Cross-protection induced in mice by immunizations with proteins of related bacteria species.

Groups of mice were immunized with detoxified protein from S. typhimurium, S. paratyphi B and S. paratyphi C. Consecutive infections with different concentrations of the homologous and heterologous strains showed that: 1. Immunizations with proteins from S. typhimurium induced protections in 65% of the mice infected with 50 LD100 of their natural pathogen, and in 80% of the mice infected with 50 LD100 of S. paratyphi B; the infection with S. paratyphi C of mice in this group afforded protection against 20 LD100 in 75% of the animals. 2. Immunization with proteins from S. paratyphi B induced protection in the mice against the infection with 20 LD100 of S. typhimurium (survival of 80% of the mice) and against 20 LD100 of the homologous S. paratyphi B (survival of 90% of the mice). 3. Immunization with proteins from S. paratyphi C protected the mice against the infection with 20 LD100 of S. typhimurium in a proportion of 80-85% of the animals; infection with the homologous S. paratyphi C did not result in protection against more than 20 LD100 of the bacteria (80-85% survivals). The survivors, in each group, when reinfected 30 days later with 50 LD100 of S. typhimurium resisted in a proportion of 100%, as a consequence of antibodies induced against more specific proteins released in the mice during the infections by the related pathogens.