Page 660 - Veterinary Immunology, 10th Edition
P. 660
VetBooks.ir The Functions of the Microbiota
Nutritional Efficiency
The composition and metabolism of the microbiota is critically
dependent on diet. Thus the microbiota in animals fed a low-fat,
high-plant polysaccharide diet is very different from that in animals
on a high-fat, high-sugar and low-plant polysaccharide diet. There
are great differences in the microbiota of African children compared
to European children or in Indian cattle compared to American
cattle. Environmentally induced fluctuations in the intestinal
microbiota can permit the host to adjust its metabolic and
immunologic performance in response to nutritional and
environmental changes.
The intestinal microbiota changes during pregnancy. In mothers
during the third trimester, there is a reduction in species richness.
When transferred to germ-free mice, late pregnancy microbiota
induced greater adiposity and reduced insulin sensitivity. This
effect is beneficial in a normal pregnancy since it supports fetal
growth and the onset of lactation. The maternal microbiota also
drives early postnatal immune development including the potential
for young animals to develop type 2 responses and hence allergies.
Intestinal Protection
The microbiota protects the body against colonization by pathogens
and prevents the overgrowth of pathobionts. They do this by
competing for essential metabolites and nutrients, and by inducing
intestinal immune responses (Fig. 21.6). By fully occupying and
exploiting the intestinal environment, commensal bacteria block
subsequent colonization by pathogenic bacteria. (It is, for example,
possible to prevent or reduce Salmonella colonization of the chicken
intestine by feeding an appropriate mixture of commensal bacteria
to birds.) The microbiota also modify local environmental
conditions by keeping the pH and oxygen tension low. This is also
influenced by the diet; for example, the intestine of milk-fed
animals contains many lactobacilli that produce bacteriostatic lactic
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