Final answer:
The lower respiratory tract is almost devoid of bacteria due to the action of alveolar macrophages and various other defenses such as IgA, lysozyme, surfactant, and defensins that protect the region from microbial colonization.
Step-by-step explanation:
The lower respiratory tract exhibits sparsity in bacterial population due to several defensive mechanisms that work in concert to protect this region. One of the primary defenses is the presence of alveolar macrophages, which are aggressive phagocytes that swiftly eliminate microbes that bypass the upper respiratory defenses. Other contributing factors to this near sterility include secreted antibodies such as IgA, the antibacterial enzyme lysozyme, surfactant that helps prevent microbial adherence, and antimicrobial peptides known as defensins.
These defenses are robust within the lower tract, which begins at the larynx and includes the trachea, bronchi, bronchioles, and alveoli. Unlike the upper tract, which has a diverse microbiota, the lower tract is left with very few bacteria. Some that are found, like species of Pseudomonas, Streptococcus, Prevotella, Fusobacterium, and Veillonella, may not even constitute a normal microbiota but could potentially be transient organisms passing through.