The Immunologically Distinct O Antigens fromFrancisella tularensisSubspeciestularensisandFrancisella novicidaAre both Virulence Determinants and Protective Antigens

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Abstract
We have determined the sequence of the gene cluster encoding the O antigen inFrancisella novicidaand compared it to the previously reported O-antigen cluster inFrancisella tularensissubsp.tularensis. Immunization with purified lipopolysaccharide (LPS) fromF. tularensissubsp.tularensisorF. novicidaprotected against challenge withFrancisella tularensissubsp.holarcticaandF. novicida, respectively. The LPS fromF. tularensissubsp.tularensisdid not confer protection against challenge withF. novicida, and the LPS fromF. novicidadid not confer protection against challenge withF. tularensissubsp.holarctica. Allelic replacement mutants ofF. tularensissubsp.tularensisorF. novicidawhich failed to produce O antigen were attenuated, but exposure to these mutants did not induce a protective immune response. The O antigen ofF. tularensissubsp.tularensisappeared to be important for intracellular survival whereas the O antigen ofF. novicidaappeared to be critical for serum resistance and less important for intracellular survival.