Experimental streptococcal endocarditis. The early vegetation.

Aortic valve vegetation were studied in rabbits before and after intravenous injection of streptococcus mitis. Uninfected vegetation sections revealed compact eosinophilic masses with smooth-appearing edges and scattered polymorphonuclear leukocytes inside and on the surface. Scanning electron microscopic views demonstrated erythrocytes, small leukocytes, and platelets; the surface was markedly irregular. Fifteen minutes after injection of 10(10) streptococci, large mononuclear cells and polymorphonuclear leukocytes lined the vegetation edge; scanning electron microscopy showed large and small leukocytes. One and six hours after infection, there were more polymorphonuclear leukocytes inside the vegetation but few superficially. Bacteria were first seen at six hours as microcolonies inside the vegetation. Twenty-four-hour sections showed large bacterial colonies inside with few bacteria at the edges. Despite the smooth appearance of the vegetation edges, all electron microscopic views revealed very rough surfaces.