Introduction:
The human skin, the body's largest organ, is home to many microorganisms. Most of these microorganisms are harmless or even beneficial to us. Factors like the location, internal factors, and the external environment influence the colonization of microorganisms on the skin. The immune system plays a role in shaping the skin microbiota, while the microbiota also helps educate the immune system. Advanced techniques for identifying microorganisms have revealed that the bacteria on our skin are highly diverse and vary from person to person. Gaining a deeper understanding of the skin microbiome is essential for comprehending its connection to skin disorders and advancing the development of innovative treatments that promote or combat microorganisms.
What Is a Skin Microbiome?
The skin microbiome refers to the complex community of microorganisms that inhabit the surface of our skin. It consists of a diverse array of bacteria, fungi, viruses, and mites that coexist harmoniously, forming a delicate balance crucial for maintaining skin health. The skin is considered the largest organ of the human body that serves as a protective barrier against external threats such as pathogens, ultraviolet (UV) radiation, and environmental pollutants. The skin microbiome plays a vital role in supporting this barrier function and contributes to the skin's overall health.
What Is the Composition of the Skin Microbiome?
Skin appendages, including hair follicles, sebaceous glands, and sweat glands, are present on the skin surface. The skin surface is populated by microorganisms, including viruses, bacteria, fungi, and mites found in hair follicles and glands. Bacterial communities on the skin consist of rod-shaped and round-shaped bacteria, such as Proteobacteria and Staphylococcus species, respectively. These bacteria form complex relationships with other microorganisms, creating intertwined communities. Some microorganisms present in the skin microbiome are listed below:
- In healthy individuals, the bacterial species on the skin consist of gram-positive bacteria from genera like Staphylococcus, Corynebacterium, Enhydrobacter, Micrococcus, and Cutibacterium.
- Transient fractions of gram-negative bacteria such as Roseomonas mucosa, Pseudomonas species, Acinetobacter species, Enterobacteriaceae, and certain anaerobes can also be found on the skin.
- Propionibacteria species and Staphylococci species dominate in sebum-rich areas, while Corynebacteria are more prevalent in moist regions. Some archaeobacteria are also present.
- Various fungal species, including Malassezia, Cryptococcus, Rhodotorula, and Candida, have been identified on the skin. Malassezia species can grow in branching filamentous hyphae or as individual cells. They are particularly abundant on the chest and arms, while the foot harbors a greater diversity of fungi.
- Skin mites, such as Demodex folliculorum and Demodex brevis, are among the smallest arthropods that reside in sebum-rich areas such as the face, eyelids, hair follicles, or nearby areas.
- Virus particles can be found both free and within bacterial cells on the skin. Bacteriophages are the predominant type of virus found on the skin, potentially regulating bacterial populations.
What Are the Factors Contributing to Variations in Skin Microbiome?
Some key factors that affect the skin microbiome include:
- Moisture: Moisture levels play a crucial role in shaping the skin microbiome. Microbes thrive in moist environments, and different skin regions have varying moisture levels. Due to the higher humidity levels in the groin and armpit regions, certain bacteria and fungi can thrive there.
- pH Levels: The skin's pH level, which measures acidity or alkalinity, significantly affects the types of microorganisms that can thrive on the skin. The skin's surface is naturally slightly acidic, with a pH ranging from 4 to 6. This acidity helps inhibit the growth of harmful bacteria. Sweating, sebum production, and using alkaline soaps or cleansers may all modify the pH of the skin, which can impact the skin microbiome.
- Sebum Production: Sebum (an oily substance secreted by sebaceous glands) contributes to the skin's hydration and acts as a protective barrier. Sebum production varies across individuals and body regions. Higher sebum production can create an environment suitable for the growth of certain bacteria, such as Propionibacterium acnes, associated with acne.
- Environmental Exposures: Exposures to different environments and external factors can impact the skin microbiome. For example, exposure to ultraviolet (UV) radiation from sunlight can affect the composition and diversity of the microbiome. Environmental pollutants, such as air pollution and chemicals, can also influence the skin microbiome and its balance.
