MICROBIOLOGY AND VIRULENCE FACTORS
Francisella tularensis is a small, non- motile, aerobic, gram-negative cocco- bacillus. It has a thin lipopolysaccha- ride-containing envelope and is a hardy non–spore-forming organism that sur- vives for weeks at low temperatures in water, moist soil, hay, straw, and de- caying animal carcasses.21,22,60,61 Fran- cisella tularensis has been divided into 2 major subspecies (biovars) by viru- lence testing, biochemical reactions, and epidemiological features.62 Francisella tularensis biovar tularensis (type A) may be highly virulent in humans and ani- mals, produces acid from glycerol, dem- onstrates citrulline ureidase activity, and is the most common biovar isolated in North America.22,60 Francisella tularen- sis biovar palaearctica (type B) is rela- tively avirulent, does not produce acid
from glycerol, and does not demon- strate citrulline ureidase activity. In Eu- rope and Asia, all human tularemia is thought to be caused by the milder type B strains, although recent studies there have identified naturally occurring F tu- larensis related to F tularensis biovar tu- larensis.63,64 A few rapidly growing strains of F tularensis have been recov- ered from the blood of immunocom- promised patients not showing serore- activity to F tularensis.65
Transformed plasmids have been en-
gineered to express chloramphenicol and tetracycline resistance in F tularen- sis.66 Virulent, streptomycin-resistant F tularensis strains have been exam- ined in biowarfare agent studies both in the United States and the Soviet Union.24,27,56 Although F tularensis viru- lence factors are poorly understood and characterized,67,68 it is possible that strain virulence could be enhanced through laboratory manipulation.
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