The Genus Francisella
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The Genus Francisella FRANCIS NANO AND KAREN ELKINS
Introduction Francisella tularensis is best known as the zoonotic, highly infectious etiologic agent of the human disease, tularemia. There are three recognized species in the genus Francisella: F. tularensis, F. novicida, and F. philomiragia. All three species can cause human infections, although F. novicida and F. philomiragia rarely do. Disease caused by F. tularensis can be mild but it usually is acute, severe and febrile; infection with a common North American strain can be life threatening.
Phylogeny Analysis of the 16S rRNA of Francisella species places them in the γ-subclass of the Proteobacteria (Forsman, 1994). All of the Francisella are closely related (98.5 to 99.9% sequence identity in the 16S rDNA sequence) and form a group together with Wobachia persica (97.85 identity) (Forsman, 1994). Recently, two groups discovered endosymbionts of the tick species, Ornithodoros moubata (Noda, 1997) and Dermacantor andersoni (Niebylski, 1997) that could be placed in the Francisella group by virtue of their 16S rDNA sequence similarity. The bacterium associated with D. andersoni could grow in Vero cells and infect rodents experimentally, although no tick-to-mammal transmission was observed. It is reasonable to assume that W. persica is misclassified as a Wobachia species and eventually will be recognized as Francisella species along with the tick endosymbionts.
Taxonomy Classically, Francisella spp. have been classified primarily by their virulence phenotypes. The most virulent form is often called F. tularensis type A but is also referred to as F. tularensis var. tularensis. This strain is found only in North America and is the usual cause of tularemia in
humans. The type B form of F. tularensis, sometimes referred to as var. palaearctic, is found throughout the Northern Hemisphere. This strain is less virulent than the type A strain. The type A form has citrulline ureidase and ferments glycerol, whereas the type B strain does not. F. novicida can be differentiated from the F. tularensis strains by its low virulence in rats, its ability to ferment sucrose, its rapid growth and its less fastidious requirements on agar media (Hollis, 1989; Owen, 1964).
Habitat F. tularensis and F. novicida probably should be considered obligate pathogens. Their fastidious growth requirements make growth outside of hosts unlikely. Many researchers have long considered the natural reservoir for F. tularensis type A to be the rabbit. In Europe and Asia, F. tularensis type B is most often associated with hares and small rodents. However, these associations are based on the observed transmission of F. tularensis to humans, and there may be other, unobserved, carriers of tularemia. One thing is clear: arthropods are simply a mechanical vector for transmission, and there is no transovarian passage of classical F. tularensis strains.
Isolation Because of the highly infectious nature of F. tularensis, its isolation usually is attempted only by
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