Vaccine potencies of the live vaccine strain of Francisella tularensis and isolated bacterial components

The live vaccine strain of Francisella tularensis rapidly lost virulence (to mice) when grown under controlled conditions in the Porton mobile enclosed chemostat but protective activity was lost much less rapidly. In contrast, bacteria maintained their virulence and vaccine potency during batch culture in shaken flasks. Attempts to identify virulence and protective factors of F.tularensis were unsuccessful. The protective activities in mice of killed suspensions of F.tularensis, bacterial fractions and extracts of live bacteria were compared with the activity of live bacteria; all the non-living potential vaccines were ineffective.