Ultrastructural observation on the bacterial capsule of Bacillus anthracis seen in the spleen of a diseased cow.

The bacterial capsule of Bacillus anthracis seen in the spleen of a cow which had died of anthrax was studied electron microscopically. It was visible around bacteria which had been fixed in a ruthenium red-containing fixative. It was composed of electron-dense spikes and extended outwards from the bacterial surface. It was not found around bacteria which had been treated with a fixative devoid of ruthenium red. Bacteria were seen scattered among blood cells. They were not phagocytized by neutrophils or macrophages.