In a primary infection, Swiss-Webster mice were injected i.p. with 10(2) or 10(3) virulent Salmonella typhimurium. Multiple microscopic acute abscesses with predominantly polymorphonuclear leucocytes (PMNs) were seen in the liver and the spleen beginning on the 4th day after infection. By the 7th day, these lesions had become enlarged and were gradually transformed into granulomas with central necrosis and peripheral mononuclear cells. The animals usually died within 12 days with massive systemic infection and degeneration of the tissues. In contrast, it was necessary to inoculate 10(6) virulent salmonella i.v. into mice immunized with avirulent S. thphimurium in order to initiate microscopically observable lesions in the liver and the spleen. These secondary lesions were characterized by the early appearance of minute granulomas composed almost entirely of histiocytic cells. They remained small and isolated, usually without central necrosis. Subsequent regression of the lesions and regeneration of normal tissue occurred after the 2nd week following infection. The animals usually survived such a challenge infection.
Animals Immunization