Page 657 - Veterinary Immunology, 10th Edition

the microbiota also changes along the gastrointestinal tract under
VetBooks.ir the influence of nutrient availability and the local

microenvironment. The Firmicutes consist of mainly Gram-positive
bacteria. Many are spore-forming. Important members include the

Clostridia that may be beneficial or pathogenic. They also include
potentially pathogenic streptococci and staphylococci. The
Actinobacteria are also Gram-positive bacteria with a different G+C
content than the Firmicutes. The Bacteroidetes are Gram-negative

bacteria that ferment indigestible plant carbohydrates to produce
short-chain fatty acids. The Proteobacteria contain Gram-negative
enterobacteria such as E. coli and Klebsiella.
The canine stomach has a microbiome dominated by Helicobacter

spp. Bacterial counts in the canine and feline duodenum are in a
9
2
range from 10 to 10 per gram of content. In the colon, the count
11
9
ranges from 10 to 10 colony forming units/g.
The Rumen
The surface of the rumen is covered by stratified squamous

epithelium. Thus its defenses have more in common with skin
rather that with the rest of the gastrointestinal tract. Although this
epithelium is largely leak-proof, the presence of such a large source

of microbial antigens suggests that some provision must be made
for defense against leakage and that it may have a profound
influence on the innate and adaptive immune systems of animals.
Disturbances in rumen metabolism, often caused by feeding very
high-energy diets, result in changes in the ruminal microbiota,

leading to an increase in fatty acid and ethanol production, a drop
in rumen pH, and the development of subacute rumen acidosis.
This in turn results in local inflammation, opening of intercellular

junctions, and a disruption in the barrier function of the ruminal
squamous epithelium. This can permit bacterial PAMPs such as
endotoxins, flagella, and other microbial products to cross the
ruminal wall and enter the bloodstream. This may lead to
endotoxemia, a systemic innate immune response, an acute-phase

response, and prolonged systemic inflammation.
While much is known about the relationships between the
intestinal immune system and lymphoid tissues in simple-

stomached animals, much less is known about the interactions

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