Virulence of the B. abortus S2308, Δmfp, Δomp19, Δmfp(pBBR1-mfp), or Δomp19(pBBR1-omp19) strain in IRF-1 KO mice. Mice were intraperitoneally injected with 106 CFU of each strain, and mouse survival was monitored for 90 days after infection. This result is representative of three independent experiments.