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.

Gram-positive bacteria are bacteria that have a thick cell wall composed of peptidoglycan and stain purple when subjected to the Gram stain technique. They are one of the most common types of bacteria found on human skin, hair, and nail. Some of the major genera of gram-positive bacteria that colonize the skin are:

• Staphylococcus: These are spherical bacteria that form grape-like clusters. They are catalase-positive, coagulase-positive or negative, and aerobic. They are the dominant species on human skin and can be found on the scalp, arms, body, legs, groin, hair, and nail. Some of the common species are S. epidermidis, S. aureus, S. hominis, S. haemolyticus, S. warneri, and S. capitis. They are usually harmless residents of the skin, but can cause infections if they enter the body through wounds or breaks in the skin.
• Streptococcus: These are spherical bacteria that form chains. They are catalase-negative, anaerobic or facultative anaerobic, and hemolytic or non-hemolytic. They are mostly found as commensals on human skin, especially in the perineum, groin, and around the mouth. Some of the common species are S. pyogenes, S. agalactiae, S. pneumoniae, and S. mutans. They can cause infections such as pharyngitis, impetigo, cellulitis, and necrotizing fasciitis if they invade deeper tissues.
• Micrococcus: These are spherical bacteria that form clusters. They are mostly non-motile, strictly aerobic, and catalase-positive. They are abundantly found on human skin, especially on the legs, arms, and scalp. They represent about 20% of the bacterial population on these areas. The dominant species is M. luteus, which is usually harmless but can cause opportunistic infections in immunocompromised individuals.
• Corynebacterium: These are rod-shaped (club-shaped) bacteria that are aerobic and catalase-positive. They have a characteristic high mesodiaminopimelic acid content and highly repeated arabinogalactan in their cell wall. They are found abundantly in moist skin areas, especially those rich in sebaceous glands. Some of the common species are C. parvum, C. xerosis, C. jeikeium, and C. minutissimum. They can cause infections such as diphtheria, endocarditis, and urinary tract infections.
• Propionibacterium: These are rod-shaped bacteria that are anaerobic and catalase-positive. They have a unique ability to produce propionic acid using transcarboxylase enzymes. They are one of the most commonly isolated bacteria from adult skin, especially in areas with high sebum production such as the face, chest, and back. They are rarely seen in children below 10 years of age. Some of the common species are P. granulosum, P. avidum, and P. acnes. They can cause acne vulgaris by clogging pores and triggering inflammation.
• Bacillus: These are rod-shaped bacteria that are motile, obligate aerobic or facultative anaerobic, and spore-forming. They are found in both dry and moist skin areas, but cover only a small portion of skin flora. Some of the common species are B. subtilis, B. cereus, and B. anthracis. They can cause infections such as anthrax, food poisoning, and eye infections.
• Aerococcus: These are spherical bacteria that are catalase-negative and aerobic. They belong to the family Aerococcaceae and are related to streptococci. The most common species is A. viridans, which is a normal flora of the skin.

These gram-positive bacteria play an important role in maintaining the health and integrity of the skin barrier by producing antimicrobial substances, competing with pathogens for nutrients and space, modulating immune responses, and influencing skin pH and moisture levels.

Gram-negative bacteria are bacteria that have a thin peptidoglycan layer in their cell wall and do not retain the crystal violet stain in the Gram staining procedure. They are usually more resistant to antibiotics than gram-positive bacteria. Some of the gram-negative bacteria that are found on human skin, hair, and nail are:

• Enterobacter spp.: These are rod-shaped, motile, lactose-fermenting, facultatively anaerobic bacteria that belong to the family Enterobacteriaceae. They are mesophilic coliform bacteria that can grow at 37°C. They are found in moist areas of the skin, mostly the perianal region. Enterobacter cloacae is the most common species. They are mostly transient flora that can migrate from the gastrointestinal tract or the environment. They can cause opportunistic infections such as urinary tract infections, wound infections, or sepsis in immunocompromised individuals.
• Klebsiella spp.: These are rod-shaped, non-motile, encapsulated, lactose-fermenting, facultatively anaerobic bacteria that also belong to the family Enterobacteriaceae. They are mesophilic coliform bacteria that can grow at 37°C. They are found in the vaginal skin, perianal area, and groins. They are also mostly transient flora that can migrate from the rectum or the environment. They can cause opportunistic infections such as pneumonia, urinary tract infections, wound infections, or sepsis in immunocompromised individuals.
• E. coli: This is a rod-shaped, motile, lactose-fermenting, facultatively anaerobic bacterium that belongs to the genus Escherichia and the family Enterobacteriaceae. It is a mesophilic coliform bacterium that can grow at 37°C. It is found in moist areas of the skin such as the intertriginous areas, perineum, and the perianal region. It is a normal flora of the human gastrointestinal tract and can migrate to the skin from there or from the environment. It can cause opportunistic infections such as urinary tract infections, wound infections, or sepsis in immunocompromised individuals.
• Proteus spp.: These are rod-shaped, aerobic and facultative anaerobic, motile bacteria that belong to the family Enterobacteriaceae. They have the capacity to produce swarming colonies on solid media. They are found in moist areas of the skin mainly around the perineum and the perianal region. They can also be found between the fingers of the leg. Proteus mirabilis is a commonly isolated species. They can occasionally cause wound infections.
• Pseudomonas spp.: These are rod-shaped, aerobic, gammaproteobacteria that belong to the family Pseudomonadaceae. They are often pigmented and produce a characteristic fruity odor. Pseudomonas aeruginosa is an infrequently isolated normal inhabitant of the human skin. It is found in moist areas of the skin such as around the nails or between the toes. It can also be found in hair follicles or sweat glands. It can cause opportunistic infections such as folliculitis, nail infections, otitis externa, or sepsis in immunocompromised individuals.
• Acinetobacter spp.: These are gram-negative, oxidase-negative, nonfermentative, strictly aerobic bacteria that have a bacilli or coccobacilli shape. They belong to the gammaproteobacteria and the family Moraxellaceae. They are found in moist areas of the skin such as around the nose or mouth. Acinetobacter lwoffii and Acinetobacter johnsonii are common species in moist skin. They can cause opportunistic infections such as wound infections or sepsis in immunocompromised individuals.

