Quorum-Sensing and BvrR/BvrS Regulation, the Type IV Secretion System, Cyclic Glucans, and BacA in the Virulence of Brucella ovis: Similarities to and Differences from Smooth Brucellae

Brucella ovis is a rough bacterium—lacking O-polysaccharide chains in the lipopolysaccharide—that is virulent in its natural host and whose virulence mechanisms remain almost unexplored. In a search for additional traits that distinguish B. ovis from smooth Brucella, which require O-polysaccharide chains for virulence, we have analyzed the significance in B. ovis of the main virulence factors described for smooth Brucella. Attempts to obtain strains of virulent B. ovis strain PA that are mutated in the BvrR/BvrS two-component regulatory system were unsuccessful, suggesting the requirement of that system for in vitro survival, while the inactivation of bacA—in contrast to the results seen with smooth Brucella—did not affect splenic colonization in mice or behavior in J774.A1 murine macrophages. Defects in the synthesis of cyclic s-1,2 glucans reduced the uptake of B. ovis PA in macrophages and, although the intracellular multiplication rate was unaffected, led to attenuation in mice. Growth of strains with mutations in the type IV secretion system (encoded by the virB operon) and the quorum-sensing-related regulator VjbR was severely attenuated in the mouse model, and although the mutant strains internalized like the parental strain in J774.A1 murine macrophages, they were impaired for intracellular replication. As described for B. melitensis, VjbR regulates the transcription of the virB operon positively, and the N-dodecanoyl-dl-homoserine lactone (C12-HSL) autoinducer abrogates this effect. In contrast, no apparent VjbR-mediated regulation of the fliF flagellar gene was observed in B. ovis, probably due to the two deletions detected upstream of fliF. These results, together with others reported in the text, point to similarities between rough virulent B. ovis and smooth Brucella species as regards virulence but also reveal distinctive traits that could be related to the particular pathogenicity and host tropism characteristics of B. ovis.