Brucella spp. are facultative intracellular bacteria that cause brucellosis in humans and other animals. Brucella spp. are taken up by macrophages, and the outcome of the macrophage-Brucella interaction is a basis for establishment of a chronic Brucella infection. Microarrays were used to analyze the transcriptional response of the murine macrophage-like J774.A1 cell line to infection with virulent Brucella melitensis strain 16M. It was found that most significant changes in macrophage gene transcription happened early following infection, and global macrophage gene expression profiles returned to normal between 24 and 48 h postinfection. These findings support the observation that macrophages kill the majority of Brucella cells at the early infection stage, but the surviving Brucella cells are able to avoid macrophage brucellacidal activity inside replicative phagosomes at the later infection stage. At 4 h postinfection, macrophage genes involved in cell growth, metabolism, and responses to endogenous stimuli were down-regulated, while the inflammatory response (e.g., tumor necrosis factor alpha and Toll-like receptor 2), the complement system, the responses to external stimuli, and other immune responses were up-regulated. It is likely that the most active brucellacidal activity happened between 0 and 4 h postinfection. Mitochondrion-associated gene expression, which is involved in protein synthesis and transport, electron transfer, and small-molecule transfer, and many other mitochondrial functions were significantly down-regulated at 4 h postinfection. Although there were both proand antiapoptosis effects, B. melitensis 16M appears to inhibit apoptosis of macrophages by blocking release of cytochrome c and production of reactive oxygen species in the mitochondria, thus preventing activation of caspase cascades.Brucellosis in humans and other animal species is caused by facultative intracellular bacteria belonging to the genus Brucella. Unlike many pathogenic bacteria, the brucellae lack classical virulence factors, such as invasive proteases, exotoxins, endotoxic lipopolysaccharide (LPS), capsules, fimbriae, pili, virulence plasmids, and lysogenic phages (29,48). Brucella virulence relies on the ability of the organism to survive and replicate within vacuolar phagocytic compartments of macrophages (37). The host macrophage-Brucella interaction is critical for establishment of chronic Brucella infections. For example, both the type IV secretion system encoded by the virB operon (7) and the two-component regulatory system encoded by the bvrRS operon (44) are necessary for successful replication of Brucella inside macrophages. Smooth Brucella strains with intact LPS O side chains are virulent and invade macrophages through lipid rafts (56). Immediately after entry into macrophages, Brucella strains reside in an acidified compartment that fuses with components of the early endosomal pathway (56). The majority of Brucella strains are killed at the early infection stage (14, 56). However, a subpopulation of virule...

Background: Brucella is an intracellular pathogen capable of infecting animals and humans. There are six recognized species of Brucella that differ in their host preference. The genomes of the three Brucella species have been recently sequenced. Comparison of the three revealed over 98% sequence similarity at the protein level and enabled computational identification of common and differentiating genes. We validated these computational predictions and examined the expression patterns of the putative unique and differentiating genes, using genomic and reverse transcription PCR. We then screened a set of differentiating genes against classical Brucella biovars and showed the applicability of these regions in the design of diagnostic tests.