What are the skin microflora?

The skin microflora are microorganisms that are resident on our skin. Microflora are frequently (and more correctly) called the skin microbiota or the skin microbiome.

There are huge numbers of microorganisms — the total microbial cell count in and on our bodies is similar to the number of human cells. After the gut, there are more microorganisms on the skin than anywhere else in the body. Bacterial species are by far the most numerous; however fungi, viruses and mites are also found on the skin of normal healthy humans.

Resident microorganisms

Resident microbiota are found in the upper parts of the epidermis and congregated in and around the hair follicles. They include:

• Staphylococcus (see coagulase negative staphylococci)
• Micrococcus
• Corynebacterium
• Brevibacterium
• Dermabacter
• Malasezzia

Transient bacteria

Some microbiota are considered transient, as they can be only isolated and cultured from skin samples from time to time. These are mainly Gram-positive bacteria, including clostridia in the perineal area. Occasionally, moist areas allow the growth of Gram-negative Acinetobacter.

Other Gram-negative bacteria are not considered part of the normal skin micobiota, as the relatively low humidity and high osmotic pressure of the skin are unfavourable for their growth.

Where on the skin are microflora found?

Microorganisms are found all over the skin surface but the species vary with anatomical site.

Skin sites can be grouped into three types:

• Dry
• Moist
• Oily.

Dry body sites

Dry sites include the forearms, hands, legs and feet. They have the most diverse microbiota, due to high exposure to the external environment. Coagulase-negative staphylococci predominate (eg, S. epidermedis and S. hominis).

Moist body sites

Corynebacteriam flourish in the moist skin of the skin folds: elbow creases, beneath the breasts, in-between the toes and the groins.

Sebaceous sites

Sebaceous body sites include the head, neck and trunk, where sebaceous glands secrete an oily substance, sebum, allowing cutibacteria to thrive. Demodex mites (Demodex folliculorum and Demodex brevis) and the fungus Malasezzia also congregate in the oily areas of the face.

Body site distribution of microbiota

Fungi

Malasezzia are commonly found all over the body except for the feet. They are more numerous in oily areas, as described above.

Viruses

Very little is known about the prevalence of viruses on normal skin. Traditionally, viruses on the skin have been termed pathogenic, ie harmful, but recent research disputes this.

What is the role of microflora in human health?

Microorganisms can be grouped according to their relationship with us:

• Commensals — organisms that reside on our skin, deriving benefit from us, but we do not benefit from them.
• Symbionts — the microorganism and humans are mutually beneficial.
• Pathogens — the microorganism benefits but causes disease to the human.

The majority of microorganisms on our skin are commensals, as they infrequently cause ill health. However, in some circumstances commensal microbes such as S. epidermidis have beneficial or pathogenic roles.

Commensal microorganisms can prevent colonisation of pathogenic microorganisms such as S. aureus. For example:

• A strain of S. epidermidis produces antibiotic-like compounds called bacteriocins
• Commensals deplete nutrients and produce toxic metabolites thus preventing adherence of harmful bacteria to skin cells
• They enhance the immune response to pathogenic bacteria via interferon, other cytokines, and phagocytosis
• Lipotechoic acid in the cell wall of Gram-positive bacteria stimulates mast cells to release cathelicidin (an antimicrobial peptide).

Does everyone have the same skin microorganisms?

Generally speaking, most people have similar but not identical microorganisms. Some variation is due to age and environment.

Initial colonisation of a newborn baby's skin usually occurs during vaginal delivery through the birth canal. The baby's skin is at first sterile when the birth is by caesarean section.

After birth, key factors influencing microbial growth on the skin include:

• Skin pH (normal is slightly acidic at ~pH 5)
• Moisture
• Temperature
• Oxygen:carbon dioxide ratio
• Sun exposure: ultraviolet radiation (UVR) damages microorganism DNA
• Interactions with other microorganisms
• Innate host defenses
• The person's genetic makeup.

Can the skin microbiota be harmful?

Microorganisms are implicated in a number of infectious and non-infectious skin conditions affecting the epidermis, hair follicles, dermis and subcutis.

Whilst commensal organisms are harmless in most people, they may cause minor or even potentially fatal disease in another. The following factors make pathogenesis more likely:

• A breach in the mechanical skin barrier due to injury, a skin disease (eg, dermatitis), or an invasive medical device (eg, central venous line)
• Immune suppressant medication
• Immunocompromise due to cancer or human immunodeficiency virus infection (HIV)
• Extremes of age (very young or very old)
• Individual genetic factors.

Non-infectious conditions associated with microorganisms

Examples include:

• Demodex mites in rosacea
• Proprionebacterium acnes in acne
• S. epidermidis in miliaria
• Mixed flora in hidradenitis suppurativa.

Skin infections

Skin infections are listed elsewhere:

• Bacterial skin infections
• Fungal skin infections
• Viral skin infections
• Laboratory tests for bacterial infections
• Laboratory tests for fungal infections
• Laboratory tests for viral infections.