- Personal Hygiene Practices: Personal hygiene practices, including the use of cleansing products, soaps, and cosmetics, can have a significant impact on the skin microbiome. Harsh soaps or cleansers with high surfactant levels can disrupt the microbiome's natural balance, while excessive use of antimicrobial products can eliminate harmful and beneficial microorganisms.
- Climate and Temperature: Different climates and temperature levels can affect the skin microbiome. Regions with higher humidity or heat provide favorable conditions for the growth of certain microorganisms. In contrast, colder and drier climates may lead to a decrease in microbial diversity.
- Genetics: Genetic factors influence an individual's susceptibility to certain skin conditions and may also shape the skin microbiome. Genetic variations can affect factors like sebum production, skin pH, and immune responses, all of which impact the skin microbiome's composition.
- Age and Hormonal Changes: The skin microbiome evolves throughout a person's life. In infants, the microbiome is initially influenced by the mode of delivery (vaginal or cesarean) and breastfeeding. Hormonal changes during puberty and menopause can also influence the skin microbiome, contributing to skin conditions such as acne.
What Are the Skin Conditions Associated With the Skin Microbiome?
Microbiota-Associated Skin Disorders -
- Seborrheic Dermatitis: A hyperproliferative, pruritic skin disorder primarily affecting the scalp. The fungal component (particularly Malassezia species) is thought to contribute to its pathogenesis. Antifungal treatments targeting these fungal species show improvements, while antibacterial agents do not. Malassezia lipase genes are implicated in promoting hyperproliferation and inflammation.
- Acne: An inflammatory disorder of the pilosebaceous unit associated with the skin bacterium P. acnes. Puberty increases the prevalence of lipophilic microorganisms, including P. acnes. P. acnes secretes enzymes that damage the tissue lining of the pilosebaceous unit. Activation of immune responses and production of pro-inflammatory factors contribute to acne development.
- Atopic Dermatitis (AD): It is a chronic relapsing disorder affecting both children and adults, associated with microbial colonization. Atopic dermatitis manifests at specific sites with similar microbial profiles compared to other body sites. Staphylococcus aureus is highly prevalent in AD patients on both lesional and non-lesional skin. Topical or systemic antibiotics, steroids, and treatments to reduce bacterial load are common therapeutic approaches.
Disorder With an Unidentified Microbial Component -
- Chronic Wounds: Chronic wounds in diabetic, elderly, and immobile individuals are susceptible to invasion by commensal skin organisms. Chronic wounds exhibit greater microbial diversity when analyzed using molecular methods. No unique organism has been identified to colonize wounds of the same etiology. Burn wounds commonly become infected with specific bacteria, fungi, and viruses.
- Animal models have helped identify longitudinal selective shifts in the microbiota of slow-healing diabetic mouse wounds.
An Invasive Skin Commensal That Causes Infection -
- Epidermidis: Staphylococcus epidermidis is a common skin commensal but can cause infections on in-dwelling medical devices. Virulent strains can form biofilms on medical devices, making them resistant to the host immune system and antibiotics. This bacteria acts as a reservoir of antibiotic-resistance genes, transferring them to S. aureus.
Conclusion:
Preserving a healthy skin microbiome is crucial for maintaining optimal skin health. To preserve the skin microbiome, adopt gentle cleansing practices using pH-neutral and mild cleansers, avoid excessive use of antibacterial products, moisturize regularly, and opt for gentle exfoliation methods. Additionally, limiting hot water exposure, maintaining a balanced diet, managing stress levels, and avoiding smoking and excessive alcohol consumption are beneficial for supporting a healthy skin microbiome. Seeking personalized advice from a dermatologist helps in preserving the skin microbiome. By integrating these practices into one's skincare routine, one can maintain a healthy skin microbiome, resulting in healthier and more radiant skin.
