Mechanisms of antibacterial action in the respiratory system.

Perhaps the most striking finding in relation to the antibacterial activity in the respiratory system is the sterility of the bronchopulmonary apparatus from the primary bronchi downward. That these structures are ordinarily sterile, despite the continuous entry of droplet nuclei containing bacteria, has been known for more than 50 years (3). The mechanism by which the bronchopulmonary tree retains its sterility under ordinary circumstances is just beginning to be understood. Our attention was drawn to the problem by the considerations that if, in fact, the bronchopulmonary tree were normally sterile, and if in chronic bronchitis the lower bronchial secretions were ordinarily heavily populated with bacteria, there might be a basis for investigating the pathogenesis of chronic bronchopulmonary infections from the standpoint of breakdown of mechanism of local antibacterial activity. Initial investigations, therefore, were directed toward the study of the bacteriological flora of the bronchopulmonary secretions, with the use of extreme precautions to avoid contamination of the cultured material by bacteria coming from the upper respiratory passages. These studies (10) confirmed earlier observations (1, 11) that bacteria were rarely found in the bronchopulmonary secretions unless there was manifest inflammation or exudation. By the same token, when such exudation was found, the bacteria tended to be present in large numbers (>106 colonies per milliliter of secretion), and often the bacteria in the bronchial secretions were