Page 649 - Veterinary Immunology, 10th Edition

development and function while they regulate the composition and
VetBooks.ir behavior of the microbes. The surfaces of the animal body consist of

many stable, nutrient-rich ecosystems where microbes thrive. As a
result, each surface is populated by enormous numbers of bacteria,

archaea, fungi, and viruses collectively termed the microbiota. The
bacteria are the best studied of these. Thus bacteria live on the skin,
in the respiratory tract, in parts of the genitourinary tract, and
sometimes within the body but mainly within the gastrointestinal

tract. It has been estimated that in an animal body, at least half of all
the cells are microbial. As a result of their life-long, intimate
association with body surfaces, the microbiota can be considered to
be an integral part of the body—a “virtual organ.” As such, they

influence both innate and adaptive immunity and conversely, they
are influenced by signals generated by its host. This has given rise
to the concept that animals and their microbiota together form
“superorganisms” that share nutrition and exchange energy and

metabolites and whose complex interactions are regulated in large
part by immune mechanisms.
By harnessing the immensely diverse genomes present in the
microbiota, animals enhance their metabolic potential and obtain

new ways to utilize food. (Mammals possess about 20,000 protein-
encoding genes, while our microbiota may collectively possess
about 10 million.) Thus they increase an animal's ability to extract
energy from plant structural carbohydrates and to obtain essential

vitamins. Because of the microbiota, animals can utilize food
sources that would otherwise be unavailable. Microbial metabolism
permits animals to adapt to otherwise noncompetitive lifestyles. For
example, mice with a conventional microflora need to eat 30%

fewer calories than germ-free mice to maintain their body weight.
This is due to the ability of the microbiota to extract more energy
from food.
Domestic mammals do, however, have some additional

complexities associated with their diet and lifestyle. The large
domestic herbivores contain massive amounts of microbial material
in their rumen and large intestine. These reflect a major role of the
microbiota—providing nutrition by extracting energy from
complex, plant-derived polysaccharides such as the celluloses (Fig.

21.1). Carbohydrate digestion is the primary function of the

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