| Summary: | Brucella abortus, a facultative intracellular pathogen causes chronic infections in agricultural animals and humans. Brucella survival inside host cells allows it to evade host immune mechanisms such as antibody and complement. This, however subjects the bacteria to a harsh intracellular environment characterized by reactive oxygen intermediates, low pH and decreased iron availability. It has been shown that this environment, especially its acidity, activates Brucella genes necessary for survival. This work examined several genes to determine if they were essential for the intracellular survival of this pathogen. The first mutant investigated containing a deletion of entC, which encodes isochorismate synthase, a 43-kDa polypeptide that catalyzes one of the first steps in the DHBA biosynthetic pathway. It was found that DHBA protects the bacteria from intracellular killing. Also investigated was the bacA gene, thought to encode a cytoplasmic membrane transport protein that may have a role in lipid A modification required for integrity of the cell envelope and for long term intracellular survival. This mutant was found to be only slightly attenuated in C57BL/6 mice, considered the resistant Brucella host. Lastly an hfq deletion mutant was investigated. This gene is thought to encode for a host-factor I (HF-I) homologue, necessary for the post-transcriptional enhancement of a sigma factor, required for transcription of approximately 50 stationary phase genes. This mutant was attenuated in all murine backgrounds studied. Also examined was the immune response elicited against a Brucella infection to determine the crucial elements necessary to generate protective cell mediated immunity. Utilized was the resistant host model of Brucella, C57BL/6 and C57BL/10 mice which produce a Th1 immune response and the susceptible host model BALB/c mice which produce a Th2 immune response. We examined the role of IFN-γ in both murine backgrounds along with those products that attempt to compensate for the absence IFN-γ in BALB/c mice. It was determined that CD8+ T-cells and tumor necrosis factor-alpha (TNF-α) can compensate for the absence of IFN-γ production. Also investigated was the cause of the IFN-γ hiatus in the BALB/c mice. It was determined that this may partially result from excessive IL-12 p40 homodimer production.
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