Identification and classification of different isolates of Francisella tularensis.

The causative agent of tularemia, Francisella tularensis, occurs in two main biovars, the highly virulent F. t. biovar tularensis, found in North America; and the less virulent biovar palaearctica, found all over the northern hemisphere. Two other biovars have been proposed, F. t. biovar mediaasiatica and F. t. biovar palaearctica var. japonica. In Sweden tularemia is most frequently observed in man and varying hares (Lepus timidus), and occasionally in other species. Tularemia in hares is normally an acute fatal disease, although less fatal infections have been reported. The diagnosis of tularemia is routinely based on immunological reactions. We studied 10 different isolates of F. tularensis from varying hares, one isolate from an Ural owl (Strix uralensis), one vaccine strain, one strain of F. t. biovar japonica, and six isolates from a virulence study of F. tularensis, by biochemical tests and by hybridization experiments with probes complementary to 16S rRNA. All isolates, except the isolate F. t. biovar japonica, were characterized as F. t. biovar palaearctica by biochemical tests. In the 16S rRNA analysis all isolates were positive to the probe for Francisella tularensis and the probe for F. t. biovar palaearctica with the exception that F. t. biovar japonica reacted with the probe specific to F. t. biovar tularensis. To further confirm that the strains used belonged to F. t. biovar palaearctica virulence tests in rabbits were performed which disclosed this phenotype. The results presented in this work show that the isolated strains from the western part of Europe were F. t. biovar palaearctica, irrespective of animal origin or virulence.