[The characteristics of a natural attenuated isolate of Francisella tularensis].

The natural isolate of F. tularensis subsp. holarctica 268 was detected and studied. The isolate possessed the properties of the vaccine strain: residual virulence for white mice, avirulence for guinea pigs and high immunogenicity for experimental animals. A significant decrease in its virulence for animals, highly sensitive to tularemia, was noted. In contrast to most virulent strains circulating in natural foci, F. tularensis strain 268 was characterized by the absence of growth at a cultivation temperature of 42 degrees C and had stable biological properties after 10-fold passage through white mice.