Immunization of Mice with Irradiated Pasteurella Tularensis.

Viable attenuated tularemia vaccines have been shown to bt. effective in immunizing mice and other animals against challenge with high].y virulent Pasteurella tularensis. Nonviable preparations, however, induced little or no resistance to challenge with even small numbers of virulent organisms. It seemed reasonable to expect that antigenicity would be retained to a greater extent if the organisms were killed by irradiation rather than by chemicals or heat. Vaccines rendered nonviable by the action of X-radiation produced levels of immunity such that 20 to 30 per cent of immunized mice survived intraperitoneal challenge with moderate doses of the highly virulent SCHU S4 strain. Cysteamine at a concentration of 0.02 M was added to the bacterial suspension prior to irradiation to minimize the indirect effects of irradiation. Vaccines contained approximately 0 nonviable organisms per milliliter. Proof of nonviability of irradiated vaccines was based on (a) absence of colonies when irradiated vaccines were plated on solid medium, (b) failure to isolate P. tularensis from sacrificed immunized animals, and (c) the inability of treatment with streptomycin during immunization to interfere with the development of immunity.