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

Normal floras are the microorganisms that live in our body in a relatively stable manner without causing any negative effect on the host’s body. These bacteria and fungi colonize in the skin and mucous membrane throughout the life or for a short period of life.

Normal Flora of Skin, Hair, and Nail
Normal Flora of Skin, Hair, and Nail

Skin is the outermost covering of our body. It not only protects our body from invading microorganisms but ironically is a home for a wide variety of bacteria and fungi. Skin is full of normal micro floras which re-establish themselves when disturbed during bathing or other physical activities. Skin floras are influenced by dryness of the skin, pH, salt concentration, oiliness, and direct exposure to the environment (clothing habit). Moist areas like the perianal region, groin areas, face, inner thigh part, and scalp have more diversity. Both Gram-positive and negative bacteria are found in such moist areas in higher numbers. The remaining dry parts are dominated by Gram-positive bacteria with a comparatively less population. Body hair also harbors lots of bacterial and fungal flora on its surface. Nail contains more fungal flora than other parts of the body.

List of Normal Flora of Skin, Hair, and Nail

Gram-Positive BacteriaGram-Negative BacteriaFungi
Staphylococcus spp.
Streptococcus spp.
Micrococcus spp.
Corynebacterium spp.
Propionibacterium spp.
Bacillus spp.
Aerococcus spp.
Enterobacter spp.
Klebsiella spp.
E. coli
Proteus spp.
Pseudomonas spp.
Acinetobacter spp.
Candida spp.
Epidermophyton spp.
Aspergillus spp.
Penicillium spp.
Cladosporium spp.
Mucor spp.
Rhodotorula spp.
Malassezia spp.

Gram +ve and Gram -ve Bacteria of Skin, Hair, and Nail

Staphylococcus spp.

  • Gram-positive
  • Catalase-positive 
  • Coagulase positive and coagulase-negative
  • Aerobic
  • Cocci bacteria 
  • Family – Staphylococcaceae 
  • Seen in a grape-like cluster under a microscope 

Staphylococcus is a dominant species in human skin. It is found in the skin of the scalp, arms, body, legs, and groin areas and even in hairs and nails. They are resident flora of human skin, and when disturbed, they quickly re-establish themselves. 

S. epidermidis is the dominant species in human skin. In some areas, it covers more than 90% of the resident flora population. 

S. aureus is another common species in the hair follicle, vulvar skin, scalp, nose, and perineum. They are present in 10 – 40% of an adult human.

S. hominis, S. haemolyticus, S. warneri, and S. capitis are other common species of Staphylococcus residing in a healthy human’s skin. 

Streptococcus spp.

  • Gram-positive
  • Anaerobic and facultative anaerobic 
  • Catalase-negative 
  • Cocci bacteria 
  • Family – Streptococcaceae 
  • Seen in a form of a chain of spheres under a microscope 

Mostly – hemolytic streptococci are found as commensals in human skin. and – hemolytic species are found in very few people as commensals. Viridans streptococci are seen in the perineum, groin areas, and around the mouth. They are mostly transient flora. 

Micrococcus spp.

  • Gram-positive 
  • Mostly non-motile (few species are motile) 
  • Strictly aerobic 
  • Cocci in clusters 
  • Family – Micrococcaceae

Micrococcus is also abundantly found in the skin of most people. They represent about 20% of the bacterial population on the skin of legs, arms, and scalp.  M. luteus is dominant species covering around 20 – 80% of skin micrococci. 

Corynebacterium spp.

  • Gram-positive 
  • Rod-shaped (club-shaped), 
  • Aerobic bacteria 
  • Family – Corynebacteriaceae
  • Characteristic high mesodiamonopimelic acid content and highly repeated arabinogalactan

Corynebacterium is found abundantly in moist skin. Areas rich in sebaceous glands contain a large number of anaerobic Corynebacteria. C. parvum, C. xerosis, C. jeikeium, and C. minutissimum are some predominant Corynebacteria in human skin. 

Propionibacterium spp.

  • Gram-positive
  • Rod-shaped
  • Anaerobic
  • Family – Propionibacteriaceae
  • Unique ability to produce propionic acid using transcarboxylase enzymes

Propionibacterium is one of the most commonly isolated bacteria from the skin of an adult. These bacteria are rarely seen in children below 10 years of age. Once a person reaches puberty, they begin to colonize skin. P. granulosum, P. avidum, and P. acne are the most common species in human skin.  

Bacillus spp.

  • Gram-positive
  • Rod-shaped
  • Motile
  • Obligate aerobic and some facultative anaerobic
  • Spore forming
  • Family – Bacillaceae

Bacillus spp. are found in both the dry and the moist skin areas. However, they cover only a small portion of skin flora. 

Aerococcus spp. 

  • Gram-positive
  • Catalase-negative
  • Cocci
  • Aerobic
  • Family – Aerococcaceae

A. viridans are the normal flora of the skin. 

Enterobacter spp.

  • Gram-negative
  • Rod-shaped 
  • Motile
  • Lactose fermenting 
  • Facultatively anaerobic 
  • Mesophillic coliform bacteria 
  • Family – Enterobacteriaceae

Enterobacter spp. are found in moist areas, mostly the perianal region. E. cloacae are the most common species. 

