A histological and ultrastructural comparison of the sulfur granule of the actinomycosis and actinobacillosis.

The sulfur granules of actinomycosis and actinobacillosis were studied histologically and ultrastructurally. The sulfur granule of actinomycosis was large, and oval or horsehoe-shaped. In it, there were a number of gram-positive, filamentous or short rod-like hyphae beneath clubs. Ultrastructurally, the center of the sulfur granule was composed of a cluster of hyphae, and the periphery was surrounded by clubs arranged radially. The clubs were made of electron-dense fine granules and had a degenerated hypha at the center. These granules varied in amount from small to large. As for the host reaction, neutrophil infiltration appeared first around the sulfur granule. Then epithelioid cells proliferated and phagocytized hyphae. Finally, proliferating fibroblasts enclosed them and formed tiny granulomas. The sulfur granule of actinobacillosis was small and lobulated. In it, a number of gram-negative, short rod-like bacteria were present beneath clubs. Ultrastructurally, the center of the sulfur granule contained intact or degenerated bacteria, and the periphery was composed of radially projecting clubs. These clubs were made of electron-dense amorphous material, in which several layers of tubular structures surrounded the bacterium conentrically. The host reaction was almost the same as that of actinomycosis. The proliferation of fibroblasts was prominent. There was a strong tendency for these cells to form many tiny granulomas.