Vaccines against Francisella tularensis--past, present and future

Francisella tularensis is a facultative intracellular bacterial pathogen capable of causing a spectrum of human diseases collectively called tularemia. The pathogen is highly infectious and some strains can cause rapidly lethal infection especially when inhaled. The latter were developed as biological weapons in the past and nowadays cause concern as potential bioterrorism agents. A live attenuated strain of the pathogen was developed more that 40 years ago and remains the sole prophylactic measure against the pathogen. Research to develop better live and subunit vaccines is under way. The former will require an understanding of the virulence factors of F. tularensis and a facile means of mutating them and the latter will require identification of the protective antigens of the pathogen. The current vaccine and its potential replacements are the focus of this review.