Formation of a Novel Phagosome by the Legionnaires' Disease Bacterium

Legionella pneumophila, the agent of Legionnaires' disease, is a facultative intracellular gram-negative bacterial pathogen (1) . The bacterium multiplies intracellularly in human blood monocytes (1) and human alveolar macrophages (2), and under tissue culture conditions, multiplication is exclusively intracellular . L. pneumophla enters monocytes by an unusual process-a monocyte pseudopod coils around the bacterium as the organism is ingested .' Thereafter, the bacterium resides in a membrane-bound cytoplasmic vacuole . Immediately after phagocytosis, cellular organelles do not appear about the L . pneumophila vacuole .' Later on, however, the bacteria appear to be multiplying within a highly unusual vacuole . A striking feature of this vacuole is that the cytoplasmic side is studded with host cell ribosomes (1) . This morphologic feature has also been observed on electron microscopic examination of leukocytes in human lung tissue specimens from patients with Legionnaires' disease (3) . In this paper, the formation of the unusual vacuole in which L . pneumophila multiplies within monocytes is examined. This study will show that the formation ofthe vacuole occurs during the first 4-8 h after phagocytosis and that it involves a complex sequence of cytoplasmic events .