Legionnaires' disease: structural characteristics of the organism.

The Legionnaires' disease organism was isolated from lung tissue taken from two fatalities of the Legionnaires' disease epidemic that occurred in Philadelphia during 1976. In yolk sac tissue the agent grew as a small coccobacillary microorgansim, which was Gram variable and Giménez positive. Intracellular coccoid and bacillary forms, detected by electron microscopy, within and without vacuoles, underwent multiplication by septate binary fission. Some of the intracellular forms resembled obligate intracellular pathogens. On defined bacteriologic media, the organisms were predominantly bacillary. The organism conforms to the morphologic criteria of a prokaryocyte.