Klebsiella spp.

  • Gram-negative 
  • Rod-shaped 
  • Non-motile
  • Encapsulated
  • Lactose fermenting 
  • Facultatively anaerobic 
  • Mesophillic coliform bacteria 
  • Family – Enterobacteriaceae

Klebsiella spp. are found in vaginal skin, perianal area, and groins. They are mostly transient flora migrated from the rectum. 

E. coli 

  • Gram-negative 
  • Rod-shaped 
  • Motile
  • Lactose fermenting 
  • Facultatively anaerobic 
  • Mesophilic coliform bacteria 
  • Family – Enterobacteriaceae
  • Genus – Escherichia  

E. coli are found in moist areas like the moist intertriginous areas, perineum, and the perianal region.  

Proteus spp.

  • Gram-negative 
  • Rod-shaped 
  • Aerobic and facultative anaerobic 
  • Motile bacteria 
  • Family – Enterobacteriaceae 
  • Capacity to produce swarming colonies

As like other Gram-negative bacteria, Proteus spp. are transient flora found in moist skin mainly around the perineum, and the perianal region. They are also found between the fingers of the leg. P. mirabilis is a commonly isolated species. They can occasionally cause wound infection.    

Pseudomonas spp.

  • Gram-negative 
  • Rod-shaped 
  • Aerobic 
  • Gammaproteobacteria 
  • Family – Pseudomonadaceae

P. aeruginosa is an infrequently isolated normal inhabitant of the human skin. 

Acinetobacter spp.

  • Gram-negative
  • Oxidase-negative
  • Nonfermentive
  • Strictly aerobic
  • Bacilli (or coccobacilli)
  • Gammaproteobacteria
  • Family – Moraxellaceae

Acinetobacter is found in moist skin. A. lwoffii, A. johnsonii are common species in the moist skin.

Fungi Microbiome of Skin, Hair, and Nail

Candida spp.

Candida is a genus of yeast in the Saccharomycetaceae family. They are true yeasts that can produce pseudohyphae. They are a normal inhabitant of the moist skin in vaginal skin, perineum, perianal region, and groin areas. C. albicans and C. parapsilosis are the most common species in moist skin.  

Epidermophyton spp.

Epidermophyton is a genus of filamentous fungi in the division Ascomycota. They are opportunistic pathogens causing superficial and cutaneous mycoses. They are a normal inhabitant of human skin, and mainly in the nail of the foot and toes. E. floccosum is the dominant transient flora in this genus. 

Aspergillus spp.

Aspergillus is a genus of filamentous fungi in the division Ascomycota. They can cause infection in humans but are found as transient flora in the dry skin and nails. They are the most common filamentous fungal species in human skin, nails, and hair. A. fumigatus, A. flavus, A. versicolor are common species isolated from the nail and the skin. 

Penicillium spp.

Penicillium is a genus of filamentous fungi in the division Ascomycota. They are normal flora in nails and dry skin. They are transient flora and enter the body from the environment. 

Cladosporium spp.

Cladosporium is a genus of filamentous fungi in the division Ascomycota. They are transient flora of the dry skin and nails. 

Mucor spp. 

Mucor is a genus of filamentous fungi in the division Mucoromycota. Mucor is the frequently isolated fungal species under the nail. 

Rhodotorula spp.

Rhodotorula is a genus of pigmented yeasts in the division Basidiomycota. They are yeast species found in nails and moist skin. 

Malassezia spp. 

Malassezia is a genus of yeasts in the division Basidiomycota. Malassezia is the most common genera of fungi present in most individuals. They are the normal flora of human skin in the scalp, hair follicles, intertriginous areas, nails, and moist skin. M. furfur, M. ovale, and M. globosa are the common species.

References

  1. Subadh Chandra Parija. Textbook of Microbiology and Immunology. 2nd edition. Elsevier, a division of Reed Elsevier India Private Limited. ISBN: 978-81-312-2810-4.
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  3. Normal flora: Introduction, types, beneficial and harmful effects (universe84a.com)
  4. Normal Human Microbiota Definition, Types, Advantages and Disadvantages. (microbiologynote.com)
  5. The Normal Bacterial Flora of Humans (textbookofbacteriology.net)
  6. Normal flora of the Skin – Online Biology Notes
  7. Isolation and Identification of normal microbial flora of skin and throat – Online Biology Notes
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  10. Somerville D. A. (1969). The normal flora of the skin in different age groups. The British journal of dermatology, 81(4), 248–258. https://doi.org/10.1111/j.1365-2133.1969.tb13976.x
  11. Fyhrquist, N., Ruokolainen, L., Suomalainen, A., Lehtimäki, S., Veckman, V., Vendelin, J., Karisola, P., Lehto, M., Savinko, T., Jarva, H., Kosunen, T. U., Corander, J., Auvinen, P., Paulin, L., von Hertzen, L., Laatikainen, T., Mäkelä, M., Haahtela, T., Greco, D., Hanski, I., … Alenius, H. (2014). Acinetobacter species in the skin microbiota protect against allergic sensitization and inflammation. The Journal of allergy and clinical immunology, 134(6), 1301–1309.e11. https://doi.org/10.1016/j.jaci.2014.07.059.

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