Page 652 - Veterinary Immunology, 10th Edition

VetBooks.ir The Locations of the Microbiota

The Skin

Normal skin harbors trillions of microorganisms. These are found
on the keratinocyte surface and extend into sebaceous glands and
hair follicles. The skin is not, however, a hospitable surface. The

outer cell layers are constantly shed and replaced by new cells from
below. It is cool in some areas and hot in others. Haired skin may be
very different from mucocutaneous junctions. Some areas of skin
may be very dry, with a high salt content, hydrophobic, and acidic,
as well as being nutrient poor. Other areas may be moist but bathed

in a complex mixture of proteases, lysozyme, and antimicrobial
peptides such as β-defensins and cathelicidins. Nevertheless, by
using modern molecular techniques, it has been estimated that up

to a billion bacteria may live on a square centimeter of human skin.
Given the sheltering effects of hairs or feathers, it is likely that the
skin microbiota in domestic animals may be even more complex.
For example, the skin microbiota of dogs varies greatly between
different dogs and different skin sites. For example, there is higher

microbial diversity in haired skin compared to mucocutaneous
junctions. The highest microbial diversity was found in the axilla
and the dorsum of the nose. On average, about 300 different

bacterial species were identified on the dorsal canine nose.
The skin microbiota can be divided into a resident population
that is relatively stable and consistent—a true commensal
population, and a changing population of transient bacteria that
may only persist on the skin for hours or days. Both populations

can contain a mixture of commensals and potential pathogens, yet
invasion and disease are relatively uncommon. Large populations
of Proteobacteria and Oxalobacteriaceae predominate (Fig. 21.2).

The precise composition of the skin microbiota thus depends on
location (hairy, wooly or bald skin; back versus skin in the axilla or
groin or ear) and the presence of disease such as seborrhea or atopic
dermatitis. There is also great variation between individuals.
Grooming activities will have some impact on these microbial

populations but their significance is unclear.

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