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 micron 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.