Francisella tularensis has a significant extracellular phase in infected mice.

The ability of Francisella tularensis to replicate in macrophages has led many investigators to assume that it resides primarily intracellularly in the blood of mammalian hosts. We have found this supposition to be untrue. In almost all cases, the majority of F. tularensis recovered from the blood of infected mice was in plasma rather than leukocytes. This distribution was observed irrespective of size of inoculum, route of inoculation, time after inoculation, or virulence of the infecting strain. Our findings yield new insight into the pathogenesis of tularemia and may have important ramifications in the search for anti-Francisella therapies.