Normal Flora of Skin, Hair, and Nail- Human Skin Microbiome


The human body is home to a diverse and complex community of microorganisms, collectively known as the microbiome. The microbiome plays an important role in various aspects of human health and disease, such as digestion, immunity, metabolism, and mood. Different parts of the body harbor different types of microbes, depending on the environmental factors such as pH, temperature, moisture, oxygen, nutrients, and host factors such as genetics, age, sex, hygiene, and lifestyle.

The skin is the largest organ of the human body and the first line of defense against external threats. It covers an area of about 2 square meters and has a surface area of about 1.8 square meters. The skin consists of three layers: the epidermis, the dermis, and the hypodermis. The epidermis is the outermost layer that contains keratinocytes, melanocytes, Langerhans cells, and Merkel cells. The dermis is the middle layer that contains blood vessels, nerves, hair follicles, sweat glands, and sebaceous glands. The hypodermis is the innermost layer that contains adipose tissue and connective tissue.

The skin is not a uniform environment but rather a mosaic of microhabitats that vary in their physical and chemical properties. Some areas are dry and acidic (such as the forearm), while others are moist and alkaline (such as the axilla). Some areas are exposed to sunlight and air (such as the face), while others are covered by clothing or hair (such as the scalp). Some areas are rich in sebum (such as the forehead), while others are poor in sebum (such as the palm). These factors influence the composition and diversity of the skin microbiome.

The skin microbiome consists of bacteria, fungi, viruses, archaea, and mites that colonize the skin surface or reside in the hair follicles or glands. The majority of the skin microbes are bacteria, followed by fungi. The most common bacterial phyla are Actinobacteria (mainly Corynebacterium and Propionibacterium), Firmicutes (mainly Staphylococcus and Streptococcus), Proteobacteria (mainly Acinetobacter and Pseudomonas), and Bacteroidetes (mainly Prevotella). The most common fungal phyla are Ascomycota (mainly Malassezia) and Basidiomycota (mainly Rhodotorula). The most common viruses are bacteriophages that infect bacteria. The most common archaea are methanogens that produce methane from hydrogen and carbon dioxide. The most common mites are Demodex folliculorum and Demodex brevis that live in the hair follicles and sebaceous glands.

The skin microbiome is not static but dynamic and responsive to various internal and external factors. The skin microbiome can change with age, sex, hormonal status, diet, stress, medication, infection, injury, cosmetics, climate, season, geography, ethnicity, and personal hygiene. The skin microbiome can also interact with other microbiomes in the body (such as the oral or gut microbiome) or with other people or animals (through physical contact or sharing of objects).

The skin microbiome has both beneficial and harmful effects on human health. On one hand, the skin microbiome can protect the skin from colonization by pathogens (through competition for nutrients or space or production of antimicrobial substances), modulate the immune system (through stimulation or suppression of inflammation or tolerance), maintain the skin barrier function (through production of lipids or acids or enzymes), influence wound healing (through promotion or inhibition of angiogenesis or fibrosis), and contribute to vitamin synthesis (such as vitamin K or B12). On the other hand, the skin microbiome can also cause or exacerbate various skin diseases (such as acne vulgaris, atopic dermatitis, psoriasis, rosacea, seborrheic dermatitis) or systemic diseases (such as endocarditis or bacteremia) by triggering inflammation or immune dysregulation or producing toxins or biofilms.