Our skin is the largest organ of our body. Weighing in at over 3.6 kilograms (8 pounds) in an average adult, our skin contains many different types of microorganisms which all contribute to its overall condition. Since our skin is constantly in contact with the outside world, it’s an easy place for microbes to colonise different areas of the skin.

In this first blog in our mini-series on skin microorganisms, we’ll be covering the many different types of microorganisms that live on our skin. We have two more technical blog posts to follow as part of this series:

• Resident Bacteria, Transient Bacteria and When Microbes Become Harmful
• The Map of You – Where Different Microorganisms Live on Your Body

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Taxonomies help us identify different kinds of organisms and allows us to divide them into multiple categories. There are a total of 8 taxonomic categories:

Bacteria is one of the three domains of life alongside Archaea and Eukaryota. This puts it at the highest level of taxonomic categorisation. Since there are so many different species of bacteria on the planet, it’s difficult to determine the exact number of them. It’s estimated that there are roughly 1,000 different species of bacteria on the human skin alone from 19 different phyla. This makes up the majority of the microorganisms on our skin.

The bacteria that live on our skin are typically either commensalistic or mutualistic. Commensalistic means that the bacteria thrive on your skin but do not directly harm you. Mutualistic means that the bacteria and the host both benefit. Also, they are usually categorised by the environments that they thrive in:

• Sebaceous areas such as the head, neck or trunk
• Moist areas such as between the toes or the crook of the elbow
• Dry areas like the arms and the legs

Most of the bacteria that live on our skin are generally harmless. However, some can occasionally cause health issues. For example, bacteria can cause problems such as a boil or cellulitis, but they can also lead to infections such as meningitis or even food poisoning. Here are a couple of examples of bacteria that live on our skin.

Cutibacterium acne lives on oily surfaces of your skin and hair follicles. These bacteria can lead to acne breakouts because they multiply when there is excess oil production and clogged pores. This is because they feed on the sebum that is produced by the sebaceous glands. Sebum itself contains a mixture of liquid substances such as fat and cholesterol. It’s an essential lipid that helps improve your skin health by moisturising and also keeping your skin and hair protected.

However, if your skin produces too much sebum, it causes the Cutibacterium acnes to multiply too quickly. This causes your white blood cells to respond to the outbreak, leading to inflammation and ultimately acne. Interestingly, the virulence of this bacteria rises at the strain levels. Not all strains of C. acnes are pathogenic and there is accumulating evidence that this group of bacteria produce antioxidants and other products that are beneficial for the skin.

Corynebacteria includes non-pathogenic and pathogenic bacteria. Corynebacterium diphtheriae bacteria produce toxins that lead to the disease diphtheria, an infection that affects the throat and mucous membranes of your nose. It can also cause skin lesions that develop as the bacteria begins to colonise damaged areas of your skin. This is a serious infection that can eventually cause difficulty breathing, paralysis, heart failure, or even be fatal. However, Corynebacteria on the skin are mostly harmless commensal that being the 3^rd most populous group on the skin, are essential to maintain a balanced microbiome.

Staphylococcus epidermidis is usually considered one of the “good guys” on the skin. It is known to produce a series of antimicrobial peptides that keep other pathogens under control. There is also evidence of its positive effect on the skin through the modulation of the inflammatory response. S. epidermidis is one of the most abundant bacteria on human skin being predominant in moist areas but also present in sebaceous and dry areas.

Like most bacteria, this bacteria can act as an opportunistic pathogen forming a thick biofilm barrier that protects bacteria from antibiotics, chemicals, and other hazardous substances. This biofilm can adhere to polymer surfaces that could potentially lead to infections. This behaviour causes the Staphylococcus epidermidis to be commonly associated with infections regarding implanted medical devices such as prostheses and pacemakers and is also a major cause of hospital-acquired blood infection. However, the Staphylococcus epidermidis bacteria are usually harmless to healthy individuals and pose no real risk of infection.

Staphylococcus aureus is a type of bacteria on the skin that is often found in the nasal cavities and respiratory tract. Methicillin-resistant Staphylococcus aureus (MRSA) is a common example of a strain that can cause serious health problems. It is often spread through physical contact, but they need to pass through the skin to cause an infection. This means it requires something like a cut in order to become a serious problem. On the skin, this bacteria can cause or is associated with several pathogens such as Staphylococcal scalded skin syndrome, cellulitis or Eczema.

M. luteus is a very common microorganism of the skin that is found in most individuals. In a similar way but at a smaller scale than S. epidermidis, M. luteus is usually considered beneficial for our skin. Also similar to S. epidermidis, this cocci can become a pathogen when the environment is appropriate.

The genus Lactobacillus is a well-known microorganism with beneficial effects. A large number of species belonging to the genus are essential for a healthy gut and vagina. Recent studies have also highlighted that certain species such as L. iners are linked to healthy skin. The absence of this bacteria is associated with atopic dermatitis and psoriasis.

There are skin fungi present at 14 different places across the body. These are:

• Between the eyebrows
• Ear canal
• Back of the head
• Behind the ears
• Heel
• Toenails
• Between the toes
• Back
• Groin
• Forearm
• Nostrils
• Chest
• Palms
• Crook of the elbow

The heel is where the most diverse collection of fungi are. Research shows that there are around 80 different species of fungi here alone. Other places with many different species include the palm, forearm, and the crook of the elbow which have anywhere from 18 to 32 different species. The head and the trunk usually have between 2 and 10. Most of those fungal species belong to the genus Malassezia but some species from the genus Candida or dermatophytes as Trichophyton are also represented.

Viruses can live freely on the skin itself, or they can be inside bacterial cells. The most common viruses on the skin are bacteriophages that infect skin bacteria so they play an essential role in the regulation of the skin microbiota. Human viruses such as herpes or the virus causing warts and verrucas are also common. They are usually known as pathogenic because they are harmful, but commensal viruses have yet to be studied in-depth and as such, it’s disputed whether all viruses on the skin are pathogenic. As such, there isn’t much information or research on this topic as of now due to the methods of isolating and identifying these viruses being in early development.

As one of the three taxonomic domains, archaea are a substantial component in areas of the human body such as the nose, lung, gut, and skin. They resemble bacteria in their shape and size, and they make up around 12% of the human microbiome. However, archaea are relatively understudied due to methodical problems so not much is known about them.

Mites are also a part of the skin’s microbiome. For example, Demodex is the name given to small mites that live in hair follicles. In humans, these are usually found in facial hair and can sometimes cause a condition known as demodicosis. They feed on dead skin cells and can exacerbate pre-existing skin conditions in large numbers. They are also seen in increased numbers during conditions such as rosacea.