The Relationship of Inoculum Size to Lung Bacterial Clearance and Phagocytic Cell Response in Mice1,2

We investigated the effect of inoculum size on early pulmonary bacterial clearance of Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae, and characterized the phagocytic cell response to these organisms. Four h after aerosolization with S. aureus, 16% of the initially deposited bacteria after a small inoculum and 25% after a large inoculum remained (p < 0.01). With P. aeruginosa, clearance was observed at 4 h after a small inoculum, whereas multiplication occurred after a large inoculum (p < 0.01). In animals aerosolized with K. pneumoniae, clearance at 4 h was greater after the large inoculum, with 35% of bacteria remaining, whereas 62% of bacteria in the small inoculum were present (p < 0.01). Clearance of high-dose S. aureus was correlated with a 5-fold increase in the number of alveolar macrophages. However, after inoculation with P. aeruginosa, a granulocyte response was observed with a 550-fold increase after the small inoculum and a 1,600-fold increase present after the large...