Microbiology, 2021

21.1 • Anatomy and Normal Microbiota of the Skin and Eyes 855
skin and reduce water loss. Although some of the lipids and fatty acids in sebum inhibit microbial growth,
sebum contains compounds that provide nutrition for certain microbes.
Figure 21.2 (a) A micrograph of a section through human skin shows the epidermis and dermis. (b) The major layers of human skin are
the epidermis, dermis, and hypodermis. (credit b: modification of work by National Cancer Institute)
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• How does desquamation help with preventing infections?
Normal Microbiota of the Skin
The skin is home to a wide variety of normal microbiota, consisting of commensal organisms that derive
nutrition from skin cells and secretions such as sweat and sebum. The normal microbiota of skin tends to
inhibit transient-microbe colonization by producing antimicrobial substances and outcompeting other
microbes that land on the surface of the skin. This helps to protect the skin from pathogenic infection.
The skin’s properties differ from one region of the body to another, as does the composition of the skin’s
microbiota. The availability of nutrients and moisture partly dictates which microorganisms will thrive in a
particular region of the skin. Relatively moist skin, such as that of the nares (nostrils) and underarms, has a
much different microbiota than the dryer skin on the arms, legs, hands, and top of the feet. Some areas of the
skin have higher densities of sebaceous glands. These sebum-rich areas, which include the back, the folds at
the side of the nose, and the back of the neck, harbor distinct microbial communities that are less diverse than
those found on other parts of the body.
Different types of bacteria dominate the dry, moist, and sebum-rich regions of the skin. The most abundant
microbes typically found in the dry and sebaceous regions are Betaproteobacteria and Propionibacteria,
respectively. In the moist regions, Corynebacterium and Staphylococcus are most commonly found (Figure
21.3). Viruses and fungi are also found on the skin, with Malassezia being the most common type of fungus
found as part of the normal microbiota. The role and populations of viruses in the microbiota, known as
viromes, are still not well understood, and there are limitations to the techniques used to identify them.
However, Circoviridae, Papillomaviridae, and Polyomaviridae appear to be the most common residents in the
healthy skin virome. 1 2 3