[Mutations in Francisella tularensis, decreasing the virulence of these bacteria and leading to a change in the immune response upon infection of experimental animals with them].

The research was aimed at isolation of Francisella tularensis mutants possessing the decreased virulence for experimental animals and mediating the changes in the animal immune response. A number of spontaneous and induced mutants of the American and European subtypes of Francisella tularensis were selected for antibiotics resistance or detergent sensitivity. All the obtained mutants have the decreased virulence and differ in their ability to induce the protective antitularemia immunity or ability to induce the humoral immune response in the laboratory animals. The dimeric immunoprecipitation in gel as well as immunoblotting have shown the mutations decreasing the virulence to cause the loss by bacteria of a number of antigenic structures (in case the virulence is completely lost) or changes in antigenic structure resulting in inability of bacteria to induce the humoral immune response when immunizing the laboratory animals. The latter occurs in partially virulent mutants of the vaccine mutant type. The concomitant changes in virulence, ability to cause protective immunity or humoral immune response of the mutants is discussed.