SYNOPSIS Two strains of pathogenic Naegleria were employed to infect mice and monkey kidney (Vero line) cell cultures. Mice were infected intranasally. Moribund mice were sacrificed and their brains processed for light and electron microscopy. The normal architecture of the infected brain was completely destroyed; the olfactory lobes and the cerebral cortex showed the heaviest damage. The inflammatory response was mainly in the form of neutrophil polymorphs (PMN) and was confined to the olfactory lobes and the superficial regions of cerebral cortex. Numerous amebas were seen interspersed with the degenerating neurons, glial processes, and PMN. Most conspicuous were the food vacuoles which contained host tissue in various stages of digestion. Amebas in the brain tissue also produced many micropinocytotic vesicles from the surface of the plasma membrane. These vesicles are interpreted as vehicles of transport of nutritive materials from the host tissue. The infected cell culture showed the characteristic cytopathic effect (CPE). The CPE was chiefly in the form of cell shrinkage, nuclear pycnosis and discontinuity of cell sheet. Amebas were often seen in an intracellular location. The Vero cells produced many fuzzy pinocytotic vesicles at these loci where the ameba plasma membrane and Vero cell membrane were in close apposition; the probable significance of this is discussed. Most impressive, however, were the pseudopodial formation and capturing of the host material which indicated the great phagocytic activity of the amebas. This was confirmed further by the presence of large numbers of food vacuoles containing host material in various stages of digestion. These observations show that the amebas invade and destroy the brain tissue by active phagocytosis.

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Spherical bodies resembling coccidian oocysts and measuring 8.0 to 9.0 ,um in diameter were seen in the stools of eight persons with explosive, watery diarrhea. Seven had recently traveled to tropical countries, mostly in the Caribbean, and four had acquired immunodeficiency syndrome. The structures were easily discernible in wet mounts by light microscopy and contained variable numbers of granular inclusions, but were refractory to, or stained partially with, 12 commonly used laboratory stains. Electron microscopy revealed an outer fibrillar coat, a thin cell wall, granules, and organelles which were not surrounded by membranes. One type of organelle was similar to the thylakoid photosynthesizing organelles of blue-green algae (cyanobacteria). These findings indicate that the bodies may be a species of blue-green algae.

This report confirms the gram-negative 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 predominantely 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 nonseptic division, and enclosure within a double envelope consisting of two three-layer "unit" membranes, each approximately 75 A wide. This structure, together with a pinching division, is typical of gram-negative bacteria. The Legionnaires' disease organism multiples 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.