A strain of Francisella species wherein a gene which encodes for part of the glutamate metabolic pathway has been inactivated, and which is able to produce a protective immune response in an animal, for use as live prophylactic or therapeutic vaccine against infection by said Francisella species. Particularly effective strains include those where the capB gene is deleted. Other embodiments of the invention describe strains which comprise a further genetic mutation wherein a gene which encodes for another component of the cell is also inactivated. Pharmaceutical compositions comprising said strains, together with methods which utilize such strains are also described and claimed.
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