Cat-scratch disease. Light micrograph of Warthin-Starry silver stain in a lymph node biopsy from a case of cat-scratch disease. The causative agent is Bartonella henselae, a gram-negative bacillus bacterium. With Warthin-Starry stain, they appear as small, black curved organisms either singly or in clusters, usually in the areas of necrosis and in blood vessels.