[The resistance of Francisella tularensis to the bactericidal action of normal serum as a criterion for evaluating the virulence of the bacterium].

The study of the sensitivity of F. tularensis to the bactericidal action of normal serum (NS) revealed that all virulent cultures were resistant, while avirulent cultures were highly sensitive to NS. The synthesis of R-lipopolysaccharide (LPS) by capsule-deficient (Cap-) clones or the synthesis of S-LPS by capsule-defective (Cap+/-) clones of the avirulent phenotype of these bacteria had no influence on the sensitivity of these forms to NS, but ensured longer survival of Cap+/- variants in the serum in comparison with Cap- cells. The antibacterial activity of NS with respect to avirulent strains was determined by the system of the complement, activated following the classical route. Only complete S-LPS characteristic of virulent strains was capable of preventing the sorption of components and the assembly of the complement on the surface of bacteria, while defective forms of LPS of avirulent strains did not protect serosensitive receptors from interaction with the complement. The opsonization of Cap+/- virulent and Cap- avirulent bacteria with tularemic antibodies did not alter [correction of after] their reaction to incubation with NS, but ensured the survival of Cap+ forms for more than 24 hours.