Electron microscopic examination of the inflammatory response to Legionella pneumophila in guinea pigs

We have described the ultrastructural morphology of splenic and pulmonary exudates from guinea pigs infected intranasally and intraperitoneally by Legionella pneumophila. Legionella pneumophila produced pneumonia and splenitis by both routes of inoculation. The microbe was also disseminated to other organs. Within neutrophils, Legionella pneumophila typically displayed degenerating forms, suggesting that this intracellular environment is somewhat hostile to the bacterium. By contrast, macrophages tended to contain intact forms, located within organelles morphologically identical with rough endoplasmic reticulum. Some bacteria were replicating at this site. In macrophages containing greater than 25 microbes per section, Legionella pneumophila was usually dispersed within the cytoplasm outside of organelles, and many of the heavily infected macrophages exhibited ultrastructural features of injury. Neutrophils phagocytosed Legionella pneumophila, but we found no ultrastructural evidence of either ingestion of Legionella pneumophila by macrophages or of localization of the microbe to primary or secondary phagosomes of macrophages. Our findings support the contention that Legionella pneumophila is an intracellular parasite of macrophages. The homing of Legionella pneumophila to cytoplasmic organelles morphologically indistinguishable from rough endoplasmic reticulum has no bacteriologic parallel. It remains to be determined how Legionella pneumophila enters this organelle, whether this structure is, in fact, functional rough endoplasmic reticulum and whether this site is actively involved in replication of the bacterium. The animal models used herein seem suitable for further delineation of these questions.