INCREASED ENCAPSULATION AND VIRULENCE OF FRANCISELLA-TULARENSIS LIVE VACCINE STRAIN (LVS) BY SUBCULTURING ON SYNTHETIC MEDIUM

Francisella tularensis live vaccine strain (LVS), taken directly from lyophilized vials, did not appear to have a capsule and had low virulence for balb/c mice. When this strain was subcultured on Chamberlain's synthetic medium (pH 6.5), it became extensively capsulated (1-5 mu m in diameter) and its virulence for Balb/c mice increased about 1000-fold. We conclude that the virulence of the attenuated vaccine strain, F. tularensis LVS, may still be influenced by growth conditions.