Early events associated with experimental infection of the murine lung with Rhodococcus equi

Pneumonia due to Rhodococcus equi was induced in the murine lung by deposition of a known dose of organisms. From serial estimations of bacterial numbers in the lungs of inoculated mice, analysis of the cellular composition of bronchoalveolar lavage fluid and morphological examination of the lungs, events in the host-parasite interaction were followed until day 7. Early bacterial clearance from the lung was dose-dependent but was not sustained. A proportion of the inoculated R. equi was susceptible to the early nonspecific phagocytic cell response, and the contribution of neutrophils to bacterial clearance appeared largely limited to the first 24 hours. A substantial fraction of the organisms survived in the alveoli, probably within macrophages. The contribution phagocytes make to resistance against R. equi is similar to that which prevails in infection with Listeria monocytogenes.