on the development of more accurate methods of microbial testing. Such techniques enable the identification of new microorganisms that inhabit both the surface and deeper layers of the skin. Studies of the influence of environmental factors on the skin microbiome also represent a popular research direction. Of great interest, is the role of the skin microbiome in the regulation of host immune mechanisms.
    Studies on the skin microbiome allow for a better understanding of the interactions between microorganisms and the impact of the external environment on the microbiome balance. In turn, functional studies allow for the development and optimization of therapeutic methods that enable the inhibition of skin pathogen growth.

    The entry is from 10.3390/microorganisms9030543


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