These are some of the gram-negative bacteria that are found on human skin, hair, and nail as normal flora or transient flora. They usually do not cause any harm to healthy individuals but can become pathogenic under certain conditions such as impaired immunity or injury.

Fungi are also part of the normal flora of skin, hair, and nail. They are mostly yeasts and filamentous fungi that can live as commensals or opportunistic pathogens. The fungal microbiome of skin, hair, and nail is influenced by various factors such as age, sex, diet, hygiene, climate, and host immunity.

The most common fungi found on skin, hair, and nail are dermatophytes and Malassezia. Dermatophytes are a group of fungi that can infect the keratinized tissues of humans and animals. They belong to three genera: Trichophyton, Microsporum, and Epidermophyton. Dermatophytes can cause superficial and cutaneous mycoses such as tinea capitis (scalp ringworm), tinea pedis (athlete`s foot), tinea unguium (nail fungus), and tinea corporis (body ringworm). Dermatophytes are usually transmitted by direct contact with infected hosts or fomites.

Malassezia are a genus of yeasts that belong to the division Basidiomycota. They are the most abundant fungi on human skin, especially in areas rich in sebaceous glands such as the scalp, face, chest, and back. Malassezia can produce lipases that hydrolyze sebum into fatty acids and glycerol. These metabolites can modulate the skin barrier function and immune response. Malassezia can also cause various skin disorders such as pityriasis versicolor (a pigmentation disorder), seborrheic dermatitis (a chronic inflammatory condition), folliculitis (inflammation of hair follicles), and atopic dermatitis (a type of eczema).

Other fungi that can be found on skin, hair, and nail include Candida, Aspergillus, Penicillium, Cladosporium, Mucor, Rhodotorula, and Acinetobacter. These fungi are usually transient flora that enter the body from the environment. They can colonize moist areas such as the perineum, perianal region, groin areas, intertriginous areas, and nails. Some of these fungi can also cause opportunistic infections in immunocompromised hosts or when the skin barrier is disrupted.

The fungal microbiome of skin, hair, and nail plays an important role in maintaining the health and integrity of the epidermis. It can interact with the host cells and other microorganisms to modulate the immune system, prevent colonization by pathogens, produce beneficial metabolites, and influence the skin pH and moisture. However, when the balance of the fungal microbiome is disturbed by external or internal factors, it can also contribute to various skin diseases. Therefore, understanding the diversity and function of the fungal microbiome is essential for developing better strategies for diagnosis, prevention, and treatment of skin disorders.

The human skin, hair, and nail are home to a diverse and dynamic community of microorganisms, collectively known as the skin microbiome. The skin microbiome consists of bacteria, fungi, and other microbes that live on the surface and in the deeper layers of the skin. The skin microbiome plays an important role in maintaining the health and function of the skin, as well as influencing the immune system and the susceptibility to infections and diseases.

The normal flora of the skin, hair, and nail can be classified into two groups: resident flora and transient flora. Resident flora are the microorganisms that are permanently or semi-permanently associated with the skin, hair, and nail. They are usually harmless or beneficial to the host and can prevent the colonization of pathogens by competing for nutrients and space. Transient flora are the microorganisms that are temporarily present on the skin, hair, and nail. They are usually acquired from the environment or other sources and can be easily removed by washing or disinfection. Transient flora can sometimes cause infections or diseases if they enter the body through wounds or mucous membranes.

The normal flora of the skin, hair, and nail vary depending on several factors, such as the anatomical location, age, sex, hormonal status, hygiene, diet, climate, genetics, and lifestyle of the host. The most common types of bacteria found on the skin, hair, and nail are gram-positive bacteria, such as Staphylococcus spp., Streptococcus spp., Micrococcus spp., Corynebacterium spp., Propionibacterium spp., Bacillus spp., and Aerococcus spp. Gram-negative bacteria are less common and mostly found in moist areas, such as Enterobacter spp., Klebsiella spp., E. coli, Proteus spp., Pseudomonas spp., and Acinetobacter spp. The most common types of fungi found on the skin, hair, and nail are yeasts, such as Candida spp., Rhodotorula spp., and Malassezia spp., and filamentous fungi, such as Epidermophyton spp., Aspergillus spp., Penicillium spp., Cladosporium spp., and Mucor spp.

The normal flora of the skin, hair, and nail have a complex and dynamic relationship with the host and each other. They can influence the immune system, modulate inflammation, produce antimicrobial substances, degrade sebum and keratin, synthesize vitamins and hormones, metabolize drugs and toxins, and affect the odor and appearance of the skin. The normal flora of the skin, hair, and nail can also be affected by external factors, such as antibiotics, cosmetics, detergents, disinfectants, pollutants, stress, and UV radiation. These factors can alter the composition and function of the skin microbiome and lead to dysbiosis or imbalance. Dysbiosis can result in various skin disorders, such as acne vulgaris, atopic dermatitis, psoriasis, rosacea, seborrheic dermatitis, and fungal infections.

Therefore, it is important to understand the normal flora of the skin, hair, and nail and their roles in maintaining the health and beauty of the human body.

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