[The capacity of avirulent forms of Francisella tularensis for dissemination and proliferation in the host body].

In this investigation isogenic avirulent variants obtained from F. tularensis standard virulent strain 503 were used. The capsule-deficient variants (cap-) were shown to have no species-specific capsular antigens and to be capable of producing R-LPS having no the polysaccharide part of the molecules. The capsule-defective forms (cap +/- ) were found to synthesize capsular antigens and S-LPS whose polysaccharide part essentially differed from the O-lateral chains of LPS of the virulent strain. The study of bacterial dissemination revealed that virulent bacteria rapidly spread in the macroorganism, and their subsequent proliferation shortly led to the death of animals. Avirulent mutants (cap- and cap +/- ) appeared in the organs of animals later and proliferated slower, parasitizing in the macroorganism without fatal outcome. The cap- variants were not capable of inducing the synthesis of antitularemic antibodies and possessed no protective properties. The cap +/- mutants were capable of inducing the synthesis of antitularemic antibodies in mice. These antibodies facilitated the elimination of avirulent strains from the macroorganism, but did not ensure protection from infection with virulent strains.