Brucella ovis is a veterinary pathogen associated with epididymitis in sheep. Despite its genetic similarity to the zoonotic pathogens B. abortus, B. melitensis and B. suis, B. ovis does not cause zoonotic disease. Genomic analysis of the type strain ATCC25840 revealed a high percentage of pseudogenes and increased numbers of transposable elements compared to the zoonotic Brucella species, suggesting that genome degradation has occurred concomitant with narrowing of the host range of B. ovis. The absence of genomic island 2, encoding functions required for lipopolysaccharide biosynthesis, as well as inactivation of genes encoding urease, nutrient uptake and utilization, and outer membrane proteins may be factors contributing to the avirulence of B. ovis for humans. A 26.5 kb region of B. ovis ATCC25840 Chromosome II was absent from all the sequenced human pathogenic Brucella genomes, but was present in all of 17 B. ovis isolates tested and in three B. ceti isolates, suggesting that this DNA region may be of use for differentiating B. ovis from other Brucella spp. This is the first genomic analysis of a non-zoonotic Brucella species. The results suggest that inactivation of genes involved in nutrient acquisition and utilization, cell envelope structure and urease may have played a role in narrowing of the tissue tropism and host range of B. ovis.

Most members of the genus Brucella show strong urease activity. However, the role of this enzyme in the pathogenesis of Brucella infections is poorly understood. We isolated several Tn5 insertion mutants deficient in urease activity from Brucella abortus strain 2308. The mutations of most of these mutants mapped to a 5.7-kbp DNA region essential for urease activity. Sequencing of this region, designated ure1, revealed the presence of seven open reading frames corresponding to the urease structural proteins (UreA, UreB, and UreC) and the accessory proteins (UreD, UreE, UreF, and UreG). In addition to the urease genes, another gene (cobT) was identified, and inactivation of this gene affected urease activity in Brucella. Subsequent analysis of the previously described sequences of the genomes of Brucella spp. revealed the presence of a second urease cluster, ure2, in all them. The ure2 locus was apparently inactive in B. abortus 2308. Urease-deficient mutants were used to evaluate the role of urease in Brucella pathogenesis. The urease-producing strains were found to be resistant in vitro to strong acid conditions in the presence of urea, while urease-negative mutants were susceptible to acid treatment. Similarly, the urease-negative mutants were killed more efficiently than the urease-producing strains during transit through the stomach. These results suggested that urease protects brucellae during their passage through the stomach when the bacteria are acquired by the oral route, which is the major route of infection in human brucellosis.Brucellosis is the most common zoonosis in humans. Transmission of this disease occurs mainly by inhalation of infected aerosols, by animal contact, and by conjunctival and gastrointestinal routes. The gastrointestinal route is the most common portal of entry of Brucella in humans through ingestion of raw milk or its products and raw liver or meat (8). Transmission of Brucella melitensis, B. abortus, B. suis, and B. canis in animals also occurs by ingestion of contaminated abortions, discharge materials, or contaminated pasture plants. In contrast, gastrointestinal transmission is not important under natural conditions for B. ovis, where the sexual route seems to be the most probable route of infection (21). Most isolates of B. ovis are urease negative (10).Urease is a multisubunit, nickel-containing enzyme that catalyzes the hydrolysis of urea, yielding ammonia and carbon dioxide. The released ammonia is used by many bacteria as a source of nitrogen, and even for the generation of ATP from a strong ammonia gradient in the case of Ureaplasma urealyticum (38). Moreover, urease is a virulence factor for several human pathogens, and it plays a major role in both urinary and gastrointestinal tract infections, although through different mechanisms (9). In urinary tract infections caused by Proteus mirabilis, urease promotes direct toxicity to renal epithelium cells and kidney stone formation (16,24). In gastrointestinal tract infections, urease allows Helicobacter pylori colonization...

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