Page 654 - Veterinary Immunology, 10th Edition

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FIG. 21.3 The role of the skin microbiota in regulating immunity.
Environmental bacteria trigger innate responses with PAMPs such
as bacterial lipopolysaccharides (LPS) and single-stranded RNA
(ssRNA) acting through toll-like receptors (TLRs).

The Respiratory Tract

Like all body surfaces exposed to the external environment, the
upper respiratory tract houses a dense and complex microbiota. It
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has been calculated that a human inhales 10 organisms/day just
breathing normally. Many nasal bacteria are also found on the skin

while others are common environmental bacteria. Deeper in the
airways, in the lower respiratory tract, many diverse bacteria are
also present. Neisseria and Gram-negative cocci are common.

Contrary to previous belief, the lung is not sterile. Healthy lungs
harbor a complex microbiota, closely related to but much less dense
than that found in the upper respiratory tract. The bronchi contain
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about 2000 bacterial genomes per cm . Lung tissues contain
between 10 and 100 bacterial cells per 1000 lung cells. These include

both aerobes and anaerobes, and like other surfaces, the
populations differ greatly between individuals. The predominant
phyla are Firmicutes with lesser numbers of Proteobacteria and

Actinobacteria. The organisms generally live within the mucus
layer and include bacteria, fungi (yeasts), and viruses including
bacteriophages. Pathobionts are also present and may induce
disease in immunodeficient individuals. Perhaps the best example

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