Toxicity of Pasteurella tularensis killed by ionizing radiation.

Approximately 2 x 10(11) viable Pasteurella tularensis cells per ml, contained in suspensions, were killed by exposure to 10(6) r of gamma-radiation. When injected intraperitoneally into mice, the irradiated suspensions initially contained about 10 ld(50) per ml, and immunized mice against challenge with fully virulent strains of P. tularensis. Toxicity and immunizing activity of the suspensions decreased significantly within a few days at 5 C. Mice were protected against the toxin by immune serum or by prior injection of endotoxin of Escherichia coli. Cortisone did not protect against the newly prepared suspension, but was effective against the aged suspension. Lethal doses of newly prepared suspension for guinea pigs and rabbits were approximately 0.5 ml and 2 ml, respectively. Cortisone protected rabbits, but not guinea pigs, against lethal challenge. Pyrogenic effects resembling those shown by endotoxin-containing suspensions were demonstrated in rabbits. The results suggested that two toxins are responsible for the toxicity of irradiated suspensions of P. tularensis: one labile and associated with the immunizing activity of the suspension, the other more stable and resembling classical endotoxin.