The skin microbiota is the term used to describe the collection of microorganisms that live on our skin. More specifically, it refers to the genomes of all these microbes including bacteria, fungi, protozoa, viruses, and mites living.
Generally, the skin microbiota is similar between different individuals, although it is never exactly the same. Environment and age are examples of factors that affect the variation that does exist.
The abundance of microbes living inside and on the human body is vast, with the total number of microbial cells outnumbering that of human cells by 10 to 1. The skin is the second most common body site for microbes, next to the gut. Bacteria are by far the most common skin microbes, although healthy, normal skin may also be home to fungi, viruses, and mites.
Resident and transient microbiota
Resident microbes are found in the upper epidermis, collected around hair follicles. Examples of resident microbes include the following:
- Staphylococcus
- Brevibacterium
- Micrococcus
- Malasezzia
- Corynebacterium
- Dermabacter
Certain bacteria are referred to as transient, since it is only possible to culture them from skin samples now and then. These are generally Gram-positive bacteria such as Clostridium, but sometimes the Gram-negative bacteria acinetobacter grows in moist skin areas. Other Gram-negative bacteria are not regarded as components of normal microbiota since the skin’s high osmotic pressure and low humidity do not favor the growth of these bacteria.
Location
Microbes are present across the entire skin surface, but bacterial species vary depending on body site, which is categorized as one of three types: dry, moist, or oily.
Dry body sites are home to the greatest variety of microbes, since they have high exposure to the surrounding environment. The most common microbes are coagulase-negative Staphylococci and examples of dry anatomical sites include the hands, feet, legs, and forearms. Moist body sites include underneath the breasts, the groin, in-between the toes, and elbow creases. Corynebacteria are abundant at these sites. Oily or sebaceous skin sites include the trunk, neck, and head. Sebum is secreted at these sites which enables Demodex mites to flourish and allows the growth of a fungus called Malasezzia.
Beneficial or harmful?
Different microorganisms on the skin are categorized according to how they affect us. Commensals are “friendly” microbes that benefit from living on our skin, usually without causing any ill effects. Symbionts are microbes that benefit from living on the skin and benefit the human host in return. Pathogens are disease-causing microbes.
Most microorganisms on the skin are commensals. These can prevent the colonization of other pathogenic microbes by using up the nutritional sources that those harmful microbes require to survive and by producing toxic substances that stop the pathogens from adhering to skin cells.
Microbes can cause various infectious and non-infectious conditions of the epidermis, dermis, subcutis, and hair follicles. Infections range from mild and harmless to severe and potentially life threatening, but the factors that make infection more likely to be pathogenic include the following:
- Broken skin barrier due to a skin condition such as dermatitis or use of an invasive medical tool
- The use of immunosuppressants
- Immunocompromisation as a result of cancer or HIV
- Being extremely young or extremely old
- Certain genetic factors
Further Reading
Gut transit time and pH shape microbiome individuality and dietary responses