The bacterial flora of the respiratory tract. Some knowns and unknowns.

that may be isolated from the mucous membranes lining these structures. Although the lower respiratory tract is in direct continuity with the secretions draining the upper respiratory tract, it is, by contrast, normally sterile.2` This remarkable difference is brought about by a series of coordinated mechanisms of the mammalian host which act to protect the tracheobronchial tree and pulmonary parenchyma from infection. Only when injury, acute or chronic, has occurred does bacterial infection of the lower respiratory tract usually arise. There are some exceptions to this generalization such as diptheria and perhaps pertussis, pneumonic plague and tularemia, and tuberculosis; but in general, this view seems valid.