from Renee M. Tsolis and David O'Callaghan writing in Brucella: Molecular Microbiology and Genomics:
The type IV secretion system (T4SS) encoded by the virB operon is a key pathogenicity determinant that is highly conserved among Brucella species, and is one of the few "classical" virulence factors identified to date. Since its first description in 1999, work on the VirB T4SS has focused on its role in intracellular survival and persistence, its architecture and assembly in the cell envelope, on the signals that induce its expression and function within the infected host cell, on identification of T4SS substrates, and on its effect on the host response to infection. In this chapter, we review the advances in our understanding of structure/function relationships of the T4SS, and of its function in infection by Brucella species.
Further reading: Brucella: Molecular Microbiology and Genomics