Pulmonary clearance of bacteria was studied using histologic, bacteriologic, and radiotracer methods. When mice were exposed to an aerosol of P32-tagged Staphylococcus aureus or Proteus mirabilis, and the rate of disappearance of viable bacteria compared with the rate of their mechanical removal, it was found that bacterial viability declined 80 to 90 per cent in 4 hours, whereas radioactivity declined by only 14 to 20 per cent. The marked disparity in these rates indicated that mechanical removal comprised a relatively small fraction of the total clearing process. The in situ bactericidal action of the lung predominated over the mechanical removal process in achieving clearance of the inhaled bacteria. By immunofluorescent methods, the inhaled bacteria were found to be localized in the alveolar spaces and within alveolar macrophages. These observations suggest that the bactericidal action of the bronchopulmonary tree is due primarily to the phagocytic activity of the alveolar macrophages, and that the action of the mucociliary stream, in relation to bacterial particles, may be largely related to the transport from the lung of phagocytes containing material of bacterial origin.

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