2. INFECTIOUS AGENT

Francisella tularensis (formerly Pasteurella tularensis), is a small, gram-negative nonmotile coccobacillus (see picture). All isolates are serologically homogeneous but are differentiated epidemiologically and biochemically into Jellison type A (F. tularensis biovar tularensis), which has an LD₅₀ in rabbits of fewer than 10 bacteria, or type B strains (F. tularensis biovar palaearctica), which have an LD₅₀ of greater than 10⁷ in rabbits.