Ultrastructure of the Agent of Legionnaires’ Disease in the Human Lung

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Abstract
This report confirms the gramnegative ultrastructural characteristics of the Legionnaires’ disease organism by direct examination of pulmonary tissue from six confirmed cases—two from the original Philadelphia epidemic of 1976 and four from more recent sporadic cases. All microorganisms seen in all six lungs were identical ultrastructurally and were predominately within intra-alveolar macrophages, as previously observed by light microscopy. They appeared as short, blunt rods that were clearly prokaryotic; i.e., they had diffuse electron-lucent nucleoid areas interspersed among areas of well-defined ribosomes, a pinching nonseptate division, and enclosure within a double envelope consisting of two three-layer “unit” membranes, each approximately 75 Å wide. This structure, together with a pinching division, is typical of gram-negative bacteria. The Legionnaires’ disease organism multiplies both intracellularly and extracellularly in tissue and has no unique ultrastructural features that would aid in its specific identification. These findings are compared with recent reports describing the ultrastructure of what was considered to be the Legionnaires’ disease organism in yolk sac and culture medium, and in one human lung.